undefined | unit 6 geological hazards groundwater and building stones

E GEO

Unit – 6

Geological Hazards, Groundwater and Building Stones

Q1. What is earthquake and causes of it?

A1. The sudden release of stored energy due to sudden vibrations of earth’s crust that creates seismic wave is termed as earthquakes. Earthquakes are measured using observations from seismographs. Earthquake may occur naturally as a result of human activities. The magnitude of an earthquake and intensity of earthquake is reported on Richter scale.

Causes of earthquakes

1) Tectonic plates movement.

2) Induced causes such as tunnel construction and implementing geothermal.

3) Volcanic causes or volcanism.

4) Collapse quakes such as phenomenon of cave-ins.

During earthquake waves are generated at focus these waves are called as sea waves and it travel in all directions with their desired velocities.

Always obey law of reflection and refraction because as they pass through Earth materials of varying density.

1) P-waves (Primary waves)

P waves are compressions pushes in nature and behave like sound wave.

Velocity = 5-7 km/s in earth crust.

= >8 km/s in earth mantle and core

= 1.5 km/s in water.

P wave is the first arriving energy on a seismogram.

Smaller and higher frequency than ∂ and surface Wave.

P motion or wave travel fastest in material.

It is capable of passing through solid, liquid as well as gases.

2) S- waves (Secondary waves)

S waves are transverse or distortional like those of light waves.

Waves travel in a direction at right angles to direction of propagation of wave.

Velocity= 3-4 km/s in earth crust.

>4.5 km/s in earth mantle.

2.5-3.0 km/s in inner core.

S waves do not travel through fluids, therefore does not exist in earth liquid outer core or molten rock.

S waves are slower than P waves in solid, hence arrive after P- waves.

3) L Waves (Long waves)

It travels along or parallel to the Earth surface. This wave is also called as love surface wave.

Velocity= 2.0-4.5 km/s in earth depending on frequency of propagating waves.

The behaviour of these waves is similar to that of sea waves and hence involves both vertical and horizontal motion.

Their largest at surface and decrease in amplitude with depth.

These waves are main waves which causes all the damage on earth surface.

Description Richter Magnitude Earthquake effect

Micro less than 2 not felt

Very minor 2.0-2.9 only recorded.

Minor 3.0-3.9 rare causes

Light 4.0-4.9 sharing of items.

Moderate 5.0-5.9 major damage

Strong 6.0-6.9 area of = 100 miles can be damaged.

Major 7.0-7.9 serious damage

Great 8.0-8.9 damage up to 100 miles across.

Rarely >9.0 1000 miles destruction.

Q2. Explain seismic zone of India?

A2.

Seismic zone map is required so that building and other structures located in different region can be designed.

Seismic zone in India is divided into four zones based on sea waves and intensity.

Previously there were 5 zones but Bureau of Indian Standards (IS 1893 Part 1:2002) has grouped the country into four zones.

The areas falling in seismic zone 1 in current map are merged with seismic zone 2.

Zone II low damage risk zone- area may * These 2 zones are covering part of KA,

have intensity of MSK VI. Andhra, Orissa, MP and Rajasthan.

*It is also known as low risk earthquake zone.

Zone III moderate damage risk zone * North including some parts of Rajasthan to

Area may expect maximum south through Konkan coast and also eastern

Intensity of MSK VII. Part.

Zone IV High damage risk zone- areas * Part of Northern belt starting from J & K to may expect maximum intensity Himachal Pradesh, also Delhi and parts of

of MSK VIII. Haryana. Kayna region is also there.

Zone V very high damage risk zone Entire north east, including all 7 sister states,

May expect maximum intensity parts of Himachal and J & K and Andaman and

of MSK IX or more. Nicobar islands.

Q3. Explain landslide and stability of hill slope and preventive measures?

A3. Landslide

The movement downslope of a mass of Rock, soil, debris is termed as landslide.

Landslides represent most extreme hazards, especially in terms of loss of life.

The factor affecting the landslide can be of various ways by both natural and human activities.

Factors which cause landslide-

i) Geology part of land such as structure, lithology and tectonic activity.

ii) Rainfall

iii) Land use – mining, forest, quarrying.

iv) Geomorphology - nature of slope.

Types of landslide are:

1) Slide

2) Flow

3) Creep

4) Slump

5) Torrent

6) Fall

7) Topple

Prevention of landslide:

1) Drainage control:

Impermeable layer.

Keep water from infiltrating slope.

2) Slope supports:

Retaining wall

vegetation

3) Grading can increase slope stability:

Material from upper slope can be moved to base.

4) Rock fall protection

5) Diverting debris pathway.

Q4. What is groundwater and its types?

A4. Groundwater

It is the water present below earth surface in pores of soil particle or in fractures of rocks.

The depth at which soil pore spaces and voids in rock becomes fully saturated is called as water table.

Uses:

It is used for agricultural, municipal and industrial use by constructing and operating extraction well.

Groundwater is mainly found in aquifers. There are two types of aquifer-

1) Unconfined

2) Confined.

Types of aquifer

1) Unconfined aquifers:

Water table can easily available upto land surface.

Water is collected or accumulates over impervious surface.

2) Confined aquifer:

In this, aquifer is sandwiched between two layers of impervious bed.

Water present in these aquifer is at under pressure.

Wells are drilled through upper confining layer.

Then water will rise in the well as it is under pressure or pressurized.

Wells are called as artesian well.

The level of water is termed as piczometric water level.

Q5. What is the hydrogeological properties of rocks?

A5.

Porosity and Hydraulic conductivity are the properties of hydrologic of rock in which groundwater occurs and through which it moves.

Rock which have properties to allow storage and transmission of water are known as hydrologic properties of rocks.

Specific yield is a part of total porosity of rock or we can say it indicates specific quantity of water released by saturated rock under the action of gravity.

Gravity force drives water out from pores of rocks.

But all the water is not removed or drive out from gravity force, some retained as specific retention.

Specific retention is part of total porosity showing the quantity of water retained by rock after gravity has taken out the water or gravity drainage is done.

Molecular force and capillarity are responsible for holding the water against Gravity.

Porosity is defined as property of material that contains voids or space.

Voids are obtained by following-

i) Lava tubes

ii) joint and fractures

iii) Vesicles in basalts

iv) Between grains- inter granular

v) Within grains- intra granular

vi) Solution channel from where Rock material is removed.

Porosity = specific yield + specific retention

Porosity is part of volume of Rock not occupied by Rock materials.

Q6. What are the types of aquifer and groundwater?

A6. Types of aquifer

Unconfined aquifers:

Water table can easily available upto land surface.

Water is collected or accumulates over impervious surface.

2. Confined aquifer:

In this, aquifer is sandwiched between two layers of impervious bed.

Water present in these aquifer is at under pressure.

Wells are drilled through upper confining layer.

Then water will rise in the well as it is under pressure or pressurized.

Wells are called as artesian well.

The level of water is termed as piczometric water level.

Types of groundwater

River

Snow

Glacier

Aquifer

Natural spring

Lakes

Rain, etc.

Q7. Write short note on water table depth zones?

A7. Water table is the table at which soils are saturated with water at the upper level of an underground surface.

Subsurface water –

Water at soil mantle is subsurface water.

Water under surface is mainly divided into two zones –

a) The unsaturated zone (Zone of aeration) which includes soil water zone.

b) Zone of saturation which includes groundwater.

Water table divides the zone of aeration from zone of saturation.

Saturated zone- pores of soil are filled with water.

Aeration zone- soil pores are partially saturated with water.

The aeration zone is divided into three parts-

a) Soil water zone

b) Intermediate zone

c) Capillary fringe

1) Soil water zone

Soil water is present between particles of soil.

This water can be removed by air drying or by plant absorption but it is not removed from gravity.

Field capacity-It is the safest maximum water content of soil, generally lies between 28 – 35℅ for most of soil. We should never supply water to the soil beyond its field capacity.

Optimum moisture content-It is the safest minimum water content of soil for healthy growth of crop.

Temporary wilting pt.-First danger signal indicating water deficiency of soil.

Permanent wilting pt,-It is the minimum moisture content of soil.If we supply external water plant, it will survive. If we don’t, plant will die instantaneously.

 By means of any magnitude of external pressure, plant cannot suck the water below P.W.P Therefore water storage capacity of soil.

Water storage capacity=FC - P.W.P

 It is also called Available moisture.

 FC to OMC is called as Readily Available moisture.

RAM = FC – OMC {RAM= (60-70%) of AM}

Example: 1) For loamy soil, FC = 28%, OMC =15%, PWP =8%

AM = FC -PWP = 8℅

RAM = FC -OMC = 13℅

2) For clayey soil, FC=25℅, PWP= 5℅, OMC=12℅

AM = FC – PWP=20℅

RAM =FC – OMC = 13℅

RAM =65℅AM

 Plant extract this water for growth until soil capillary force is equal to extractive force of plant root.

2) Intermediate zone

It is grown in between soil water zone and capillary zone.

3) Capillaryzone

Capillary fringe is subsurface layer in which groundwater seeps up from water table by capillary action.

Saturated zone is classified into 4 groups-

i) Aquifer

ii) Aquiclude

iii) Aquifuge

iv) Aquitard

Aquifer- It is porous layer that contains and transmit groundwater. Aquifer is present at various depth.

Types of aquifer– (a) Confined aquifer (b) unconfined aquifer

II. Aquiclude- It is impermeable layer underlying or overlying an aquifer. It absorbs water but cannot transmit it in significant amount.

III. Aquifuge- It is impermeable body rock that contains no interconnected opening. Hence, neither absorb nor transmit water.

IV. Aquitard-Low permeability adjacent to aquifer or serves as storage unit for groundwater.

Q8. Explain cone of depression and circle of infulnce?

A8. Radius of influence

It is defined as maximum distance at which drawdown can be detected with usual measure device in field.

Area of the base of this cone is also defined as area of influence and boundary of area of influence is termed as circle of influence.

Drawdown is the distance from original water table to water level in well.

Cone of depression or cone of exhaustion

When water is pumped out from well, level of water goes down decreasing or leading to depression in water table around the well in form of inverted cone. This phenomenon is called as cone of depression.

This is a temporary fluctuation because due to sea page of groundwater from sides of well it again (i.e. well)gets restored.

If soil or bed or side of well is highly permeable, then cone of depression is nearly flat, while loop permeable leads to very steep. The distance between well and groundwater divide is termed as radius of influence.

Q9. What are the artificial recharge method of groundwater?

A9. Artificial Recharge Techniques

1) Direct surface techniques

Flooding

Basins or percolation tank.

Ditch and furrow system.

2) Direct subsurface techniques

Recharge well

Dug well recharge

borehole flooding

natural opening

recharge pits and shaft

3) Combination surface

Subsurface techniques

Basin tank with pit, shaft or wells.

4) Indirect techniques

Induced recharge from surface water source.

Aquifer modification.

Ditch and furrow method

This technique requires less soil preparation than recharge basin.

It is provided where land is with irregular topography, shallow, flat bottomed and closely spaced ditches or furrow provide maximum water contact area for recharge water from source stream or canal.

Percolation Tank

It is most common method used in India to recharge groundwater both in Alluvial as well as hard rocks.

Tanks are constructed in basaltic Lava flow and crystalline rock.

States like Gujarat, Karnataka, Andhra Pradesh, Madhya Pradesh, Maharashtra, use this types of tank only.

For speedy recharge, it is constructed in fractured and weathered rock.

Cut of trench is provided to minimise seepage losses.

Q10. What are the requirement of good building stones?

A10. Following are the quality requirements of good building stones:-

1) Durability: Building stones should be able to resist the adverse effect of forces such as wind, rain and heat. The stone should not be deteriorate due to adverse effect and hence must be durable.

2) Hardness:

 Hardness of stone is determined by Mohs scale.

 Stones are subjected to wearing and abrasion action caused by movement of machine or men repeatedly, so it requires stone which is hardened strong to resist all this action.

3) Strength-Most of the building stones have high strength to resist load coming from superstructure or substructure. Hence, it is the first step to check the quality of stone. Compressive strength of building stone generally falls in the range of 60 to 200 N / .

4) Specific gravity-

 For more heavier and strongest stone, specific gravity is also to be high.

 For good building stone, the specific gravity is in between 2.4 and 2.8

5) Toughness-

 Toughness is defined as sudden load or impact load

 Stone should be tough enough to resist all this above load. For road constructions, stone aggregate which is tough is used.

6) Appearance-

 In building stone, appearance is a primary requirement, its colour and ability to receive polish is an important factor.

 Light colour stones are more used and preferred over dark colour stones.

7) Fire resistance-

 Stones should be free from fire or fireproof if used in building stones.

 It should be free from calcium carbonate, minerals and oxides of iron. All these have different coefficient of thermal expansion.

 Limestone can with stand temperature upto 800°C

8) Porosity and absorption-

 Building stone should not be porous otherwise it will cause a major problem.

 Water absorption is directly proportional to porosity.

Types of stones Max. limit of water absorption

Sandstone 10

Limestone 10

Shale 10

Quartzite 3

Granite 1

Slate 1

9) Dressing-

 Giving proper and required shape is called as dressing in stone. Care should be taken so that dressing will not cost of required strength.

10) Fracture-

 For good building stone, its fracture should be

Sharp

Even

Bright and

Clear

With grains well cemented together.

 Dull and chalky fracture give or indicate early future decay.

11) Texture-

A good building stone should have cavity free crystalline structure which are fully compacted. Stones with this type of texture are strong and durable (cavities= loose or soft material).

12) Mineral-

 Minerals like(Calcium carbonate), clay, quartz should be present in a good building stone.

Dolomite, quartz and all are cementing agents and used for proper formation of stone

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