Unit-5

Geological Hazards

5.1           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 5.1 Rockfall at Upper Island Cove

Fig 5.2 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 5.3 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 (Fig5.4 a). Translational slide: In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting (Fig 5.4 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 5.4 c).

Fig 5.4 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 5.5). 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 5.5 (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 5.6).

Fig 5.6 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 5.7a,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 5.7a, it is greater than the shear force, so the block should not move. In Fig 5.7b, 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 5.7c, the slope is steeper still, so the shear force is considerably greater than the shear strength, and the block will very likely move.

Fig 5.7 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 5.8. 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 5.8 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 5.9(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 5.9, right).

Fig 5.9 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 5.9, 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 5.10). 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 5.10 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 5.11: Simple slope slip section

As seen in Fig 5.11, 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 5.12 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 5.13, 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 5.13 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 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.

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).

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:

Landslides can be grouped by:

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

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.

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 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.

Landslides in regolith

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

Three types of creep occur:

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.

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 5.14 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:

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:

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:

5.2 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:

Reinforced Gravity Walls

Brick Masonry retaining wall

Gravity Retaining Walls

Fig 5.15

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 5.16

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:

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.

5.3 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 5.17) with the aim of:

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

Fig 5.17: 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 5.18):

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):

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 5.18 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 5.19). 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 5.19). 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 5.19 Grassed Waterways

Grassed waterways are used as

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.

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

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.

5.4 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.).

5.5 Groundwater

Some water underlies the Earth's surface almost everywhere, beneath hills, mountains, plains, and deserts. It is not always accessible, or fresh enough for use without treatment, and it's sometimes difficult to locate or to measure and describe. This water may occur close to the land surface, as in a marsh, or it may lie many hundreds of feet below the surface, as in some arid areas of the western United States. Water at very shallow depths might be just a few hours old; at moderate depth, it may be 100 years old; and at great depth or after having flowed long distances from places of entry, the water may have been in the ground for several thousand years.

Groundwater is a part of the natural water cycle. Some part of the precipitation that lands on the ground surface infiltrate into the subsurface. The part that continues down through the soil until it reaches rock material that is saturated is groundwater recharge. Water in the saturated groundwater system moves slowly and may eventually discharge into streams, lakes, and oceans.

Fig 5.20

Fig 5.20 is a simplified diagram showing how the ground is saturated below the water table (the solid blue area at the bottom). The ground above the water table (the pink area) may be wet to a certain degree, but it does not stay saturated. The dirt and rock in this unsaturated zone contain air and some water and support the vegetation on the Earth. The saturated zone below the water table has water that fills the tiny spaces (pores) between rock particles and the cracks (fractures) of the rocks.

Occurrence

A couple of important factors are responsible for the existence of groundwater:

Gravity

Gravity pulls water and everything else toward the center of the Earth. That means that water on the surface will try to seep into the ground below it.

The rocks below our feet

The rock below the Earth's surface is the bedrock. If all bedrock consisted of a dense material like solid granite, then even gravity would have a hard time pulling water downward. But Earth's bedrock consists of many types of rock, such as sandstone, granite, and limestone. Bedrocks have varying amounts of void spaces in them where groundwater accumulates. Bedrock can also become broken and fractured, creating spaces that can fill with water. And some bedrock, such as limestone, are dissolved by water -- which results in large cavities that fill with water.

Fig 5.21

Fig 5.21 shows groundwater seeping out from between rock layers and freezing in the winter temperatures. They were taken in a gorge at Buttermilk Falls State Park in Ithaca, NY.

In many places, if you looked at a vertical cross-section of the earth you would see that rock is laid down in layers, especially in areas of sedimentary rocks. Some layers have rocks that are more porous than others, and here water moves more freely (in a horizontal manner) through the earth. Sometimes water can be seen seeping out through exposed rock layers as seen in the photo.

Gravity doesn't pull water to the center of the Earth. Deep in the bedrock, there are rock layers made of dense material, such as granite, or material that water has a hard time penetrating, such as clay. These layers may be underneath the porous rock layers and, thus, act as a confining layer to retard the vertical movement of water. Since it is more difficult for the water to go any deeper, it tends to pool in the porous layers and flow in a more horizontal direction across the aquifer toward an exposed surface-water body, like a river.

5.6 Groundwater Decline and Depletion

Groundwater is a valuable resource both in the United States and throughout the world. Where surface water, such as lakes and rivers, is scarce or inaccessible, groundwater supplies many of the hydrologic needs of people everywhere. In the United States, it is the source of drinking water for about half the total population and nearly all of the rural population, and it provides over 50 billion gallons per day for agricultural needs. Groundwater depletion, a term often defined as long-term water-level declines caused by sustained groundwater pumping, is a key issue associated with groundwater use.

Excessive pumping can overdraw the groundwater "bank account"

The water stored in the ground can be compared to money kept in a bank account. If you withdraw money at a faster rate than you deposit new money you will eventually start having account-supply problems. Pumping water out of the ground faster than it is replenished over the long-term causes similar problems. The volume of groundwater in storage is decreasing in many areas of the United States in response to pumping. Groundwater depletion is primarily caused by sustained groundwater pumping. Some of the negative effects of groundwater depletion:

Effects of groundwater depletion

Pumping groundwater at a faster rate than it can be recharged can have some negative effects on the environment and the people who make use of the water:

The most severe consequence of excessive groundwater pumping is that the water table, below which the ground is saturated with water, can be lowered. For water to be withdrawn from the ground, water must be pumped from a well that reaches below the water table. If groundwater levels decline too far, then the well owner might have to deepen the well, drill a new well, or, at least, attempt to lower the pump. Also, as water levels decline, the rate of water the well can yield may decline.

There is more of an interaction between the water in lakes and rivers and groundwater than most people think. Some, and often a great deal, of the water flowing in rivers comes from seepage of groundwater into the streambed. Groundwater contributes to streams in most physiographic and climatic settings. The proportion of stream water that comes from groundwater inflow varies according to a region's geography, geology, and climate.

Groundwater pumping can alter how water moves between an aquifer and a stream, lake, or wetland by either intercepting groundwater flow that discharges into the surface-water body under natural conditions, or by increasing the rate of water movement from the surface-water body into an aquifer. A related effect of groundwater pumping is the lowering of groundwater levels below the depth that streamside or wetland vegetation needs to survive. The overall effect is a loss of riparian vegetation and wildlife habitat.

The basic cause of land subsidence is a loss of support below ground. In other words, sometimes when water is taken out of the soil, the soil collapses, compacts, and drops. This depends on several factors, such as the type of soil and rock below the surface. Land subsidence is most often caused by human activities, mainly from the removal of subsurface water.

As the depth of water increases, the water must be lifted higher to reach the land surface. If pumps are used to lift the water (as opposed to artesian wells), more energy is required to drive the pump. Using the well can become prohibitively expensive.

5.7 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 5.22 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 5.23

The fig 5.23 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 5.24 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 5.25 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 5.1

5.8 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 5.26 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 5.27 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.

5.9 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 5.28

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 5.29

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 5.30Seismic ray paths (perpendiculars to seismic wavefronts) in the Earth

5.10 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.

5.11 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 5.31 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.

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