Unit - 3

Geological Hazards

3.1 Rock Instability and Slope movement

Slope-stability or mass-movement problems occur where either sediment and/or rock and/or snow move downslope in response to gravity. Potential slope-stability problems exist wherever development has taken place at the base of steep slopes.

Fig 1 Rockfall at Upper Island Cove

Fig 2 Upper Island Cove

Downslope movement is a natural process but can be accentuated by undercutting of the base of slopes, clearance of stabilizing vegetation, or diversion of natural drainage. Types of downslope movement include landslide, avalanche, rockfall, rock slip, and rotational slumps. The first three are rapid events, and generally the most dangerous to life and property.

Fig 3 Mass Movement Types

Mass movement types

Variations in water content and rates of movement produce a variety of forms. A rockfall is simply a volume of rock made up of individual pieces that fall independently through the air and hit a surface. A debris avalanche is a mass of falling and tumbling rock, debris, and soil. It is differentiated from a slower landslide by the tremendous velocity of onrushing material. The extreme danger of a debris avalanche results from its high speed and consequent lack of warning.

A landslide is a sudden rapid movement of a cohesive mass of material (soil, rock, etc) that is not saturated with moisture. It involves a large amount of material failing simultaneously. A common type of slide is the rotational slide or slump which occurs when surface material moves along a concave surface. Frequently water is present along this movement plane and acts as a lubricant.

The simplest form of a rotational slump is when a small block of land shifts downward. The upper surface of the slide appears to rotate backward and often remains intact. When the moisture content of moving material is high, the term flow is used. Flows include earthflows and more fluid mudflows.

Block Sliding

Although many types of mass movements are included in the general term “landslide,” the more restrictive use of the term refers only to mass movements, where there is a distinct zone of weakness that separates the slide material from the more stable underlying material. The two major types of slides are rotational slides and translational slides. Rotational slide: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide (Fig 4 a). Translational slide: In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting (Fig 4 b). A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass (Fig 4 c).

Fig 4 Major types of landslide movement.

When large amounts of rock suddenly break loose from a cliff or mountainside, they move quickly and with tremendous force (Fig 5). Air trapped under the falling rocks acts as a cushion that keeps the rock from slowing down. Landslides and avalanches can move as fast as 200 to 300 km/hour.

Fig 5 (a) Landslides are called rock slides by geologists. (b) A snow avalanche moves quickly downslope, burying everything in its path.

Landslides are exceptionally destructive. Homes may be destroyed as hillsides collapse. Landslides can even bury entire villages. Landslides may create lakes when the rocky material dams a stream. If a landslide flows into a lake or bay, it can trigger a tsunami (Fig 6).

Fig 6 The 1958 landslide into Lituya Bay, Alaska

Landslides often occur on steep slopes in dry or semi-arid climates. The California coastline, with its cliffs and years of drought punctuated by seasons of abundant rainfall, is prone to landslides. At-risk communities have developed landslide warning systems.

Controlling Factors

Mass wasting happens because tectonic processes have created uplift. Erosion, driven by gravity, is the inevitable response to that uplift, and various types of erosion, including mass wasting, have created slopes in the uplifted regions. Slope stability is ultimately determined by two factors: the angle of the slope and the strength of the materials on it.

In Fig 7a,a block of rock situated on a rock slope is being pulled toward Earth’s center (vertically down) by gravity. We can split the vertical gravitational force into two components relative to the slope: one pushing the block down the slope (the shear force), and the other pushing into the slope (the normal force). The shear force, which wants to push the block down the slope, has to overcome the strength of the connection between the block and the slope, which may be quite weak if the block has split away from the main body of rock, or maybe very strong if the block is still a part of the rock. This is the shear strength, and in Fig 7a, it is greater than the shear force, so the block should not move. In Fig 7b, the slope is steeper and the shear force is approximately equal to the shear strength. The block may or may not move under these circumstances. In Fig 7c, the slope is steeper still, so the shear force is considerably greater than the shear strength, and the block will very likely move.

Fig 7 Differences in the shear and normal components of the gravitational force on slopes with differing steepness.

Slopes are created by uplift followed by erosion. In areas with relatively recent uplift, slopes tend to be quite steep. This is especially true where glaciation has taken place because glaciers in mountainous terrain create steep-sided valleys. In areas without recent uplift, slopes are less steep because hundreds of millions of years of erosion (including mass wasting) have made them that way. However, some mass wasting can happen even on relatively gentle slopes.

The strength of the materials on slopes can vary widely. Solid rocks tend to be strong, but there is a very wide range of rock strength. If we consider just the strength of the rocks and ignore issues like fracturing and layering, then most crystalline rocks — like granite, basalt, or gneiss — are very strong, while some metamorphic rocks — like schist — are moderately strong. Sedimentary rocks have variable strength. Dolostone and some limestone are strong, most sandstone and conglomerate are moderately strong, and some sandstone and all mudstones are quite weak.

Fractures, metamorphic foliation, or bedding can significantly reduce the strength of a body of rock, and in the context of mass wasting, this is most critical if the planes of weakness are parallel to the slope and least critical if they are perpendicular to the slope. This is illustrated in Fig 8. At locations A and B the bedding is nearly perpendicular to the slope and the situation is relatively stable. At location D the bedding is nearly parallel to the slope and the situation is quite unstable. At location C the bedding is nearly horizontal and the stability is intermediate between the other two extremes.

Fig 8 Relative stability of slopes

Internal variations in the composition and structure of rocks can significantly affect their strength. Schist, for example, may have layers that are rich in sheet silicates (mica or chlorite) and these will tend to be weaker than other layers. Some minerals tend to be more susceptible to weathering than others, and the weathered products are commonly quite weak (e.g., the clay formed from feldspar). The side of Johnson Peak that failed in 1965 (Hope Slide) is made up of chlorite schist (metamorphosed sea-floor basalt) that has feldspar-bearing sills within it. The foliation and the sills are parallel to the steep slope. The schist is relatively weak, to begin with, and the feldspar in the sills, which has been altered to clay, makes it even weaker.

Unconsolidated sediments are generally weaker than sedimentary rocks because they are not cemented and, in most cases, have not been significantly compressed by overlying materials. This binding property of sediment is sometimes referred to as cohesion. Sand and silt tend to be particularly weak, clay is generally a little stronger, and sand mixed with clay can be stronger still. The deposits that make up the cliffs at Point Grey in Vancouver include sand, silt, and clay overlain by sand. As shown in Fig 9(left) the finer deposits are relatively strong (they maintain a steep slope), while the overlying sand is relatively weak, and has a shallower slope that has recently failed. Glacial till — typically a mixture of clay, silt, sand, gravel, and larger clasts — forms and is compressed beneath tens to thousands of meters of glacial ice so it can be as strong as some sedimentary rock (Fig 9, right).

Fig 9 Left: Glacial outwash deposits at Point Grey, in Vancouver. Right: Glacial till on Quadra Island, B.C

Apart from the type of material on a slope, the amount of water that the material contains is the most important factor controlling its strength. This is especially true for unconsolidated materials, like those shown in Fig 9, but it also applies to bodies of rock. Granular sediments, like the sand at Point Grey, have lots of spaces between the grains. Those spaces may be completely dry (filled only with air); or moist (often meaning that some spaces are water-filled, some grains have a film of water around them, and small amounts of water are present where grains are touching each other); or completely saturated (Fig 10). Unconsolidated sediments tend to be strongest when they are moist because the small amounts of water at the grain boundaries hold the grains together with surface tension. Dry sediments are held together only by the friction between grains, and if they are well sorted or well rounded, or both, that cohesion is weak. Saturated sediments tend to be the weakest of all because a large amount of water pushes the grains apart, reducing the mount friction between grains. This is especially true if the water is under pressure.

Fig 10 Depiction of dry, moist, and saturated sand

Instability of rock structures

Slope stability refers to the condition of inclined soil or rock slopes to withstand or undergo movement. The stability condition of slopes is a subject of study and research in soil mechanics, geotechnical engineering, and engineering geology. Analyses are generally aimed at understanding the causes of an occurred slope failure, or the factors that can potentially trigger a slope movement, resulting in a landslide, as well as at preventing the initiation of such movement, slowing it down, or arresting it through mitigation countermeasures.

The stability of a slope is essentially controlled by the ratio between the available shear strength and the acting shear stress, which can be expressed in terms of a safety factor if these quantities are integrated over a potential (or actual) sliding surface. A slope can be globally stable if the safety factor, computed along any potential sliding surface running from the top of the slope to its toe, is always larger than 1. The smallest value of the safety factor will be taken as representing the global stability condition of the slope. Similarly, a slope can be locally stable if a safety factor larger than 1 is computed along any potential sliding surface running through a limited portion of the slope (for instance only within its toe). Values of the global or local safety factors close to 1 (typically comprised between 1 and 1.3, depending on regulations) indicate marginally stable slopes that require attention, monitoring, and/or an engineering intervention (slope stabilization) to increase the safety factor and reduce the probability of a slope movement.

A previously stable slope can be affected by several predisposing factors or processes that make the safety factor decrease - either by increasing the shear stress or by decreasing the shear strength - and can ultimately result in slope failure. Factors that can trigger slope failure include hydrologic events (such as intense or prolonged rainfall, rapid snowmelt, progressive soil saturation, an increase of water pressure within the slope), earthquakes (including aftershocks), internal erosion (piping), surface or toe erosion, artificial slope loading (for instance due to the construction of a building), slope cutting (for instance to make space for roadways, railways or buildings), or slope flooding (for instance by filling an artificial lake after damming a river).

Example

Fig 11: Simple slope slip section

As seen in Fig 11, earthen slopes can develop a cut-spherical weakness area. The probability of this happening can be calculated in advance using a simple 2-D circular analysis package. A primary difficulty with analysis is locating the most-probable slip plane for any given situation. Many landslides have only been analyzed after the fact. More recently slope stability radar technology has been employed, particularly in the mining industry, to gather real-time data and assist in determining the likelihood of slope failure.

Fig 12 Real life landslide on a slope

Real-life failures in naturally deposited mixed soils are not necessarily circular, but before computers, it was far easier to analyze such a simplified geometry. Nevertheless, failures in 'pure' clay can be quite close to circular. Such slips often occur after a period of heavy rain, when the pore water pressure at the slip surface increases, reducing the effective normal stress and thus diminishing the restraining friction along the slip line. This is combined with increased soil weight due to the added groundwater. A 'shrinkage' crack (formed during prior dry weather) at the top of the slip may also fill with rainwater, pushing the slip forward. At the other extreme, slab-shaped slips on hillsides can remove a layer of soil from the top of the underlying bedrock. Again, this is usually initiated by heavy rain, sometimes combined with increased loading from new buildings or removal of support at the toe (resulting from road widening or other construction work). Stability can thus be significantly improved by installing drainage paths to reduce the destabilizing forces. Once the slip has occurred, however, weakness along the slip circle remains, which may then recur at the next monsoon.

Slope stability issues can be seen with almost any walk down a ravine in an urban setting. An example is shown in Fig 13, where a river is eroding the toe of a slope, and there is a swimming pool near the top of the slope. If the toe is eroded too far, or the swimming pool begins to leak, the forces driving a slope failure will exceed those resisting failure, and a landslide will develop, possibly quite suddenly.

Fig 13 The relationship between the angle of repose and material on the slope.

Angle of Repose

The angle of repose is defined as the steepest angle of granular unconfined material measured from the horizontal plane on which the granular material can be heaped without collapsing, ranging between 0-90°. For granular material, the angle of repose is the main factor that influences the slope's stability under different conditions with the cohesiveness/friction of the material, the grain size, and the particle shape.

The angle of Repose and Slope stability

The angle of repose and the stability of a slope are impacted by climatic and non-climatic factors.

• Water Content

Water content is an important parameter that could change the angle of repose. Reportedly, the higher water content can stabilize a slope and increase the angle of repose. However, water saturation can result in a decrease in the slope's stability since it acts as a lubricant and creates a detachment where mass wasting can occur.

Water content is dependent on soil properties such as grain size, which can impact infiltration rate, runoff, and water retention. Generally, finer-grained soils rich in clay and silt retain more water than coarser sandy soils. This effect is mainly due to capillary action, where the adhesive forces between the fluid, particle, and the cohesive forces of the fluid itself counteract gravitational pull. Therefore, a smaller grain size results in a smaller surface area on which gravitational forces can act. Smaller surface area also leads to more capillary action, more water retention, more infiltration, and less runoff.

• Vegetation

The presence of vegetation does not directly impact the angle of repose, but it acts as a stabilizing factor in a hillslope, where the tree roots anchor into deeper soil layers and form a fiber‐reinforced soil composite with a higher shear resistance (mechanical cohesion).

• Roundness of Grains

The shape of the grain can have an impact on the angle of repose and the stability of the slope. The more rounded the grain is, the lower the angle of repose. A decrease in roundness, or an increase in angularity, results in interlocking via particle contact. This linear relationship between the angle of repose and the roundness of grain can also be used as a predictor of the angle of repose if the roundness of the grain is measured.

Applications of Angle of Repose in engineering

The angle of repose is related to the shear strength of geologic materials, which is relevant in construction and engineering contexts. For granular materials, the size and shape of grains can impact the angle of repose significantly. As the roundness of materials increases, the angle of repose decreases since there is less friction between the soil grains.

When the angle of repose is exceeded, mass wasting and rockfall can occur. Many civil and geotechnical engineers need to know the angle of repose to avoid structural and natural disasters. As a result, the application of retaining walls can help to retain soil so that the angle of repose is not exceeded.

Slope Stabilization

Since the stability of the slope can be impacted by external events such as precipitation, an important concern in civil/geotechnical engineering is the stabilization of slopes.

Application of Vegetation

The application of vegetation to increase the slope stability against erosion and landslide is a form of bioengineering that is widely used in areas where the landslide depth is shallow. Vegetation increases the stability of the slope mechanically, by reinforcing the soils through plant roots, which stabilize the upper part of the soil. Vegetation also stabilizes the slope via hydrologic processes, by the reduction of soil moisture content through the interception of precipitation and transpiration. This results in drier soil that is less susceptible to mass wasting.

Stability of slopes can also be improved by:

• Flattening of slopes results in a reduction in weight which makes the slope more stable
• Soil stabilization
• Providing lateral supports by piles or retaining walls
• Grouting or cement injections into special places
• Consolidation by surcharging or electro-osmosis increases the stability of the slope

Types of Landslides

Landslides can be grouped by:

• Scale
• Where they occur
• Cause – either a one-off event or long-term movement
• The dominant type of movement – i.e. fall, topple, slide, spread, flow, or creep

Individual landslides are often formed by more than one type of movement, so classifying distinct groups of landslides has proved challenging.

Different types of features are grouped below according to where they occur and whether they mainly involve bedrock or regolith (rock and soil debris).

All landslides are potential hazards, but most occur in remote mountain areas away from people. Debris flows in particular are a regular hazard that disrupts transport routes in the Highlands during severe weather conditions.

Landslides in bedrock

Rock falls

Single and small rock-falls from cliffs build up to form aprons of scree or talus, sometimes developing over long periods. Some scree slopes are relict, where the scree apron almost buries the crags that once released rockfalls that formed them.

A trait of actively forming rock fall screes is the sorting of rock debris, with the largest stones at the base of the scree and the smallest at the top. Reworking of the scree slopes by other processes – e.g. Snow avalanching, debris flows, and gully erosion by water – disrupts this sorting.

Rock slope failures

This group of landslides varies greatly in features. Many rock slope failures in Scotland are so large (involving whole mountainsides) that most people don’t even notice them.

For example, the rock slope failure on BeinnFhada in Kintail is huge – 3km long, from near sea level to 1,000m, with a failed mass of perhaps more than 100 million cubic meters.

Rotational landslides

Rotational landslides occur where more resistant rocks founder over underlying weaker rocks. Multiple failures may produce spectacular whole mountainside collapse, as at Trotternish in Skye and Hallaig in Raasay. At Hallaig, the landslide complex continues underwater, down to the seabed.

Debris flows

Debris flows are Scotland’s most common type of landslide hazard. They range in character from sloppy wet mudflows to slurries of rock debris similar in texture to wet concrete.

Debris flows begin on steep slopes of more than 20 degrees. But they can continue to travel over much gentler ground that slopes at only 10 degrees. How far a debris flow can travel depends on how much debris it carries compared to the volume of water.

A debris flow often starts as a translational slide, but the water and rubble mix as the slide moves downslope, forming a slurry that flows. Debris flows often leave a trail of rubble in their wake, forming distinctive ridges or levees.

Debris flows range in size from 1m to 10m across and may carry up to several cubic meters of debris. One-off debris flows may occur on open hillsides, but repeated debris flows are more common below gullies.

Debris cones form where repeated debris flows build up. Many debris cones in the Highlands have periods of activity and quiet. To trigger them, suitable weather conditions are needed and enough mud and rock must have collected higher up in the gully floor.

Creep

Creep is the slow downslope movement of material under gravity. It generally occurs over large areas.

Three types of creep occur:

• Seasonal movement or creep within the soil – due to seasonal changes in soil moisture and temperature, e.g. Frost heave processes
• Continuous creep – where the soil and rock debris isn’t strong enough to resist gravity
• Progressive creep – where the soil and rock debris suddenly reaches a critical state such that it will fail due to other factors

Solifluction

Solifluction is a mixture of creep and flow, which forms distinctive sheets, terraces, and lobes of debris and boulders. Solifluction sheets and lobes are found on steeper slopes where the process has moved loosened boulders and soil downslope.

Scotland’s larger boulder lobes were last active during the Loch Lomond Readvance (12,900 to 11,500 years ago). Some smaller solifluction features are still active most winters.

Translational slides

Such slumps and slides occur in very wet weather, when the near-surface soil and rock debris gets saturated with water, and slides and flow downslope. They can develop into more dangerous debris flows on steep ground, where the landslide may travel a long distance, churning up the debris into a slurry.

Prevention by surface drainage

Every precaution should be taken to prevent surface runoff water from entering a potentially unstable area. Any sags, depressions, or ponds above the slope line of either an embankment or a cut should be drained to minimize the possibility of surface water percolating into a weak or unstable area. If the new construction crosses an old landslide its surface should be reshaped as necessary to provide good surface drainage, but unnecessary removal of vegetation should be avoided lest excessive erosion may occur. Sealing of all surface cracks in any type of slide will be of benefit, both by preventing the entrance of surface water into the slide mass and by reducing frost action in areas subject to freezing and thawing. Although surface drainage alone will seldom correct an active landslide, any improvement in surface drainage will be beneficial. In the case of potential landslides, where no movement has occurred before construction, surface drainage may result in greater returns from the investment than any other type of preventive treatment, even though other preventive measures may be required in conjunction with the surface drainage. Surface runoff or the water flowing from springs or seeps should never be allowed to drain into or across an unstable area or potential landslide.

Methods of improving surface drainage include reshaping of slopes, construction of paved ditches, installation of flumes or conduits, and paving or bituminous treatment of slopes.

Subdrainage

If the preliminary investigation reveals the presence of groundwater which may induce slide movement, adequate subdrainage should be included in the plans. Such subdrainage is equally important in cut areas and under proposed embankments. The effectiveness and frequency of use of the various types of drainage treatment vary according to the geologic formation and climatic conditions; they probably are influenced by local custom also. It is generally agreed, however, that for the majority of landslides groundwater constitutes the most important single contributory cause; and in many areas of the country, the most generally used successful methods for both prevention and correction of landslides consist entirely or partially of groundwater control. This is especially true of the Pacific Coastal region. Although most of the types of subdrainage treatment apply to the prevention and correction of both embankment slip-outs and landslides in excavation areas, the differences in methods are considered of sufficient importance to justify the separate discussion of subdrainage treatments applied to these two general types of landslides.

Drainage in Embankment Areas

Slip out may occur whenever the imposed embankment load results in shear stress that exceeds the shear strength of the foundation soil; or where the construction of the embankment interferes with the natural movement of groundwater and results in the development of pore pressure or hydrostatic pressures. Two factors must, therefore, be considered in the investigation of possible slipouts: weak zones in the foundation soil, which may be overstressed by the proposed embankment load, and subsurface water, which may either result in the development of hydrostatic pressure or may reduce the shear strength of the soil sufficiently to induce slide movement. Careful exploration will usually reveal these conditions before construction, but the investigator must be of a suspicious and inquisitive nature, as there may be no readily apparent surface indications of the unstable conditions. Some of the methods of preventing roadway slip-outs are listed and discussed hereinafter. As previously noted, if a surface layer of weak soil is relatively shallow and is underlain by stable rock or soil, the most economical treatment is usually that of stripping and wasting the unsuitable material. If seepage is evident after stripping or if there is a possibility that it may develop during wet cycles, a layer of pervious material should be placed before the embankment is constructed. This may consist of clean pit run gravel, free-draining sand, or other suitable local materials. If springs or concentrated flows are encountered, a drain pipe may be required also. Where subsurface water or soil of questionable strength is found at such great depths that stripping is uneconomical, deep drainage or stabilization trenches have been used successfully to prevent slip-outs. Such stabilization trenches are usually excavated with power equipment with the steepest side slopes that will be stable for the minimum construction period; they should extend below any water-bearing layers and into firm material.

Horizontal drains have, since their development during the past few years, supplanted drainage tunnels in many cases. As was the case with drainage tunnels, they were first installed as a corrective treatment. Although they are still used principally for this purpose, they have been installed at several locations as a preventive treatment. Horizontal drains usually consist of perforated metal pipe, often 2 in. In diameter, forced into a predrilled hole (generally 3 to 4 in. In diameter) at its slight angle to the horizontal; the gradient of horizontal drains may range from 5 to 25 percent. The length of these drains may be as great as 200 to 300 ft or more.

Vertical drain wells have also been installed under embankments to accelerate the consolidation, through the removal of water, of weak compressible foundation soil. Such drains, usually 15 to 24. In diameter, are drilled or driven through and to the bottom of the saturated, compressible soil layers, then backfilled with coarse sand or other suitable filter material. A layer of filter material is placed over the area in which the vertical drains are installed, with outlets leading beyond the embankment slope line. The design of this latter type of vertical drain well should be based on laboratory tests of undisturbed soil samples, from which the consolidation and strength characteristics of the soil are determined.

Drainage trenches are sometimes installed as interceptors of subsurface water above the limits of the excavations, too often with indifferent success. There is seldom any assurance that such intercepting trenches will effectively cut off all groundwater which might contribute to slope failure. If deep trenches are required the cost frequently becomes prohibitive, considering the probable effectiveness of the drainage trenches. The most widely used successful method of subdrainage for preventing slides in cut slopes is probably the horizontal drain treatment. These horizontal drains are the same as previously described for slip-out prevention.

Rock Bolting

Rock bolting is the systematic reinforcement and/or anchorage of rock slopes by the insertion and grouting of steel bars into holes predrilled into the more or less fractured rock mass, improving its stability. The deformed steel bars are typically 25 to 50 mm in diameter and up to 12 to 15 m in length. Long bolts are typically formed by joining shorter threaded bars using special couplers, to facilitate handling. For the convenience of installation, strand anchors are normally used where longer bolts are required. Bolts are installed across the discontinuities or the potential failure surfaces at a dip angle flatter than the normal and typically work mainly in tension and only subordinately in shear and bending.

Fig 14 Schematic detail (source: SGI-MI project files)

Typically, drillholes in the rock are self-supporting. However, critical drilling conditions with a potential loss of borehole stability may be encountered when drilling through highly fractured or mylonitic zones, especially if the water is also encountered in the drillhole. In this case, it may be simpler to grout and redrill the hole, rather than using a casing.

With the degree of relaxation or loosening of the fractured rock to be reinforced and/or to be tied to the more competent rock below the bolts can be un-tensioned or tensioned. Relaxation and or loosening of the rock mass is a process that takes place as a result of unloading and weathering; once relaxation or loosening has been allowed to take place there is a loss of interlock between the blocks of rock and a significant decrease in the shear strength along the discontinuities and the rock mass as a whole. Once relaxation or loosening has taken place, it is not possible to reverse the process. For this reason:

• Where the degree of relaxation or loosening is relatively modest, it is possible to use passive (untensioned) rock bolting acting as pre-reinforcement; the deformations necessary to activate the bolts are sufficiently small not to result in a significant reduction of the shear strength characteristics of the discontinuities and the rock mass as a whole;
• Where significant relaxation and loosening have already taken place, it may be necessary to install tensioned bolts to prevent further displacements and loss of interlock.

The advantages of using un-tensioned bolts are the lower costs and quicker installation compared with tensioned bolts.

From a conceptual point of view, un-tensioned (passive) rock bolts work in the same way as nails of soil nailing structures.

They are grouted for their full length in a single operation both below and above the potential failure surface. In slope applications, where the drillhole dips into the ground, there is no need for anchoring the distal end of the bolt. Even though in many situations a head plate is not strictly required, an endplate is normally fitted to the bolt at the surface and this may be useful to anchor netting and or other facings that may be required.

From a conceptual point of view, tensioned (active) rock bolts work like anchors in tieback retaining structures.  They are characterized by an anchor head, a free-stressing length, and a bond length, located beneath the discontinuity or the potential failure surface.

Tensioned (active) bolts must satisfy three basic requirements:

1. There must be a suitable method of anchoring the distal end of the bolt in the drill hole;
2. A known tension must be applied to the bolt without creep and loss of load over time;
3. The complete bolt assembly must be protected from corrosion for the design life of the project.

Methods of securing the distal end of a bolt in the drill hole include mechanical devices, resin, and cement grout. The selection of the appropriate method depends on several factors such as the required capacity of the bolt, speed of installation, strength of the rock in the bond zone, access to the site for drilling and tensioning equipment, and the level of corrosion protection required (Wyllie and Norrish, 1996).

The most appropriate method to ensure that bolts are not susceptible to creep and loss of load over time is to set operating loads significantly lower than the pullout resistance and below the level at which significant creep or fluage is observed in load tests.

Methods of protecting steel against corrosion include galvanizing, applying an epoxy coating, and encapsulating the steel in cement grout. Because of the brittle nature of the grout and its tendency to crack, particularly when loaded in tension and in bending, the protection system is usually composed of a combination of grout and a plastic sleeve.

Grout mix can be readily pumped down a small-diameter grout tube, so that grouting proceeds from the distal end of the drill hole towards the surface, displacing any water or debris and producing a continuous grout column. Grouting is continued until clean grout flows out of the hole at the surface. Hollow bars can be used in place of solid bars, in which case the grout is injected through the bar itself, avoiding the need for the grout tube.

When bolting is carried out in anunweathered rock mass with relatively widely spaced discontinuities, the spacing between bolts may be commensurably wide and there is no need for any facing. In this case, the end of the bolt is fitted with a small steel plate, typically embedded in a small concrete slab for corrosion protection.

Where the rock mass is highly fractured and/or the fractured rock may degrade and ravel from under and in between the reaction plates of the bolts, a structural facing must form an integral part of the rock bolting scheme. Different solutions may be foreseen for the structural facing, including for example:

• Reinforced concrete walls: the wall acts both as a protection against raveling of the rock and as a large reaction plate for the rock bolts; the rock bolt will be drilled through sleeves in the concrete; it is also important that there be drain holes through the concrete to prevent the buildup of water behind the wall.
• Shotcrete, reinforced with reinforcing mesh (typically steel, but other materials may be equally suitable).
• Reinforced wire mesh, with a network of steel cables.
• Reinforced wire mesh associated with reinstatement of vegetation.

Key takeaways:

Slope-stability or mass-movement problems occur where either sediment and/or rock and/or snow move downslope in response to gravity. Potential slope-stability problems exist wherever development has taken place at the base of steep slopes.

3.2  Concept of sliding blocks, Different controlling factors

Mass, it's far referred to as a block slide. A translational slide is from time to time referred to as a dust slide whilst it happens alongside lightly sloping, discrete shear planes in fine-grained rocks (together with fissured clays) and the displaced mass is fluidized via way of means of an boom in pore water pressure.

A falling-block motion (additionally called a sliding-block or dropping-block motion) is a single-shot firearm motion wherein a strong steel breechblock slides vertically in grooves reduce into the breech of the weapon and is actuated with the aid of using a lever.[

When the breechblock is withinside the closed (top) position, it seals the chamber from the excessive pressures created whilst the cartridge fires and competently transfers the flinch to the movement and stock. When the breechblock is withinside the diminished position, the rear (breech) give up of the chamber is uncovered to permit ejection or extraction of the fired case and reloading of an unfired cartridge.

It is a totally robust movement; whilst the breech is closed, the receiver basically turns into a unmarried piece of steel (rather than different movements which depend on lugs to fasten the breech). This kind of movement is utilized in heavy artillery in addition to small arms. An extra benefit is the unobstructed loading path, which imposes no restrict on cartridge length; this changed into vast withinside the late-nineteenth century technology of very long "buffalo" and "express" big-sport cartridges.[citation needed] Rifles the use of this movement consist of the M1870 Belgian Comblain, M1872 Mylonas, Sharps rifle, Farquharson rifle, 1890 Stevens, Sharps-Borchardt Model 1878, Winchester Model 1885, Browning version 1885, Browning M78 and Ruger No. 1 & No. 3. Falling-block movement army rifles have been not unusualplace withinside the nineteenth century. They have been changed for army use with the aid of using the quicker bolt-movement rifles, which normally reloaded from a mag protecting numerous cartridges. A falling block pistol changed into additionally produced in 1847 with the aid of using the French gunsmith Gastinne Renette.

As properly as getting used for artillery, falling-block movement rifles are nevertheless synthetic and used for looking and goal taking pictures and business shotguns (8ga) for taking pictures clinkers in boilers. The falling-block movement is intently associated with that of the Martini–Henry rifle, the Peabody movement (comparable to, however now no longer equal with, that of the Martini–Henry), the Ballard movement, and the Madsen–Rasmussen (uniquely, a repeater), which use a pivoting instead of a sliding block.

A sliding puzzle, sliding block puzzle, or sliding tile puzzle is a aggregate puzzle that demanding situations a participant to slide (often flat) portions alongside sure routes (commonly on a board) to set up a sure end-configuration. The portions to be moved can also additionally include easy shapes, or they will be imprinted with colors, patterns, sections of a bigger picture (like a jigsaw puzzle), numbers, or letters. Sliding puzzles are basically two-dimensional in nature, even though the sliding is facilitated with the aid of using automatically interlinked portions (like in part encaged marbles) or three-d tokens. As this case shows, a few sliding puzzles are mechanical puzzles. However, the mechanical furnishings are commonly now no longer vital to those puzzles; the components ought to as properly be tokens on a flat board which are moved consistent with sure rules. Unlike different excursion puzzles, a sliding block puzzle prohibits lifting any piece off the board. This assets separates sliding puzzles from rearrangement puzzles. Hence, locating movements and the trails unfolded with the aid of using every pass in the two-dimensional confines of the board are critical components of fixing sliding block puzzles.

Key takeaways:

Mass, it's far referred to as a block slide. A translational slide is from time to time referred to as a dust slide whilst it happens alongside lightly sloping, discrete shear planes in fine-grained rocks (together with fissured clays) and the displaced mass is fluidized via way of means of an boom in pore water pressure.

3.3 Instability in vertical rock structures and measures to prevent collapse

Stability is right here used to specific the behaviour of rock loads associated with their "chance of being constant in position" (Webster's dictionary). Stability can be felt as a relative expression. In difficult rock tunnelling in which frequently a widespread a part of the tunnel may be left unsupported throughout the development length as well as throughout operation, any instability that calls for assist can be seemed as being a balance problem. Tunnelling in negative rock situations in which non-stop use of assist or lining is required, "balance problems" are frequently related most effective with the ones components of the floor in which the "standard" excavation system and approach of assist is insufficient and special measures or answers are required. In both case, a primary goal is to evaluate the balance behaviour efficaciously and pick secure and least expensive strategies for excavation and assist. Stability is a relative time period additionally in different respects as it could be linked to the specified stage of protection, which might also additionally range with the usage of the development. The stage of protection may additionally be exclusive withinside the numerous international locations consistent with policies for running situations and protection, in addition to the enjoy the contractor possesses. Failure, 'the dropping of strength', might also additionally, in evaluation to instability ('the shortage of being constant in position') be seemed because the follower of instability. It can be honestly stated that failure is the end result of instability. Both failure and instability are used instead erratically withinside the literature as they frequently overlap. The time period floor is often used on this chapter. By floor is right here meant 'the in situ rock mass subjected to strain and water'. Rock mass is as stated earlier 'rocks penetrated through discontinuities', i.e. the structural fabric that is being excavated and wherein the underground establishing is located.

There aren't anyt any general wellknown analyses for figuring out rock assist layout, due to the fact every layout is particular to the circumstances (scale, depth, presence of water, etc.) on the real web website online and the country wide rules and enjoy. Support layout for a tunnel in rock frequently includes problems which are of pretty very little subject in maximum different branches of strong mechanics. The material and the underground commencing bureaucracy an incredibly complicated shape. "It is seldom viable, neither to collect the correct mechanical records of the floor and forces acting, nor to theoretically decide the precise interplay of these" (Hoek and Brown, 1980). Therefore, the rock engineer is usually confronted with the want to reach at some of layout choices wherein judgement and sensible enjoy ought to play an essential part. Prediction and/or assessment of assist necessities for tunnels is essentially primarily based totally on observations, enjoy and private judgement of these worried in tunnel construction (Brekke and Howard, 1972). Often, the estimates are sponsored via way of means of theoretical tactics in assist layout of which 3 predominant groups were practised in current years, namely - the category structures, - the floor-assist interplay evaluation (and the Fenner-Pacher curves utilized in NATM), - the important thing block evaluation. The complicated predicament of structural analyses of tunnels is defined withinside the recommendations for the layout of tunnels of an ITA Working Group edited via way of means of Duddeck (1988), from which the subsequent is extracted: "The end result of an evaluation relies upon very lots at the assumed version and the values of the giant parameters. The predominant functions of the structural evaluation are to offer the layout engineer with: 1) A higher knowledge of the floor-shape interplay brought about via way of means of the tunnelling process. 2) Knowledge of what varieties of primary dangers are worried and wherein they're located. 3) A device for deciphering the web website online observations and in-situ measurements. The to be had mathematical strategies of evaluation are lots extra delicate than are the residences that represent the structural version. Hence, in maximum instances it's miles extra suitable to investigate opportunity viable residences of the version, or maybe unique models, than to purpose for a extra delicate version." The layout of excavation and assist structures for rock, despite the fact that primarily based totally on a few scientific principles, has to satisfy sensible necessities. In order to pick out and integrate the parameters of significance for balance in an underground commencing the primary functions figuring out the steadiness are reviewed along with numerous modes of failure.

INSTABILITY AND FAILURE MODES IN UNDERGROUND EXCAVATIONS

Basically, the instability of rock hundreds surrounding an underground commencing can be divided into predominant agencies (Hudson, 1989): 1. One is block failure, in which pre-present blocks withinside the roof and aspect partitions emerge as loose to circulate due to the fact the excavation is made. These are called 'structurally managed disasters' via way of means of Hoek and Brown (1980) and contain a first rate style of failure modes (as loosening, ravelling, block falls etc.). 2. The different is in which disasters are prompted from overstressing, i.e. the stresses advanced withinside the floor exceed the nearby energy of the material, which may also arise in predominant forms, namely: a. Overstressing of big or intact rock (which takes location withinside the mode of spalling, popping, rock burst etc.). b. Overstressing of particulate substances, i.e. soils and heavy jointed rocks (in which squeezing and creep may also take location). Various modes of disasters are linked to those agencies. Terzaghi (1946) has in his classification labored out a behaviouristic description primarily based totally on failure modes. Also the brand new Austrian tunnelling method (NATM) carries a comparable description of the floor behaviour which summarizes the predominant kinds of instability in underground openings. Both descriptions are proven in Table 6-1. Additional modes of rock mass behaviour in underground excavations defined via way of means of Terzaghi (1946) are: Spalling 1 1 This time period is regularly utilized by different authors as synonymous with popping or slight rock burst. , which refers back to the falling out of character blocks, often because of damage for the duration of excavation. Running floor, which happens whilst a fabric invades the tunnel till a strong slope is formed on the face. Stand-up time is 0 or almost 0. Examples are smooth medium to coarse sands and gravels above floor water level. Flowing floor, that is a aggregate of water and solids, which collectively invade the tunnel from all sides, which includes the bottom. It is encountered in tunnels beneath floor water desk in substances with very little coherence. This has been envisaged in which the numerous modes of disasters withinside the 6 predominant agencies of floor are indicated.. Most kinds of rock hundreds fall inside this scheme. In addition to the continuity and the competency of the floor the time factor, the manner the particle or blocks circulate, and the presence of water decide the improvement and mode of a failure. Input from enjoy and know-how of the behaviour of numerous kinds of rock hundreds in underground openings is essential in balance evaluation and rock help evaluations. Further, the knowledge of the way viable failure modes are associated with floor situations is a prerequisite withinside the estimates of rock help.

Key takeaways:

Stability is right here used to specific the behaviour of rock loads associated with their "chance of being constant in position" (Webster's dictionary). Stability can be felt as a relative expression

3.4 Types of landslide

Rock falls. Single and small rock falls from cliffs building up to shape aprons of scree or talus, every so often growing over long term periods.

Rock slope failures

This institution of landslides varies substantially in features

Rotational landslides

Debris flows

Creep

Solifluction

Translational slides.

Landslides may be grouped with the aid of using: scale wherein they arise cause – both a one-off occasion or long-time period movement dominant sort of movement – i.e. fall, topple, slide, spread, glide or creep Individual landslides are regularly shaped with the aid of using multiple sort of movement, so classifying awesome corporations of landslides has proved challenging.

Different kinds of capabilities are grouped under in step with wherein they arise and whether or not they particularly contain bedrock or regolith (rock and soil debris). All landslides are capacity hazards, however maximum arise in far flung mountain regions farfar from people. Debris flows especially are a everyday threat that disrupts delivery routes withinside the Highlands for the duration of excessive climate conditions.

Landslides in bedrock Rock falls Single and small rock falls from cliffs building up to shape aprons of scree or talus, every now and then growing over long term durations. Some scree slopes are relict, wherein the scree apron nearly buries the crags that when launched rock falls that fashioned them. A trait of actively forming rock fall screes is the sorting of rock particles, with the biggest stones at the bottom of the scree and the smallest on the top. Reworking of the scree slopes via way of means of different processes – e.g. snow avalanching, particles flows and gully erosion via way of means of water – disrupts this sorting.

Rock slope disasters This organization of landslides varies substantially in features. Many rock slope disasters in Scotland are so large (concerning entire mountainsides) that maximum human beings don’t even observe them. For example, the rock slope failure on Beinn Fhada in Kintail is huge – 3km long, from close to sea degree to 1,000m, with a failed mass of possibly greater than a hundred million cubic metres. Rotational landslides Rotational landslides arise wherein greater resistant rocks founder over underlying weaker rocks. Multiple disasters can also additionally produce wonderful entire mountainside collapse, as at Trotternish in Skye and at Hallaig in Raasay. At Hallaig, the landslide complicated maintains below water, right all the way down to the seabed. Landslides in regolith Debris flows Debris flows are Scotland’s maximum not unusualplace sort of landslide hazard. They variety in individual from sloppy moist mudflows to slurries of rock particles comparable in texture to moist concrete. Debris flows start on steep slopes of greater than 20 degrees. But they could maintain to journey over an awful lot gentler floor that slopes at best 10 degrees. How some distance a particles waft can journey relies upon on how an awful lot particles it contains as compared to the quantity of water. A particles waft regularly begins offevolved off as a translational slide, however the water and rubble blend because the slide actions downslope, forming a slurry that flows. Debris flows regularly go away a path of rubble of their wake, forming exclusive ridges or levees. Debris flows variety in length from 1m to 10m across, and can bring as much as numerous cubic metres of particles. One-off particles flows can also additionally arise on open hillsides, however repeated particles flows are greater not unusualplace under gullies. Debris cones shape wherein repeated particles flows building up. Many particles cones withinside the Highlands have durations of hobby and quiet. To cause them, appropriate climate situations are wanted and sufficient dust and rock should have accrued better up withinside the gully floor.

Creep Creep is the sluggish downslope motion of cloth beneathneath gravity. It commonly takes place over huge areas. Three varieties of creep arise: seasonal motion or creep in the soil – because of seasonal modifications in soil moisture and temperature, e.g. Frost heave processes non-stop creep – wherein the soil and rock particles isn’t robust sufficient to face up to gravity revolutionary creep – wherein the soil and rock particles all of sudden reaches a essential nation such that it'll fail because of different factors Solifluction Solifluction is a combination of creep and flow, which paperwork exclusive sheets, terraces and lobes of particles and boulders. Solifluction sheets and lobes are observed on steeper slopes wherein the technique has moved loosened boulders and soil downslope. Scotland’s large boulder lobes had been ultimate energetic for the duration of the Loch Lomond Readvance (12,900 to 11,500 years ago). Some smaller solifluction functions are nonetheless energetic maximum winters. Translational slides Such slumps and slides arise in very moist weather, while the near-floor soil and rock particles receives saturated with water, and slides and flows downslope. They can grow to be extra risky particles flows on steep ground, wherein the landslide may also tour a protracted distance, churning up the particles right into a slurry.

Key takeaways:

Rock slope disasters This organization of landslides varies substantially in features. Many rock slope disasters in Scotland are so large (concerning entire mountainsides) that maximum human beings don’t even observe them

3.5 Slope reinforcement by Rock bolting and Rock Anchoring

Rock bolting is the systematic reinforcment and/or anchorage of rock slopes via way of means of the insertion and grouting of metallic bars into holes predrilled into the extra or much less fractured rock mass, enhancing its stability. The deformed metallic bars are generally 25 to 50 mm in diameter and as much as 12 to fifteen m in length.

Long bolts are commonly shaped with the aid of using becoming a member of shorter threaded bars the use of unique couplers, to facilitate handling. For comfort of installation, strand anchors (see fact-sheet 6.8) are generally used wherein longer bolts are required. Bolts are hooked up throughout the discontinuities or the capacity failure surfaces at a dip perspective flatter than the regular and commonly paintings especially in anxiety and simplest subordinately in shear and bending.

Typically, drillholes in rock are self supporting. However, essential drilling situations with ability lack of borehole balance can be encountered while drilling via higly fractured or milonitic zones, particularly if water is likewise encountered withinside the drillhole. In this case, it could be less difficult to grout and redrill the hollow, as opposed to the usage of a casing. In relation to the diploma of rest or loosening of the fractured rock to be strengthened and/or to be tied to the greater equipped rock beneath the bolts may be un-tensioned or tensioned. Relaxation and or loosening of the rock mass is a method that takes location as a consequences of unloading and weathering; as soon as rest or loosening has been allowed to take location there's a lack of interlock among the blocks of rock and a substantial lower withinside the shear electricity alongside the discontinuities and withinside the rock mass as a whole. Once rest or loosening has taken location, it isn't feasible to opposite the method. For this reason: wherein the diploma of rest or loosening is notably modest, it's far feasible to apply passive (untensioned) rock bolting appearing as pre-reinforcement (Moore and Imrie, 1982; Spang and Egger, 1990); the deformations important to spark off the bolts are small enough now no longer to bring about a substantial discount of the shear electricity traits of the discontinuities and of the rock mass as a whole; wherein substantial rest and loosening have already taken location, it could be important to put in tensioned bolts with a purpose to save you similarly displacements and lack of interlock. The benefits of the usage of un-tensioned bolts are the decrease fees and faster set up as compared with tensioned bolts. From a conceptual factor of view, un-tensioned (passive) rock bolts paintings withinside the equal manner as nails of soil nailing structures. They are grouted for his or her complete period in a unmarried operation each beneath and above the ability failure floor. In slope applications, wherein the drillhole dips into the ground, there's no want for anchoring the distal stop of the bolt. Even aleven though in lots of state of affairs a head plate isn't strictly required, a stop plate is generally suited to the bolt on the floor and this could be usefull to anchor netting and or different facings that can be required. From a conceptual factor of view, tensioned (active) rock bolts paintings like anchors in tieback keeping structures.

They are characterised with the aid of using a anchor head, a free-stressing period and a bond period, placed underneath the discontinuity or the ability failure floor. Tensioned (active) bolts have to fulfill 3 fundamental requirements: There have to be a appropriate technique of anchoring the distal stop of the bolt withinside the drill hollow; A recognized anxiety have to be implemented to the bolt with out creep and lack of load over time; The whole bolt meeting have to be included from corrosion for the layout lifestyles of the project. Methods of securing the distal stop of a bolt withinside the drill hollow consist of mechanical devices, resin and cement grout. The choice of the precise technique relies upon on numerous elements along with the specified ability of the bolt, speed

Where the rock mass is enormously fractured and/or the fractured rock might also additionally degrade and ravel from beneathneath and in among the response plates of the bolts, a structural going through should shape an quintessential a part of the rock bolting scheme.

Different answers can be foreseen for the structural going through, such as for example: Reinforced concrete walls: the wall acts each as a safety towards raveling of the rock and as a massive response plate for the rock bolts; the rock bolt could be drilled thru sleeves withinside the concrete; it's also crucial that there be drain holes thru the concrete to save you buildup of water in the back of the wall. Shotcrete, bolstered with reinforcing mesh (normally metallic, however different substances can be similarly suitable). Reinforced twine mesh, with a community of metallic cables. Reinforced twine mesh related to reinstatement of vegetation.

A Rock anchor is a excessive electricity metal tendon that's positioned in conjunction with a stressing anchorage on one end, while a way for moving pressure to the grout & rock on the opposite end. A hollow is drilled of a right period and diameter, and then the rock anchor tendon is inserted & constant to the rock.

In order to understand the idea of rock anchoring, one desires to apprehend what rock anchors are. A Rock anchor is a excessive power metallic tendon that is located in conjunction with a stressing anchorage on one end, while a method for shifting pressure to the grout & rock on the opposite end. A hollow is drilled of a right period and diameter, and then the rock anchor tendon is inserted & constant to the rock. In order to to a a success rock anchoring, one ought to recognise sure elements inclusive of the carrier existence of an anchor, layout load in step with anchor, sort of the soil & rock to be able to be encountered with appreciate to depth, undrained shear power & bulk density at exclusive depths and permeability of rock amongst different things. What is the motive of Rock Anchoring? The most important motive of rock anchoring is to stabilize the muse of the development and keep away from its movement. Several elements inclusive of hydrostatic pressure, wind loads, seismic sports amongst others, can set off an uplifting pressure on the muse. It can endanger the constructional integrity of the shape. Apart from the rock anchors also are set up to hold in take a look at the tilting of Piles in conditions like shoring structures for resisting lively and passive earth pressure. Types of Rock Anchors: Rock anchors are of kinds: Prestressed Rock Anchors Passive Rock Anchors Materials Required: For the manner of rock anchoring following substances are required: Bearing Plate Rock anchors of your choice Drill and drilling bits Grouting material Restraining bolt Tendon Anchor Head Bolts How to do rock anchoring? The manner of rock anchoring is determined primarily based totally on what type of shape is needed to be anchored. If the shape in query is a as a substitute huge inclusive of a bridge or dams, the favored technique of anchoring is passive rock anchoring. Thus it's miles honestly essential to gauge the size, wight and different elements cited above earlier than beginning the manner of rock anchoring. The popular steps to be observed for rock anchoring are indexed below:

Drilling the Hole Injecting the Grout Inserting threaded sheathed rock anchors Installing Bearing Plate Tensing the anchor Rock anchoring is a totally essential technique which calls for professional experts to be completed successfully.

Key takeaways:

Rock bolting is the systematic reinforcment and/or anchorage of rock slopes via way of means of the insertion and grouting of metallic bars into holes predrilled into the extra or much less fractured rock mass, enhancing its stability.

3.6 Retaining Walls

Definition

A retaining wall is a structure that retains (holds back) any material (usually earth) and prevents it from sliding or eroding. It is designed so that to resist the material pressure of the material that it is holding back.

Types of Retaining Wall

Earth retaining structure can be considered to have the following types:

1. Gravity Walls

Reinforced Gravity Walls

• Concrete Cantilever retaining wall
• Counter-fort / Buttressed retaining wall
• Precast concrete Ret wall
• Pre-stressed retaining wall

Brick Masonry retaining wall

• Soil Nailing
• Reinforced soil

• Anchored Earth ret wall
• Tailed Gabion
• Tailed Concrete Block
• Miscellaneous

Gravity Retaining Walls

Fig 15

Gravity retaining walls rely on their huge weight to retain the material behind them and achieve stability against failures. Gravity Retaining Wall can be constructed from concrete, stone, or even brick masonry. Gravity retaining walls are much thicker in sections. The geometry of these walls also helps them to maintain stability. Mass concrete walls are suitable for retained heights of up to 3 m. The cross-section shape of the wall is affected by stability, the use of space in front of the wall, the required wall appearance, and the method of construction.

Reinforced Retaining Walls

Reinforced concrete and reinforced masonry walls on spread foundations are gravity structures in which the stability against overturning is provided by the weight of the wall and reinforcement bars in the wall. The following are the main types of wall:

Concrete Cantilever retaining wall

A cantilever retaining wall consists of a wall that is connected to the foundation. A cantilever wall holds back a significant amount of soil, so it must be well engineered. They are the most common type used as retaining walls. Cantilever wall rest on a slab foundation. This slab foundation is also loaded by back-fill and thus the weight of the back-fill and surcharge also stabilizes the wall against overturning and sliding.

Counter-fort / Buttressed retaining wall

Counterfort walls are cantilever walls strengthened with counter forts monolithic with the back of the wall slab and base slab. The counter-forts act as tension stiffeners and connect the wall slab and the base to reduce the bending and shearing stresses. To reduce the bending moments in vertical walls of great height, counterforts are used, spaced at distances from each other equal to or slightly larger than one-half of the height Counter forts are used for high walls with heights greater than 8 to 12 m.

Fig 16

Precast concrete retaining wall.

Prestressed ret wall.

Reinforced Soil Retaining Wall

Mechanically stabilized earth walls are those structures that are made using steel or GeoTextiles soil reinforcements which are placed in layers within a controlled granular fill. Reinforced soils can also be used as retaining walls if they are built as:

1. As an integral part of the design.
2. As an alternative to the use of reinforced concrete or other solutions on the grounds of economy or as a result of the ground conditions.
3. To act as temporary works.
4. As remedial or improvement works to an existing configuration.

This category covers walls that use soil, reinforced with reinforcing bars, to provide a stable earth retaining system and includes reinforced soil and soil nailing.

Soil Nailing

Constructing a soil nailed wall involves reinforcing the soil as work progresses in the area being excavated by the introduction of bars which essentially work in tension, called Passive Bars. These are usually parallel to one another and slightly inclined downward. These bars can also work partially in bending and in shear. The skin friction between the soil and the nails puts the nails in tension.

Hybrid Systems Retaining Wall

The type of retaining walls that use both factors that is their mass and reinforcement for stability are called Hybrid or Composite retaining wall systems.

Anchored Earth walls

Any wall which uses facing units tied to rods or strips which have their ends anchored into the ground is an anchored earth wall. The anchors are like abutments. The cables used for tieing are commonly high-strength, pre-stressed steel tendons. To aid anchorage, the ends of the strips are formed into a shape designed to bind the strip at the point of the soil.

Tailed Gabion

Gabions are cages, cylinders, or boxes filled with earth or sand that are used in civil engineering, road-building, and military application, and many others. OR Gabion elements fitted to geo-gridtails' extending into supporting soil. For erosion control, caged riprap is used. For dams or foundation building, metal structures are used.

Sheet Pile Walls

Steel sheet pile walls are constructed by driving steel sheets into a slope or excavation up to the required depth. Their most common use is within temporary deep excavations. They are considered to be most economical where retention of higher earth pressures of soft soils is required. It cannot resist very high pressure.

Key takeaways:

A retaining wall is a structure that retains (holds back) any material (usually earth) and prevents it from sliding or eroding. It is designed so that to resist the material pressure of the material that it is holding back.

3.7 Slope Treatment

Slope treatment methods can be divided broadly into measures to reduce runoff (terracing, diversions, grassed waterways, conservation ponds), methods to stabilize slopes and reduce erosion (retaining walls, drop structures, sabo dams), and integrated methods to address specific problems (gully control, trail improvement), although they all tend to have multiple functions.

Terracing

Terracing is the technique of converting a slope into a series of horizontal step-like structures (Fig 17) with the aim of:

• Controlling the flow of surface runoff by guiding the runoff across the slope and conveying it to a suitable outlet at a non-erosive velocity;
• Reducing soil erosion by trapping the soil on the terrace; and
• Creating flat land suitable for cultivation.

Terracing helps prevent the formation of rills, improves soil fertility through reduced erosion, and helps water conservation.

Fig 17: A terraced slope in Nepal

Types of terrace

Terraces can be made in a variety of ways. The best approach depends on many factors including the steepness of the slope, the intended use, and the soil. The terraces are constructed with light equipment or by hand. The spacing between the terraces depends on the slope of land; the distance between terraces goes down as the slope increases. The three main types of the terrace are bench, level or contour, and parallel or channel.

Level or contour terraces are constructed along slope contours with the main aim of retaining water and sediment. The terrace edge is planted with trees, small plants, and grass, usually with trees on the outward-facing edge to increase stability.

Bench terracing is similar to contour terracing with the difference that the terraces do not strictly follow the contour line and runoff may run along as well as across the terrace. Bench terraces are primarily constructed to enable crops to be grown on sloping land, rather than to retain water and sediment. Bench terraces are recommended for slopes with a gradient of up to 33%, but as a result of pressure on land is constructed on slopes up to 50–60% (Sharda et al. 2007).

Parallel or channel terraces are mainly used in heavy rainfall areas. They are also known as graded terraces as they have a constant slope or gradient along their length which is used to convey excess runoff at a safe velocity into a grassed waterway or channel.

Of these three, bench terraces are the most common type found in the mountain and hill areas of the Hindu Kush Himalayan region.

Bench terraces

Bench terraces are particularly suitable where marked seasonal variations exist in the availability of water. The approach consists of converting relatively steep land into a series of horizontal steps running across the slope. These steps can be constructed by simply digging out the clayey soil, or they can be reinforced with locally available mud, stone, or brick. The terraces help conserve moisture during the long dry season, which is especially important where there are sandy and loam types of soil, and they help to slow and drain away runoff during the heavy rainfall monsoon season, which also helps counteract the tendency for sliding. There are three main types (Fig 18):

• Outward sloping terraces, which are used to reduce a steep slope to a gentle slope;
• Level terraces, which are used to impound water for paddy cultivation; and
• Inward sloping terraces, which are the most suitable for steep slopes because they guide the surface runoff towards the hillside rather than down the slope.

Rainwater can be drained from outward sloping terraces along a ditch constructed along the toe of the riser. In inward sloping terraces, the riser is kept free from flowing water and is protected by a cover of grass. Terrace design is influenced by the following factors (Sharda et al. 2007):

• Soil depth and distribution of the topsoil;
• The slope of land;
• Amount and distribution of rainfall; and
• Farming practices and proposed crops to be grown.

Contour terraces

The main aim of contour terraces is to retain water and sediment. Contour terraces are similar to bench terraces, with the major difference that the terrace is formed along the contour, so that runoff flows across but not along the terrace. Besides, the terrace edge is planted with trees, small plants, and grass to stabilize it and trap sediment. The terraces can be constructed by excavating soil from the upper half and using it to fill in the lower half as for bench terraces or can be allowed to form naturally using a technique called sloping agricultural land technology (SALT), or contour hedgerow intercropping (agroforestry) technology (CHIAT).

SALT combines the strengths of terracing with the strengths of natural vegetation to stabilize sloping land and make it available for farming. Double hedgerows of fast-growing perennial nitrogen-fixing tree or shrub species are planted along the contour lines on a slope at a distance of 4–6 meters to create a living barrier that traps sediment carried downslope by runoff (Tang 1999; Tang and Murray 2004). As the sediment builds up, the sloping land is gradually transformed into terraced land. The space between the contour hedgerows is used for subsistence and cash crops. The hedgerows both markedly reduce soil erosion and contribute to improving and/or maintaining soil fertility through nitrogen fixation at the roots and incorporation of the hedgerow trimmings into the soil. SALT can be established on farmland slopes with gradients of 5–25% or more.

A combined approach has also been developed for improved terraces in which retaining walls are first constructed along the contours using cement bags filled with soil supported by bamboo cuttings along the contour. The soil is then excavated from the upper part of the terraces and used to build up the lower part above and behind the terrace riser wall to create a level bed; the fertile topsoil is kept aside and later spread over the newly terraced fields. Grass and hedgerow species are then planted on the outermost margins of the terraces above the risers. The vegetation improves the terrace stability and increases moisture retention, while the construction means that the terraces are immediately ready for use, unlike the original SALT technique.

Fig 18 Types of bench terrace

Diversions

Diversions are ridges of soil or channels with a supporting ridge on the lower side. They are built across the slope to intercept runoff and dispose of it at a selected location. They are used to break up long slopes, to direct water away from active erosion sites, to direct water around agricultural fields or other sites, and to channel surface runoff to suitable outlet locations. Safe passage of the surface runoff to prevent slope failure can be achieved by installing drainage ditches, or by cross drainage work for road structures.

Slope drainage

The simplest way to safely drain offsprings and surface water is to use an open ditch (drain) or a system of open ditches. The main ditch is located in the direction of the slope gradient (downhill); secondary or lateral ditches are located in a fishbone pattern (Fig 19). Water should be collected as close as possible to its origin and channeled to a side drain, culvert, or any other nearby watercourse. Ditch excavation should start at the lowest point and work up so that the accumulating water may drain off immediately. The most common types of the drain are stone or gravel-filled drains with or without pipes. Pipe drains are the most efficient and effective, but they are more expensive and often not locally available. Normal stone drains may silt up over time and it is advisable to form a drainage channel of stones or place a bundle of brushwood, at the bottom of the drain. The top of the drain should be covered with a layer of grass to prevent siltation.

Simple drainage ditches

Drainage ditches can be constructed with different shapes and sizes. The appropriate size and shape for a particular site depend upon such factors as the expected runoff, site condition, and availability of resources and construction materials.

Grassed Waterways

Grassed waterways are natural or artificially constructed watercourses shaped or graded to the required dimensions and planted with suitable vegetation (Fig 19). Grassed waterways generally run down a slope and are designed to conduct surplus water safely into natural drainage courses. They are usually made broad and shallow, although the shape and size can vary depending on the size of the drainage area, the slope of the land, and soil type. The channels help surface water flow across the land without causing soil erosion. They are used as outlets to prevent rill and gully formation. The vegetation in the channel helps control the water flow and reduces channel surface erosion. Properly designed grassed waterways can safely transport large volumes of water to the downslope. They are also used as filters to prevent sediments from entering nearby water bodies.

Fig 19 Grassed Waterways

Grassed waterways are used as

• Outlets for diversions and emergency spillways;
• To safely convey runoff from contour and graded bunds and bench terraces;
• As outlets for surface and sub-surface drainage systems on sloping land;
• To carry runoff from natural drains and prevent the formation of gullies; and
• To dispose of water collected in road ditches or discharged through culverts.

Conservation Ponds

Conservation ponds, also known as farm ponds, are small reservoirs constructed to collect and store water from surface runoff. Storing water runoff during excessive rainfall helps to reduce the peak flow and surface erosion and thus reduce the probability of floods. It is also useful for providing supplemental irrigation for agriculture, water for domestic purposes, and fish farming. Conservation ponds play a significant role in areas with rainfed agriculture, and the construction of a large number of ponds in a catchment area can have a significant effecton downstream flow and control of floods. Water storage is a topic of increasing importance in the Hindu Kush Himalayas as the focus turns toward adaptation to climate change, and conservation ponds and other storage mechanisms are likely to play an increasing role in future development activities. Conservation ponds can be broadly classified into embankment type ponds and dugout type ponds.

Dugout ponds

The types of dugout ponds range from the very simple, which require no explanation, to forms specifically designed to collect water on a slope for infiltration and recharge purposes. Some selected types are described below. Simple earthen ponds are cheap and durable but have high seepage losses that reduce effectiveness. Vertical and horizontal seepage loss can be significantly reduced by compacting a 30 cm deep layer of heavy clay on the floor and walls of the pond. The addition of cow-dung and puddling will help seal seepage pores. Watering buffaloes in such ponds also help reduce seepage. High-density polythene sheets or SILPAULIN (multi-layered, cross-laminated, ultraviolet stabilized plastic sheets) can also be used to reduce seepage.Eyebrow pits are a special type of dugout pond used to reduce runoff across a slope and thus stabilize degraded slopes and increase infiltration and recharge of springs. Small curved trenches around 2 m long and 50 cm wide in the shape of an eyebrow are dug at intervals facing inward to the slope to catch water and slowly return it to the soil. Grass and fodder species are planted along the lower ridges of the pits.

Embankment type ponds

Embankment-type ponds are constructed to collect runoff at the base of a slope. They can be constructed across dry water beds or courses with a gentle to a steep slope that fills when it rains. Usually, an earthen dam is constructed between two hillsides to hold back the water from overland runoff. The pond bottom and dam should be made up of soil that prevents excess seepage. Embankment ponds should not be built by damming any stream with the permanent flow, no matter how small. The dimensions depend on the volume of water to be stored. The location should be sufficiently depressed to enable the maximum storage volume to be obtained with the least requirement for earthworks. 

Seepage control. Seepage is almost inevitable in any dam or embankment structure. It mainly occurs through the embankment and under the foundation. The following methods can be used to control seepage.

• Core wall: A core wall is constructed using impervious materials and checks the flow of water in the dam. The main purpose of the wall is to add strength to the bund and increase the seepage gradient length through the embankment.
• Filter and toe drain: These drains are constructed by including a layer of coarse material in the dam section to attract the flow of water and bring the saturation line down to the level of the drain.
• Berm: A berm is a horizontal structure built on the lower part of the downstream (outward) face of the dam to prevent seepage from the face and keep the seepage line within the base of the dam section. It increases the base width of the embankment.
• Cut off the wall: A cut-off wall is constructed to join the impervious foundation to the base of the dam and prevent piping.

Spillway

A spillway is an opening constructed in the embankment to allow water to exit the pond once it is at the desired level. The size and type of spillway depend upon the size and characteristics of the watershed and the site conditions. There are two types of spillway: mechanical and emergency. A mechanical spillway is used to let outexcess storage water safely; whereas an emergency spillway is used as a safeguard for the earthen embankment against overtopping when the inflow exceeds the capacity of the mechanical spillway.

Retaining Walls

Retaining walls are artificial structures that hold back soil, rock, or water from a building, structure, or area. Retaining walls prevent downslope movement and soil erosion, and provide support for vertical or near-vertical changes in gradient. The walls are generally made from timber, masonry, stone, brick, concrete, vinyl, steel, or a combination of these. Retaining walls act to support the lateral pressure exerted by a soil mass which may cause slope failure. Retaining walls are strongly recommended where the toe of the slope has collapsed and the slope failure is likely to progress upward along the slope. Retaining walls should be constructed on a stable foundation.Retaining walls are categorized in two ways: based on the mechanics of performance, and based on the construction material.

Drop Structures

Drop structures, also known as grade control structures, are structures placed at intervals along a channel reach to change a continuous steep slope into a series of gentle slopes and vertical (or steep and roughened) drops, like a series of steps. They control erosion and river channel degradation by reducing the slope of the channel and preventing the development of high erosive flow velocities and allow water to drop safely from one level to another without gouging out gullies. They can also help to control flooding and trap the sediment moving with runoff water.

Drop structures include sills, weirs, chute spillways, drop pipes, and check dams. A weir allows water to run over the edge like a miniature waterfall, dropping down onto a concrete apron. The apron absorbs the impact of the falling water and then the water streams to an outlet. When the drop in grade is more dramatic, a chute can be used to prevent severe erosion. As the name implies, water moves down a chute made of concrete or lined with rocks or concrete blocks. Like chutes, pipes are effective in handling water when the drop in grade is dramatic. They are designed to carry water through or under an earth embankment to a lower elevation. With a drop inlet, water drops down into the inlet and then flows through the pipe. Because of the high energies that must be dissipated, preformed scour holes or plunge pools may be required below these structures.

Check dams

Check dams are small low drop structures built across a gully or channel to prevent it from deepening further. These small dams decrease the slope gradient and reduce the velocity of water flow and the erosive power of the runoff. They also promote the deposition of eroded materials to further stabilize the gully or channel. Gully plugging using check dams, accompanied by planting between the dams to stabilize the channel, can be one of the most effective ways to conserve soil and water and rehabilitate land degraded by gullies. The effectiveness of different check dams depends upon the design, location, and construction materials. Check dams can be constructed from a wide range of materials including rock, wood, bamboo, gravel bags, sandbags, concrete, masonry, and fiber rolls.

Sabo Dams

Sabo dams are a common measure to limit debris flows. They are in some ways similar to check dams, but they are intended to limit debris flow rather than runoff velocity. The word sabo comes from Japanese and means soil conservation (sa means soil and bo means conservation); sabo dams are a Japanese technology that is now becoming popular beyond Japan. Sabo dams are relatively small structures built across the river bed in upstream areas in the form of a cross dike. They look like a normal small dam, except that they have a lower open section at the center which allows debris to pass through during normal conditions but prevents large-scale debris flow during flash floods. Sabo dams built in the upstream areas of mountain streams accumulate sediment and suppress the production and flow of sediment. Those built at the exits of valleys work as a direct barrier to a debris flow that has occurred. A sabo dam with slits is particularly effective in capturing a debris flow because it has a larger capacity thanthe sand pool under normal conditions. Sabo dams are usually constructed using masonry, concrete, reinforced concrete, or steel cribs according to the conditions in the planned area.

The main functions of a sabo dam are to

• Reduce erosion of the river bed and bank;
• Trap sediment discharge;
• Control sediment discharge;
• Grade the effect of sediment; and
• Reduce the energy of debris flows.

Black soils

Among the in situ soils of India, the black soils found in the lava-covered areas are the most conspicuous. Those soils are often referred to as regur but are popularly known as “black cotton soils,” since cotton has been the most common traditional crop in areas where they are found.

Black soils are derivatives of trap lava and are spread mostly across interior Gujarat, Maharashtra, Karnataka, and Madhya Pradesh on the Deccan lava plateau and the Malwa Plateau, where there is both moderate rainfall and underlying basaltic rock. Because of their high clay content, black soils develop wide cracks during the dry season, but their iron-rich granular structure makes them resistant to wind and water erosion. They are poor in humus yet highly moisture-retentive, thus responding well to irrigation. Those soils are also found on many peripheral tracts where the underlying basalt has been shifted from its original location by fluvial processes. The sifting has only led to an increased concentration of clastic contents.

Key takeaways:

Slope treatment methods can be divided broadly into measures to reduce runoff (terracing, diversions, grassed waterways, conservation ponds), methods to stabilize slopes and reduce erosion (retaining walls, drop structures, sabo dams), and integrated methods to address specific problems (gully control, trail improvement), although they all tend to have multiple functions.

3.8 Case study on black clay

A very beneficial approach withinside the stu dy of clay mineral s is differential thermal evaluation, a technique that's qualitative in addition to quantitative in nature. This approach of evaluation det ects and shows the thermal effects that accom pany the lack of water, ox idation, decomposition, or adjustments in crystal shape that can arise whilst a substa nce is warmth ed. A differential th ermocouple (-head ed th ermocouple) is used as th e warmth sensitive de vice. The th ermocouple junction may be made withinside the laboratory by truly becoming a member of wires of various metals (chromel-alumel) a t their ends and heating them till a be advert is formed. Two such junctions are made. One junction is inserted in an inert mat eri al (A 12°:1) which does now no longer go through both exothermic or endothermi c reactions thru the tem - perature c language this is studied; the opposite is located in th e pattern holder containing the soil pattern this is to be tested. Both junctions are then -heated in a furnace to a temperature of 950"C . at a consta n t charge of 15 'stages consistent with minute. An endothermic or exothermic response at th e junction of the soil pattern will crea te a poten tial among those junctions, "therefore inflicting an electrical curren t to float from th e warmer to the colder junction. The capability is proportional to the con centration of the mineral giving the response. A reflecting galvanometer is used as a measuring tool for detecting any temperature differen ce among the pattern and the alumin um oxide. A mineral ma y show off a characteristic rea ction or reactions a t a particular temperature range, therefore permitting an ea sy manner of identification. A plot of galvanometer readings against temperature offers a curve which suggests a quantitative in addition to a qualitativepicture of the soil minerals withinside the pattern.

Since the numerous clay minerals in a soil pattern yield sufficiently one-of-a-kind peaks, the temperature at whi ch a height happens is regularly indicative of which mineral is present. The charge of reaction, the amplitude and the form of a height are all depending on the form of alternate occurring withinside the pattern. The 2: I layered silicates, I: I layered silicates, and the hydrous oxides deliver primary peaks at one-of-a-kind temperatures in order that identity is easy and accurate. The identity of a mineral through differential thermal evaluation isn't always final.

A high-quality identity would require numerous one-of-a-kind analytical methods, including elemental evaluation, x-ray diffraction, and electron microscopy. Familiarity with the soil and a truthful understanding of the soil forming technique is regularly useful in looking ahead to the soil minerals. The software of the differential thermal evaluation to the quantitative remedy of clay minerals has been mentioned. Speil (34) and Kerr et at. (18) have proven in particular mathematical phrases that the attention of a clay mineral in an unknown pattern may be associated with the vicinity among a height and the bottom line of a thermal curve. More specific dedication of relatively easy structures may be made through strolling known combinations and getting ready a calibration curve of vicinity as opposed to percentage of every component.

However, if a aggregate of or extra components has peaks withinside the equal temperature c program languageperiod, the vicinity of that height will be an additive characteristic of the components. Fortunately, maximum clay minerals display as a minimum one height that's unique for itself. In this take a look at the absolute attention of the mineral in query is of little import, alternatively it's far the capacity to expose comparative concentrations among samples this is of primary interest. It suffices to mention that because the attention of the reacting substance giving the height in creases, the vicinity beneathneath the height increases. The equipment become standardized with natural kaolinite from the McNamee mine, South Carolina. The resistance become adjusted in order that this clay gave an endothermic deflection of 30 gadgets at the height of reaction. The pattern to be examined through the differential thermal equipment become exceeded thru a 65-mesh display and maintained at 50 percentage relative humidity till equilibrium become reached. Kerr et al. (17) endorse that any try to acquire equilibrium with a detailed humidity is unnecessary, however thinking about the one-of-a-kind levels of wetness of the soil samples studied and the significance of the adsorbed water peaks on this work, this kind of technique become believed justified. After the pattern become loaded, the specimen holder become protected with a steel plate and enclosed withinside the furnace. The galvanometer become switched to the "on" role and zeroed. A watch with a massive minute hand become used, considering the fact that readings have been made each 60 seconds. Because mistakess withinside the wide variety of ·examine ings in line with given time c program languageperiod can without difficulty occur, the minute hand become synchronized with the lengthy hour hand so thatt'n" wide variety of revolutions through the minute hand equaled the wide variety to which the lengthy

Hour hand turned into pointing. This system simply facilitated the detection of error. The furnace turned into then started; the price of heating turned into controlled through a calibrated voltage transformer and the temperature turned into study from a pyrometer stressed to the pattern thermocouple.

Galvanometer readings had been made each minute, with care being taken to hold the heating price at 15 levels in keeping with minute. The file turned into crafted from room temperature to about 950°C. The primary shape of the montmorillonite organization minerals is related to the mineral pyrophyllite (25) which includes a unmarried gibbsite sheet among sheets of silica tetrahedrons. In montmorillonite (18) magnesium is substituted for aluminum in octahedral coordination. This outcomes in poor expenses at the three-layer sheet which in flip are balanced through intersheet cations which include calcium and sodium. The structural gadgets are stacked one above every other and are loosely held with water molecules gift among them. This interlayer water may be removed with out disruption of the crystal shape and is vital in the identity of the montmorillonite organization minerals through thermal means.

Numerous isomorphous replacements make advantageous identity of minerals tough through chemical methods. Jackson (14) prefers to apply the time period montmorin to encompass the isomorphous collection of 2: 1 layer silicates which include montmorillonite, beidellite, saponite, nontronite, and other isomorphous relatives. The montmorillonite organization minerals are characterised through having high cation iron trade capacities (ISO m.e.flOO grams of day). Soils having montmorillonite organization minerals are fantastically plastic, will swell upon wetting, and decrease and shape massive cracks while dried. These minerals are usually observed in soils having impartial to alkaline reaction. The differential thermal curves of the montmorillonite organization minerals had been reviewed through many workers (9,17,18,34). The salient feature of the thermal curve of the montmorillonite organization is the adsorbed water top among 100° and 200°C. Initial destruction of lattice starts at approximately 5000 e. For nontronite and approximately 6000 e. For montmorillonite. This decomposition is specially because of the lack of the hydroxyl businesses in the shape of water (18) and is represented at the thermal curve through a vulnerable endothermic top. A 1/3 and very last endothermic top at approximately 900 levels which corresponds to the breakdown of the lattice is also function of this organization. None of the Hawaiian clays analyzed show the endothermic peaks at 600° or 900 zero e. The thermal curve for the montmorillonite organization of minerals is unlucky in that the principle endothermic top among 100° and 200°C. Isn't always unique for this organization. Other materials which include natural matter, extra moisture, allophane, halloysite, and 2: 1

Key takeaways:

The thermal curve for the montmorillonite organization of minerals is unlucky in that the principle endothermic top among 100° and 200°C. Isn't always unique for this organization. Other materials which include natural matter, extra moisture, allophane, halloysite, and 2: 1

3.9 Groundwater: Factors controlling water-bearing capacity of rock

The two most important forces controlling water movement in the rock are gravity and molecular attraction.  Gravity causes water to infiltrate until it reaches impermeable zones where it is diverted laterally.  Gravity generates the flow of springs, rivers, and wells.  If the pores in rocks and sediments are connected, gravity allows the water to move slowly through them.  However, the smaller the opening, the harder it is for gravity to cause water movement.  The second force, molecular attraction, slows the flow of water through small pores.  Water is attracted to the surface of every particle with which it comes in contact.  The force results from the attraction of the molecules of two substances for each other.

The molecular attraction of water in rocks

The attraction between water and soil or rock particles is termed adhesion.  It is effective only over short distances.  Thus, only a thin film of water is locked to the outside of each grain resisting the flow downward in response to gravity. It is this adhesion that helps hold water in the soil for plants.  If gravity were the only force involved, all water would drain through the soil to some depth.  In fine-grained sediments such as silt and clay, the aggregate surface area which can attract water molecules is very great.  Fine-grained materials hold more water over a longer time than the same volume of coarse-grained materials such as sand or gravel.

Surface tension and capillarity

The attraction of water molecules for each other is termed cohesion.  It can be demonstrated by immersing a pencil in water and noting the drop that remains at the base of the pencil, seemingly held there by the water above it. This attraction is due to the surface tension characteristic of water, caused by cohesion.  Water will also rise in a small tube if it is immersed.  This phenomenon is called capillary action or capillarity.  The smaller the tube, the higher the water will rise.  Capillary action is important in rocks and sediments because pores immediately above the saturated zone are filled with capillary water.  The more fine-grained the sediment or rock, the higher the water is pulled.  The diameter of the pore opening and the degree of connection with the saturated zone is very important.  So much water is drawn into the pores above the water table that this zone is given a special term, the capillary fringe.

Permeability of rocks

Permeability is the capacity of a rock to transmit water under pressure.  If no pressure exists, a static equilibrium is present and there is no tendency for water to move.  This condition is very rare in nature.  Most water can be thought to be in a dynamic state or moving in response to a pressure gradient.

Meinzer defines permeability as follows: "The permeability of a rock is measured by the rate at which it will transmit water through a given cross-section under a given difference of pressure per unit of distance."  In a sequence of sedimentary rock with varying permeability, it commonly can be shown that horizontal permeability or permeability that is parallel to the bedding of rocks such as sandstone and conglomerate is greater than permeability at right angles to bedding.  This is because some beds in the sequence have such low permeability that vertical infiltration is slow whereas lateral permeability in units below confining beds is good.

No rocks near the surface of the earth are impermeable if enough pressure is applied in forcing the water through the natural openings in the rock.  However, the forces generated by nature are insufficient in some cases to produce detectable permeability, and rocks with such characteristics are said to be relatively impermeable.  Examples of such rocks are found in shales that contain clays that swell on wetting and thus close off natural openings that may exist when the rock is dry.  On the other hand, coarse, clean gravel contains such large openings that it readily transmits water.  Ordinarily, such deposits function as the best aquifers where they can be easily recharged.  Dirty or clay-rich gravels have much less permeability because the fine silt and clay between the larger particles effectively slow down or block completely the flow of water through some of the pores between the sand grains.

Coefficient of permeability

The coefficient of permeability (P) used by the USGS may be expressed as the number of gallons of water a day, at 60 °F, that is conducted laterally through each square foot of water-bearing material (measured at right angles to the direction of flow), under a hydraulic gradient of 1 foot per foot.  It has the units of gallons per day per square foot (gpd per sq. Ft.).

Key takeaways:

The attraction of water molecules for each other is termed cohesion.  It can be demonstrated by immersing a pencil in water and noting the drop that remains at the base of the pencil, seemingly held there by the water above it.

3.10 Pervious & impervious rocks and ground water

The rocks that don't allow water to byskip via them freely are called Impervious Rocks. They can be porous, like clay, or non-porous, like unfissured granitic rocks. Impervious rocks additionally called Impermeable Rocks. Rock which permits water to byskip via it because of cracks or defects are called pervious rocks.

Rainwater now no longer most effective flows down the streams or rivers however additionally slowly is going down into the soil. This water accumulates below the floor withinside the gaps among rocks, pebbles, sand etc. This is the groundwater, which we attain via wells and tubewells. Rocks that have cracks or pores (minute holes) in them and may contain water are referred to as pervious rocks. In Andhra Pradesh we've got few districts like Khammam wherein such rocks like sandstones are found. Some rocks like granite, Kadapa limestone, are very compact and do now no longer have pores in them. Water can not input into them. Groundwater generally accumulates above such rocks. Since the water can not move underneath them, those are referred to as impervious rocks. Most of the rocks underlying the soil in our state are of this kind. A small part of the state that's subsequent to rivers has deep layers of sand, soil and pebbles. Water additionally accumulates in those layers. The layer of water which accumulates below the floor amongst rocks is referred to as aquifer. The thickness of the aquifer determines the supply of groundwater withinside the area.

Groundwater is saved withinside the open areas inside rocks and inside unconsolidated sediments. Rocks and sediments close to the floor are beneathneath much less stress than the ones at enormous intensity and consequently have a tendency to have greater open area. For this reason, and due to the fact it’s steeply-priced to drill deep wells, maximum of the groundwater this is accessed with the aid of using man or woman customers is withinside the first a hundred m of the floor. Some municipal, agricultural, and commercial groundwater customers get their water from extra intensity, however deeper groundwater has a tendency to be of decrease fine than shallow groundwater, so there's a restriction as to how deep we are able to go. Porosity is the proportion of open area inside an unconsolidated sediment or a rock. Primary porosity is represented with the aid of using the areas among grains in a sediment or sedimentary rock. Secondary porosity is porosity that has advanced after the rock has shaped. It can encompass fracture porosity — area inside fractures in any sort of rock. Some volcanic rock has a unique sort of porosity associated with vesicles, and a few limestone has greater porosity associated with cavities inside fossils. Porosity is expressed as a percent calculated from the quantity of open area in a rock in comparison with the overall quantity of rock. The usual degrees in porosity of some of exclusive geological substances. Unconsolidated sediments have a tendency to have better porosity than consolidated ones due to the fact they don't have any cement, and maximum have now no longer been strongly compressed. Finer-grained substances (e.g., silt and clay) have a tendency to have extra porosity — a few as excessive as 70% — than coarser substances (e.g., gravel). Primary porosity has a tendency to be better in well-looked after sediments in comparison to poorly looked after sediments, in which there's a number of smaller debris to fill the areas made with the aid of using the bigger debris. Glacial till, which has a extensive variety of grain sizes and is commonly shaped beneathneath compression below glacial ice, has quite low porosity. Consolidation and cementation all through the manner of lithification of unconsolidated sediments into sedimentary rocks reduces number one porosity. Sedimentary rocks typically have porosities withinside the variety of 10% to 30%, a number of which can be secondary (fracture) porosity. The grain size, sorting, compaction, and diploma of cementation of the rocks all have an effect on number one porosity. For example, poorly looked after and well-cemented sandstone and well-compressed mudstone will have very low porosity. Igneous or metamorphic rocks have the bottom number one porosity due to the fact they generally shape at intensity and feature interlocking crystals. Most in their porosity comes withinside the shape of secondary porosity in fractures. Of the consolidated rocks, well-fractured volcanic rocks and limestone that has cavernous openings produced with the aid of using dissolution have the very best ability porosity, even as intrusive igneous and metamorphic rocks, which shaped beneathneath superb stress, have the bottom.

Key takeaways:

The rocks that don't allow water to byskip via them freely are called Impervious Rocks. They can be porous, like clay, or non-porous, like unfissured granitic rocks. Impervious rocks additionally called Impermeable Rocks. Rock which permits water to byskip via it because of cracks or defects are called pervious rocks.

3.11 Lowering of water table and Subsidence

Excessive pumping can decrease the groundwater table, and reason wells to not be capable of attain groundwater. ... When groundwater is overused, the lakes, streams, and rivers related to groundwater also can have their deliver diminished. Land Subsidence. Land subsidence happens whilst there may be a lack of assist underneath ground.

Land subsidence is the decreasing of the land-floor elevation from adjustments that take area underground. Common reasons of land subsidence from human pastime are pumping water, oil, and fueloline from underground reservoirs; dissolution of limestone aquifers (sinkholes); fall apart of underground mines; drainage of natural soils; and preliminary wetting of dry soils (hydrocompaction)

Overdrafting of aquifers is the important purpose of subsidence withinside the southwestern United States, and as floor-water pumping increases, land subsidence will also increase. In many aquifers, floor water is pumped from pore areas among grains of sand and gravel. If an aquifer has beds of clay or silt inside or subsequent to it (determine 2), the decreased water strain withinside the sand and gravel reasons sluggish drainage of water from the clay and silt beds. The decreased water strain is a lack of assist for the clay and silt beds. Because those beds are compressible, they compact (grow to be thinner), and the results are visible as a decreasing of the land floor. The decreasing of land floor elevation from this technique is permanent. For example, if decreased floor-water degrees triggered land subsidence, recharging the aquifer till floor water again to the authentic degrees could now no longer bring about an considerable restoration of the land-floor elevation.

Groundwater-associated subsidence is the subsidence (or the sinking) of land as a result of groundwater extraction. It is a developing trouble withinside the growing global as towns boom in populace and water use, with out good enough pumping law and enforcement. One estimate has 80% of significant U.S. Land subsidence troubles related to the immoderate extraction of groundwater,[1] making it a developing trouble at some stage in the global. Groundwater may be taken into consideration one of the remaining unfastened resources, as everybody who can have the funds for to drill can normally draw up simply in line with their cappotential to pump (relying on nearby regulations). However, as visible withinside the figure, pumping-precipitated draw down reasons a melancholy of the groundwater floor across the manufacturing well. This can in the long run have an effect on a big location through making it greater tough and high-priced to pump the deeper water. Thus, the extraction of groundwater will become a tragedy of the commons, with ensuing monetary externalities.

The arid regions of the arena are requiring increasingly more water for developing populations and agriculture. In the San Joaquin Valley of the United States, groundwater pumping for plants has long past on for generations. This has resulted withinside the complete valley sinking an top notch amount, as proven withinside the figure. This has now no longer come with out consequences. Any massive-scale extrade of topography, irrespective of how moderate it is able to seem, has the capacity to extensively extrade the floor-water hydrology. This has took place withinside the Joaquin Valley and different areas of the arena, together with New Orleans and Bangkok. These regions are actually difficulty to extreme flooding because of subsidence related to groundwater removal.[3][4] Total subsidence can normally be decided via way of means of ground-floor elevation surveys and GPS measurements. Potential effect at the aquifers and different ensuing geohazards together with fissures may be assessed thru long-time period hydrologic research and models. Mexico City subsidence Groundwater-associated subsidence frequently outcomes in most important harm to city regions. In Mexico City, the homes engage with the settlement, and motive cracking, tilting, and different most important harm.[5] In many places, massive sinkholes open up, in addition to floor cavities. Damage from Hurricane Katrina become exacerbated because of coastal sinking, related to groundwater withdrawal. The motive of the long-time period floor modifications related to this phenomenon are pretty properly recognized.[6] As proven withinside the USGS figure, aquifers are often related to compressible layers of silt or clay. Gwcause.jpg As the groundwater is pumped out, the powerful strain modifications, precipitating consolidation, that is frequently non-reversible. Thus, the entire quantity of the silts and clays is decreased, ensuing withinside the reducing of the floor. The harm on the floor is lots extra if there may be differential settlement, or massive-scale features, together with sinkholes and fissures. Aquifer compaction is a massive problem along side pumping-prompted land subsidence. A massive part of the groundwater garage capacity of many aquifers may be substantially decreased whilst longterm groundwater extraction, and the ensuing groundwater stage decline, reasons everlasting compaction of satisfactory sediment layers (silts and clays). A have a look at in an arid agricultural place of Arizona [7] confirmed that, inspite of a water stage recuperation of one hundred feet after groundwater pumping become stopped, the land floor endured to subside for decades. This is a end result of the ongoing dewatering of aquitards (satisfactory-grain layers that sluggish the motion of groundwater) from stresses referred to withinside the preceding paragraph. The simplest recognized technique to save you this situation is via way of means of pumping much less groundwater, that is extraordinarily tough to put into effect whilst many human beings very own water wells. Attempts are being made to at once recharge aquifers however that is nonetheless a initial effort

Key takeaways:

Excessive pumping can decrease the groundwater table, and reason wells to not be capable of attain groundwater. ... When groundwater is overused, the lakes, streams, and rivers related to groundwater also can have their deliver diminished. Land Subsidence. Land subsidence happens whilst there may be a lack of assist underneath ground

3.12 Earthquake: Magnitude and intensity of earthquake

Earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earth’s rocks. Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip.” Earthquakes occur most often along geologic faults, narrow zones where rock masses move with one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up Earth’s crust.The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterward in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock.

Fig 22 A simplified cartoon of the crust (brown), mantle (orange), and core (liquid in light gray, solid in dark gray) of the earth.

Causes of earthquakes

The earth has four major layers: the inner core, outer core, mantle, and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet.

But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it.

Earthquake recording

Fig 23

The fig 23 sketch of the seismograph shows how the instrument shakes with the earth below it, but the recording device remains stationary (instead of the other way around).

Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavyweight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead, the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

Measuring the earthquake size.

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was. A short wiggly line that doesn’t wiggle very much means a small earthquake and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about theintensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

Fig 24 An example of a seismic wave with the P wave and S wave labeled.

Locating Earthquakes

Seismograms come in handy for locating earthquakes too and being able to see the P wave and the S wave is important. P & S waves each shake the ground in different ways as they travel through it. P waves are also faster than S waves, and this fact is what allows us to tell where an earthquake was. To understand how this works, let’s compare P and S waves to lightning and thunder. Light travels faster than sound, so during a thunderstorm, you will first see the lightning and then you will hear the thunder. If you are close to the lightning, the thunder will boom right after the lightning, but if you are far away from the lightning, you can count several seconds before you hear the thunder. The further you are from the storm, the longer it will take between the lightning and the thunder.

P waves are like the lightning, and S waves are like thunder. The P waves travel faster and shake the ground where you are first. Then the S waves follow and shake the ground also. If you are close to the earthquake, the P and S waves will come one right after the other, but if you are far away, there will be more time between the two.

Fig 25 P and S waves

By looking at the amount of time between the P and S waves on a seismogram recorded on a seismograph, scientists can tell how far away the earthquake was from that location. However, they can’t tell in what direction from the seismograph the earthquake was, only how far away it was. If they draw a circle on a map around the station where the radius of the circle is the determined distance to the earthquake, they know the earthquake lies somewhere on the circle.

Scientists then use a method called triangulation to determine exactly where the earthquake was (see image below). It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter

Magnitude and Intensity

Magnitude and Intensity measure different characteristics of earthquakes. Magnitude measures the energy released at the source of the earthquake. Magnitude is determined from measurements on seismographs. Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined by effects on people, human structures, and the natural environment.

Calculating Earthquake Magnitude

The magnitude of an earthquake is a number that allows earthquakes to be compared with each other in terms of their relative power. For several decades, earthquake magnitudes were calculated based on a method first developed by Charles Richter, a seismologist based in California. Richter used seismograms of earthquakes that occurred in the San Andreas fault zone to calibrate his magnitude scale.

Two measurements are factored together to determine the Richter magnitude of an earthquake: the amplitude of the largest waves recorded on a seismogram of the earthquake, and the distance to the epicenter of the earthquake. The maximum amplitude seismic wave – the height of the tallest one – is measured in mm on a seismogram. The distance to the epicenter must also be taken into account because the greater the distance from the earthquake, the smaller the waves get. The effect of distance is factored out of the calculation. There is no upper limit defined for the Richter scale, but after a century of seismograph measurements, it appears that rocks in the earth release their stress before building up enough energy to reach magnitude 10.

The Richter scale was found to not transfer very well from the San Andreas fault zone, a transform plate boundary, to the much more powerful earthquakes that occur at convergent plate boundaries, particularly subduction zone earthquakes. Therefore, the Richter scale has been replaced by the moment magnitude scale, symbolized as Mw.

The moment magnitude scale is broadly similar to the Richter scale, but it takes more factors into account, including the total area of the fault that moves during the earthquake, and how much it moves. This produces a magnitude number that is a better indicator of the total amount of energy released by the earthquake. Because the moment magnitude scale has replaced the Richter scale, we will assume from here on that we are referring to moment magnitude, not Richter magnitude, when we speak of earthquake magnitude.

The magnitude scale portrays energy logarithmically to approximately base 32. For example, a magnitude 6.0 earthquake releases about 32 times as much energy as a magnitude 5.0 earthquake. A magnitude 7.0 releases about 32 × 32 = 1024 times as much energy as a magnitude 5.0 earthquake. A magnitude 9.0 earthquake, which rarely occurs, releases over a million times as much energy as a magnitude 5.0 earthquake.

Ranking Earthquake Intensity

Earthquake intensity is very different from earthquake magnitude. Earthquake intensity is a ranking based on the observed effects of an earthquake in each particular place. Therefore, each earthquake produces a range of intensity values, ranging from highest in the epicenter area to zero at a distance from the epicenter. The most commonly used earthquake intensity scale is the Modified Mercalli earthquake intensity scale. Refer to the Modified Mercalli Intensity Scale page on the US Geological Survey Earthquake Hazards Program website for an abbreviated version.

Table 5.1 below shows approximately how many earthquakes occur each year in each magnitude range and what the intensity might be at the epicenter for each magnitude range.

Table 1

3.13 Seismic Sea waves

Tsunamialso called seismic sea wave or tidal wave, catastrophic ocean wave, usually caused by a submarine earthquake, an underwater or coastal landslide, or a volcanic eruption. The term tidal wave is frequently used for such a wave, but it is a misnomer, for the wave has no connection with the tides.

Fig 26 The aftermath of the December 2004 tsunami in Aceh, Indon.

Origin And Development

After an earthquake or other generating impulse occurs, a train of simple, progressive oscillatory waves is propagated great distances over the ocean surface in ever-widening circles, much like the waves produced by a pebble falling into a shallow pool. In deep water, a tsunami can travel as fast as 800 km (500 miles) per hour. The wavelengths are enormous, about 100 to 200 km (60 to 120 miles), but the wave amplitudes (heights) are very small, only about 30 to 60 cm (1 to 2 feet). The waves’ periods (the lengths of time for successive crests or troughs to pass a single point) are very long, varying from five minutes to more than an hour. These long periods, coupled with the extremely low steepness and height of the waves, enable them to be completely obscured in deep water by normal wind waves and swell. A ship on the high seas experiences the passage of a tsunami as an insignificant rise and fall of only half a meter (1.6 feet), lasting from five minutes to an hour or more.

Fig 27 Tsunami

As the waves approach the coast of a continent, however, friction with the rising sea bottom reduces the velocity of the waves. As the velocity lessens, the wavelengths become shortened and the wave amplitudes (heights) increase. Coastal waters may rise as high as 30 metres (about 100 feet) above normal sea level in 10 to 15 minutes. The continental shelf waters begin to oscillate after the rise in sea level. Between three and five major oscillations generate most of the damage, frequently appearing as powerful “run-ups” of rushing water that uproot trees, pull buildings off their foundations, carry boats far inshore, and wash away entire beaches, peninsulas, and other low-lying coastal formations. Frequently the succeeding outflow of water is just as destructive as the run-up or even more so. In any case, oscillations may continue for several days until the ocean surface reaches equilibrium.

Much like any other water wave, tsunamis are reflected and refracted by the topography of the seafloor near shore and by the configuration of a coastline. As a result, their effects vary widely from place to place. Occasionally, the first arrival of a tsunami at a coast may be the trough of the wave, in which case the water recedes and exposes the shallow seafloor. Such an occurrence took place in the bay of Lisbon, Portugal, on November 1, 1755, after a large earthquake; many curious people were attracted to the bay floor, and a large number of them were drowned by the wave crest that followed the trough only minutes later.

Key takeaways:

Tsunamialso called seismic sea wave or tidal wave, catastrophic ocean wave, usually caused by a submarine earthquake, an underwater or coastal landslide, or a volcanic eruption. The term tidal wave is frequently used for such a wave, but it is a misnomer, for the wave has no connection with the tides.

3.14 Revelation from Seismic Records of the structure of the earth

When an earthquake occurs the seismic waves (P and S waves) spread out in all directions through the Earth's interior. Seismic stations located at increasing distances from the earthquake epicenter will record seismic waves that have traveled through increasing depths in the Earth.

Seismic velocities depend on the material properties such as composition, mineral phase and packing structure, temperature, and pressure of the media through which seismic waves pass. Seismic waves travel more quickly through denser materials and therefore generally travel more quickly with depth. Anomalously hot areas slow down seismic waves. Seismic waves move more slowly through a liquid than a solid. Molten areas within the Earth slow down P waves and stop S waves because their shearing motion cannot be transmitted through a liquid. Partially molten areas may slow down the P waves and attenuate or weaken S waves.

When seismic waves pass between geologic layers with contrasting seismic velocities (when any wave passes through media with distinctly differing velocities) reflections, refraction (bending), and the production of new wave phases (e.g., an S wave produced from a P wave) often result. Sudden jumps in seismic velocities across a boundary are known as seismic discontinuities.

The Crust

Mohorovicic Seismic Discontinuity

Seismic stations within about 200 km of a continental earthquake (or other seismic disturbance such as a dynamite blast) report travel times that increase regularly with distance from the source. But beyond 200 km the seismic waves arrive sooner than expected, forming a break in the travel time vs. Distance curve. Mohorovicic (1909) interpreted this to mean that the seismic waves recorded beyond 200 km from the earthquake source had passed through a lower layer with significantly higher seismic velocity.

Fig 28

This seismic discontinuity is now known as the Moho (much easier than "Mohorovicic seismic discontinuity") It is the boundary between the felsic/mafic crust with a seismic velocity around 6 km/sec and the denser ultramafic mantle with a seismic velocity of around 8 km/sec. The depth to the Moho beneath the continents averages around 35 km but ranges from around 20 km to 70 km. The Moho beneath the oceans is usually about 7 km below the seafloor (i.e., ocean crust is about 7 km thick).

Properties of the Crust

Continental Crust

Depth to Moho: 20 to 70 km, average 30 to 40 km
Composition: felsic, intermediate, and mafic igneous, sedimentary, and metamorphic rocks

Age: 0 to 4 b.y.

Summary: thicker, less dense, heterogeneous, old

Oceanic Crust

Depth to Moho: ~7 km

Composition: mafic igneous rock (basalt & gabbro) with a thin layer of sediments on top

Age: 0 to 200 m.y.

Summary: thin, more dense, homogeneous, young 

The Mantle

Low-Velocity Zone

Seismic velocities tend to gradually increase with depth in the mantle due to the increasing pressure, and therefore density, with depth. However, seismic waves recorded at distances corresponding to depths of around 100 km to 250 km arrive later than expected indicating a zone of low seismic wave velocity. Furthermore, while both the P and S waves travel more slowly, the S waves are attenuated or weakened. This is interpreted to be a partially molten zone, probably one percent or less (i.e., greater than 99 percent solid).  Alternatively, it may simply represent a zone where the mantle is very close to its melting point for that depth and pressure that it is very "soft."  Then this represents a zone of weakness in the upper mantle. This zone is called the asthenosphere or "weak sphere." 

The asthenosphere separates the strong, solid rock of the uppermost mantle and crust above from the remainder of the strong, solid mantle below.  The combination of the uppermost mantle and crust above the asthenosphere is called the lithosphere.  The lithosphere is free to move (glide) over the weak asthenosphere. The tectonic plates are, in fact, lithospheric plates.

670 km Seismic Discontinuity

Below the low-velocity zone are a couple of seismic discontinuities at which seismic velocities increase.  Theoretical analyses and laboratory experiments show that at these depths (pressures) ultramafic silicates will change phase (atomic packing structure or crystalline structure) from the crystalline structure of olivine to tighter packing structures.  A discontinuity at around 670 km depth is particularly distinct. The 670 km discontinuity results from the change of spinel structure to the perovskite crystalline structure which remains stable to the base of the mantle. Perovskite (the same chemical formula as olivine) is then the most abundant silicate mineral on Earth. The 670 km discontinuity is thought to represents a major boundary separating a less dense upper mantle from a more dense lower mantle. 

Fig 29

The Core

Gutenberg Seismic Discontinuity / Core-Mantle Boundary
Seismic waves recorded at increasing distances from an earthquake indicate that seismic velocities gradually increase with depth in the mantle (exceptions: see Low-Velocity Zone and 670 km Discontinuity above). However, at arc distances of between about 103° and 143° no P waves are recorded. Furthermore, no S waves are record beyond about 103°. Gutenberg (1914) explained this as the result of a molten core beginning at a depth of around 2900 km. Shear waves could not penetrate this molten layer and P waves would be severely slowed and refracted (bent).

Lehman Seismic Discontinuity / The Inner Core

Between 143° and 180° from an earthquake, another refraction is recognized (Lehman, 1936) resulting from a sudden increase in P wave velocities at a depth of 5150 km. This velocity increase is consistent with a change from a molten outer core to a solid inner core.

Fig 30 Seismic ray paths (perpendiculars to seismic wavefronts) in the Earth

Key takeaways:

Seismic velocities depend on the material properties such as composition, mineral phase and packing structure, temperature, and pressure of the media through which seismic waves pass. Seismic waves travel more quickly through denser materials and therefore generally travel more quickly with depth

3.15 Case Study on Elevation and Subsidence in the Himalayan region in India

Rishikesh - Gangotri highway has been breached and dislocated at several locations due to slope instability problems, particularly during the rainy season. Increased pore water pressure and downslope acting forces together with reduced frictional force provide favorable conditions for the mass movement during this season. Numerous massive landslides took place on the Uttarkashi-Harsil road section, particularly on a 42 km stretch between Uttarkashi and Bhatwari, believed to be the area of intense shaking. Fissures were most prominent on the Maneri, Bhatwari stretch. Abundance of active landslide zones, alluvial/colluvial deposits, development of depositional anderosional terraces, ground subsidence, off-set drainage, narrow-deep gorges, and formation of landslide dammed lakes witness the neotectonic activity in the area. Steep slopes, high relief, several structural discontinuities, and underlying geology combined with anthropogenic activities constitute a propensity towards failure affecting both massive country rock and the Quaternary cover. Lata-Malla landslide is an old slide zone that is reactivated recently in 2013; however, land subsidence along the road has been witnessed for a long time, according to local people and earlier field investigation. Subsidence involves the settling or sinking of a body of rock or sediment. Subsidence is a type of mass wasting or mass movement-transport of large volumes of earth material primarily by gravity. Subsidence may occur as the result of either natural or human-caused events. Ground subsidence is of concern to a geologist, geotechnical engineers, and surveyors. The slow onslaught disasters do not normally catch media attention as these often do not result in human casualties. Inadequate media attention results in insufficient rehabilitation support for the victims (Rautela 2005). It is a well-known fact that there is some relationship between the tectonic forces and the processes of slope failures. Therefore, an attempt is made to define this relationship by finding how slope processes are related to the reactivation of various faults/thrusts.

Key takeaways:

Increased pore water pressure and downslope acting forces together with reduced frictional force provide favorable conditions for the mass movement during this season. Numerous massive landslides took place on the Uttarkashi-Harsil road section, particularly on a 42 km stretch between Uttarkashi and Bhatwari, believed to be the area of intense shaking.

3.16 Seismic Zone in India

The Indian subcontinent has a history of devastating earthquakes. The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year. Geographical statistics of India show that almost 54% of the land is vulnerable to earthquakes.

A World Bank and United Nations report shows estimates that around 200 million city dwellers in India will be exposed to storms and earthquakes by 2050. The latest version of the seismic zoning map of India given in the earthquake-resistant design code of India [IS 1893 (Part 1) 2002] assigns four levels of seismicity for India in terms of zone factors. In other words, the earthquake zoning map of India divides India into 4 seismic zones (Zone 2, 3, 4, and 5) unlike its previous version, which consisted of five or six zones for the country. According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity.

Fig 31 Revised earthquake hazard zone map of India

Zone 5

Zone 5 covers the areas with the highest risk of suffering earthquakes of intensity MSK IX or greater. The IS code assigns a zone factor of 0.36 for Zone 5. Structural designers use this factor for the earthquake-resistant design of structures in Zone 5. The zone factor of 0.36 (the maximum horizontal acceleration that can be experienced by a structure) is indicative of effective (zero periods) level earthquakes in this zone. It is referred to as the Very High Damage Risk Zone. The regions of Kashmir, the Western and Central Himalayas, North and Middle Bihar, the North-East Indian region, the Rann of Kutch, and the Andaman and Nicobar group of islands fall in this zone.

Generally, the areas having trap rock or basaltic rock are prone to earthquakes.

Zone 4

This zone is called the High Damage Risk Zone and covers areas liable to MSK VIII. The IS code assigns a zone factor of 0.24 for Zone 4. Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim, parts of the Indo-Gangetic plains (North Punjab, Chandigarh, western Uttar Pradesh, Terai, North Bengal, the Sundarbans), and the capital of the country Delhi fall in Zone 4. In Maharashtra, the Patan area (Koynanagar) is also in Zone 4. In Bihar, the northern part of the state in areas such as Raxaul, near the border of India and Nepal, is also in Zone 4.

Zone 3

This zone is classified as a Moderate Damage Risk Zone which is liable to MSK VII. The IS code assigns a zone factor of 0.16 for Zone 3. Several megacities like Chennai, Mumbai, Kolkata, and Bhubaneshwar lie in this zone.

Zone 2

This region is liable to MSK VI or lower and is classified as the Low Damage Risk Zone. The IS code assigns a zone factor of 0.10 for Zone 2.

Zone 1

Since the current division of India into earthquake hazard zones does not use Zone 1, no area of India is classed as Zone 1.

Future changes in the classification system may or may not return this zone to use.

Key takeaways:

The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year. Geographical statistics of India show that almost 54% of the land is vulnerable to earthquakes.

References:

1. Engineering and General Geology, Parbin Singh, 8th Edition (2010), S K Kataria & Sons.
2. Text Book of Engineering Geology, N. Chenna Kesavulu, 2nd Edition (2009), Macmillan Publishers India.