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Revision of Soil Mechanics27-02-2019

Origin of Soil and soil water relationships

• Soil is composed of particles found from the disintegration of rocks.

• Formation of Soil takes place by two methods: 1. Physical Weathering2. Chemical Weathering

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Soil Formation

Physical Weathering

Erosion of rock by Wind , water glaciers, alternate freezing and

defreezing

Soil formed retain minerals and composition

Exp- sand and gravel

Chemical Weathering

Chemical actions of acids and alkalies in

water, air, glaciers, etc

Mineral composition is changed

Exp- clay

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Soil Deposits• Residual Soils: Located at location of formation

• Transported Soil: Transported from parent location to a new location

• Alluvial deposits: deposited by river

• Lacustrine deposit: deposited by still water of lakes

• Marine deposit: deposited by sea Water

• Aeolian deposit: deposited by wind , example Loess

• Glacial deposit: deposited by glaciers , example drift, till

• Colluvial deposit: transported by Gravity expect Talus

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Some important Soils• Bentonite clay:

– Has high percentage of montmorillonite

– Highly plastic, high swelling and shrinkage

– Formed due to volcanic ash, used as drilling mud

• Black cotton soil: – Contains high percentage of montmorillonite

– Has high swelling and shrinkage potential

– Has very low bearing capacity

– Formed from chemical weathering of basalt

• Loam: mixture of sand silt and clay, known as Garden soil

• Indurated clay: hardening of clay due to heat and pressure

• Organic clays: soil gets mixed with decomposed vegetation and dead and decayed matter– Muck: inorganic + organic matter

– Peat: fully decomposed organic matter

– Humus: Top soil, it contains partly decomposed organic material

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Soil Water Relationships

Phase System

Three phase Two Phase

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• Porosity (n) is the ratio of the volume of voids to the total

volume of soil (V )

• Degree of saturation (S) The volume of water (Vw) in a soil can vary between zero (i.e. a dry soil) and the volume of voids. This can be expressed as the degree of saturation (S) in percentage.

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Soil Water Relationships• Bulk Unit Weight (ɣt): It is total weight by total volume of soil

• Dry Unit Weight of soil (ɣd): It is weight of solids divided by total volume of soil

• Saturated Unit Weight (ɣsat) of soil:

• Submerged unit Weight of soil (ɣsub) :

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Soil Water Relationships• Unit Weight of Solids(ɣs): It is weight of solids per unit volume of

solids

• Unit weight of water(ɣw): It is weight of water per unit volume of water

• True Specific Gravity (Gs):

• Mass Specific Gravity (Gm):

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Relation between different terms

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

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Methods of determination of Water content (w)

1. Oven Drying method

2. Pycnometer Method

3. Sand Bath method

4. Calcium Carbide Method

5. Torsion Balance Method

6. Radiation Method

7. Alcohol Method

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2. Pycnometer Method

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4. Calcium Carbide Method•Water in soil reacts with

calcium carbide and forms acetylene

•Pressure is co related with w/c

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5. Torsion Balance Method

– Lab method

– Used for soil which quickly absorbs moisture

– Equipment is costly

6. Radiation Method•Radioactive isotope

Cobalt 60 is used

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Methods of determination of Unit Weight (ɣ):

1. Core cutter method

2. Water Displacement Method

3. Sand Replacement Method

4. Water balloon method

5. Radiation method

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1. Core cutter method

– Wt of empty cutter = W1

– Wt of Cutter+ soil= W2

– Wt of soil= W2-W1

– Vol of soil =1000cc

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2. Water Displacement Method

• Wt of sample = W1• Wt of sample + wax = W2• We need to find volume of

sample • Wax coating is done so as to put

it in water and we can find the volume of Wax+ sample

• Then using ‘G’ of wax we find volume of Wax which is then subtracted from Total water displaced

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3. Sand Replacement Method

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Index Properties of Soil

1. Soil Aggregate Properties

– Depends on soil mass as a whole

2. Soil Grain Properties

– Depends upon soil grain size, shape, etc

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Grain Size Distribution

1. Helps to find gradation and uniformity

2. Separates out soil into different fractions based on particle size

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Gra

in S

ize

Dis

trib

uti

on

Coarse Grain Sieve Analysis

Fine GrainSedimentation

Analysis

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1. Sieve Analysis

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Sedimentation Analysis (Pippette Method)

• For particles less than size 75micron (clay and silt)

• Based on Stokes’ law

• Particle size is from 0.0002mm to 0.2mm

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Consistency

• The consistency of a fine-grained soil refers to its firmness, and it varies with the water content of the soil.

• Used mainly for fine soils

• Water content influences consistency of soil

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Consistency Limits

• The three limits are known as the shrinkage limit (WS), plastic limit (WP), and liquid limit (WL) as shown. The values of these limits can be obtained from various methods

• Liquid limit (WL) - change of consistency from plastic to liquid statePlastic limit (WP) - change of consistency from brittle/crumbly to plastic state laboratory tests.

• The difference between the liquid limit and the plastic limit is known as the plasticity index (IP)

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Methods to find Liquid Limit

1. Casagrande Method– When Groove of 2mm is

filled with 25 No. of blows, then that water content is called Liquid limit

2. Cone penetrometer Method• 30 sec – 25 mm

• That water content is liquid limit

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• Consistency Index:

• Liquidity Index:

• Activity:

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• Toughness Index:

• Flow Index:

• Relative Density:

• Thixotrophy : Property by virtue of which loss of shear strength on remolding can be regained if soil is left undisturbed for sometime. The increase in shear strength is due to regain of chemical equilibrium and reorientation of water molecules.

• Sensitivity:

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Consistency

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Clay Minerals

Clay Mineral atomic structure

Silica Sheet Alumina Sheet

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Clay Minerals

• Kaolinite (1:1)– H-bonding between layers– Less swelling and shrinkage– Antidiarrheal medicine

• Montmorrillonite (2:1)– Weak vanderwaal forces– High swelling and shrinkage– Highly plastic

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• Illite(2:1)• Isomorphous substitution

• consists of the basic montmorillonite units but are bonded by secondary valence forces and potassium ions

Clay Minerals

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• Total StressWhen a load is applied to soil, it is carried by the solid grains and the water in the pores. The total vertical stress acting at a point below the ground surface is due to the weight of everything that lies above, including soil, water, and surface loading. Total stress thus increases with depth and with unit weight.

• Pore Water PressureThe pressure of water in the pores of the soil is called pore water pressure (u). The magnitude of pore water pressure depends on:

– the depth below the water table.

– the conditions of seepage flow.

Effective Stress

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• Effective Stress: The principle of effective stress was enunciated by Karl Terzaghi . This principle is valid only for saturated soils, and consists of two parts:

Effective Stress

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Effective Stress

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Quick Sand Condition

• Effective stress becomes zero

• For Cohesionless soil (c=0), and hence Shear strength becomes zero

• Contact force between grain particles becomes zero.

• Critical Hydraulic gradient:

• Factor of Safety:

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• In soils, the interconnected pores provide passage for water. A large number of such flow paths act together, and the average rate of flow is termed the coefficient of permeability, or just permeability. It is a measure of the ease that the soil provides to the flow of water through its pores.

• Darcy stated that discharge in One dimensional flow Q is proportional to hydraulic gradient and area of cross section

• Q= K i A

• v = q/A = k.i

where k = permeability of the soili = Dh/LDh = difference in total headsL = length of the soil mass

Permeability

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Permeability

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Factors affecting Permeability

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Methods to determine Permeability1. Constant Head Permeability Method

2. Variable/Falling Head Permeability Method

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Permeability of Stratified Deposits

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Seepage in 2-D

• This is the Laplace equation governing two-dimensional steady state flow

• Flow NetsGraphical form of solutions to Laplace equation for two-dimensional seepage can be presented as flow nets. Two orthogonal sets of curves form a flow net:– Equipotential lines connecting points of equal total head h– Flow lines indicating the direction of seepage down a hydraulic gradient

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• The space between two adjacent flow lines is known as a flow channel, and the figure formed on the flownet between any two adjacent flow lines and two adjacent equipotential lines is referred to as a field.

• Total seepage discharge of Flownet:

Seepage in 2-D

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Uses of Flownet

• Estimation of seepage losses from reservoirs

• Determination of uplift pressures below dams

• Checking the possibility of piping beneath dams

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Compaction

• Compaction is the application of mechanical energy to a soil so as to rearrange its particles and reduce the void ratio

• The objectives of compaction are:

– To increase soil shear strength and therefore its bearing capacity.

– To reduce subsequent settlement under working loads.

– To reduce soil permeability making it more difficult for water to flow through.

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Tests of Compaction

• Standard Proctor test:– Wt of hammer = 2.5 kg

– Height of fall= 305mm

– Volume of mould= 944cc

– 3layers->25 no. Of blows

• Modified Proctor test– Wt of hammer = 4.5 kg

– Height of fall= 457.2mm

– Volume of mould= 944cc

– 5layers->25 no. Of blows

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Tests of Compaction

• Indian Standard Light Compaction TestSoil is compacted into a 1000 cm3 mould in 3 equal layers, each layer receiving 25 blows of a 2.6 kg rammer dropped from a height of 310 mm above the soil. The compaction is repeated at various moisture contents.

Indian Standard Heavy Compaction TestIt was found that the Light Compaction Test (Standard Test) could not reproduce the densities measured in the field under heavier loading conditions, and this led to the development of the Heavy Compaction Test (Modified Test). The equipment and procedure are essentially the same as that used for the Standard Test except that the soil is compacted in 5 layers, each layer also receiving 25 blows. The same mould is also used. To provide the increased compactiveeffort, a heavier rammer of 4.9 kg and a greater drop height of 450 mm are used.

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Dry Density - Water Content Relationship

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Factors affecting Compaction

• The various factors which affect the compacted density are as follows:

(i) Moisture content

(ii) Compactive effort

(iii) Type of soil

(iv) Method of compaction

(v) Addition of admixture.

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Field Compaction Equipment

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Compression and Consolidation of Soils

• Components of Total SettlementThe total settlement of a loaded soil has three components:

– Elastic settlement/Immediate settlement

– Primary consolidation

– Secondary compression

Compressibility Characteristics

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Some Parameters related to Compression

1. Compression Index:

2. Coefficient of compressibility, av.

3. Coefficient of volume compressibility, mv

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7. Overconsolidation ratio (OCR): is defined as the ratio of the preconsolidation stress to the current effective stress.

Some Parameters related to Compression

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Time rate of Consolidation

• Terzaghi's one-dimensional consolidation equation

• During the consolidation process, the following are assumed to be constant:1. The total additional stress on the compressible soil layer is assumed to remain constant.2. The coefficient of volume compressibility (mV) of the soil is assumed to be constant.3. The coefficient of permeability (k) for vertical flow is assumed to be constant.

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• In solution of the consolidation equation, non-dimensional parameters are provided :1. Drainage path ratio (Z):

H = drainage path which is the longest path taken by the pore water to reach a permeable sub-surface layer above or below.

z= depth at any point from top of clay layer

For single drainage H= D, for double drainage H=D/2


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• In solution of the consolidation equation, non-dimensional parameters are provided :

2. Time factor


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Shear Strength of Soil

• Soils consist of individual particles that can slide and roll relative to one another. Shear strength of a soil is equal to the maximum value of shear stress that can be mobilized within a soil mass without failure taking place.

• The Mohr-Coulomb failure criterion can be written as the equation for the line that represents the failure envelope. The general equation is

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Shear Strength Tests

1. Direct Shear TestThe test is carried out on a soil sample confined in a metal box of square cross-section which is split horizontally at mid-height

The soil is sheared along a predetermined plane by moving the top half of the box relative to the bottom half. The box is usually square in plan of size 60 mm x 60 mm.

Tests on sands and gravels can be performed quickly

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Shear Strength Tests

1. Direct Shear Test

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Shear Strength Tests2. Triaxial Test

The triaxial compression test consists of two stages:First stage: In this, a soil sample is set in the triaxial cell and confining pressure is then applied.Second stage: In this, additional axial stress (also called deviator stress) is applied which induces shear stresses in the sample. The axial stress is continuously increased until the sample fails.

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The triaxial test has many advantages over the direct shear test:• The soil samples are subjected to uniform stresses and strains.

• Different combinations of confining and axial stresses can be applied.

• Drained and undrained tests can be carried out.

• Pore water pressures can be measured in undrained tests.

• The complete stress-strain behaviour can be determined

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Shear Strength Tests3. Vane Shear TestUsed in plastic cohesive clays where obtaining undisturbed sample is difficult

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Vertical Stresses

• As per Boussinesq Equation:

• As per westergaards solution

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Soil Exploration

• Field and lab studies that obtain necessary information about soil characteristics including position of ground water table is called Soil Exploration.

• The investigation is performed in the following phases:

– 1. Preliminary exploration 2. Detailed exploration 3. Special exploration

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Methods of making Boreholes

1. Auger boring

1. Most effective in clay soil

2. Saturated sand, silt, medium to stiff clay

3. Usually performed for small depths exp highway, shallow foundation,etc

2. Wash boring1. All types of soil except hard rock2. Not suitable for taking good quality

undisturbed samples

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3. Percussion boring

1. Best suitable for boulder and gravel strata

2. Used for all types of rocks

3. Difficult in soft sticky clays

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4. Rotatory boring

1. All soils except rocks

2. Suitable for soils resistant to auger and wash boring

3. Diamond bits are used

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SAMPLER

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Soil Exploration

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Expansive Soils• The soils that have tendency to increase in

volume on addition of water and decrease volume on its removal are known as expansive soils

• Expansive soil causes a lot of problems to structures constructed on them

• Tests on Expansive Soil:

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Expansive Soils

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Foundation

General requirements of Foundation: Should not settle in excess to permissible value Safe against Shear Should not get affected by seasonal water table

fluctuation

Foundation

Foundation

Shallow

Df/Bf <= 1

Deep

Df/Bf > 15

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Shallow Foundation

• Isolated (Spread Footing)– Supports only one

column

• Strap Footing– Helps to join two

footings

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• Strip/Continuous Footing– L>>>B

• Combined Footing– Footing supports more

than one column

• Raft/ Mat Foundation– Single slab supports all

columns

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Shallow Foundation

• Gross Pressure Intensity:

• Net Pressure Intensity: When Backfill is provided

When no backfill is provided

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Bearing Capacity

• Ultimate Bearing Intensity:

• Net Ultimate Pressure Intensity:

• Gross Safe Bearing Capacity

• Net Safe Bearing Capacity

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Bearing Capacity

Note: No factor of safety is considered while dealing with

settlement criteria No FOS is considered while dealing with unit wt of soil

• Safe Bearing Pressure:

– Maximum intensity of loading that can be allowed on soil without settlement exceeding the permissible value

• Allowable bearing Capacity/pressure:

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Bearing Capacity

Bearing Capacity

Bearing Capacity

Analytical Methods

General Shear failure Φ > 36®

RD> 70 %

Local Shear Failure Φ < 29®

35<RD<70 %

Punching Shear Failure Φ < 29®

RD<35 %

Field Test Methods

Standard Penetration Test

Plate load test

Static Cone Penetration test

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Terzaghi Bearing capacity in GSF

Note: Terzaghi assumed GSF, so in case of Normal Shear Failure1. cm = 2/3 c tan(φm) = 2/3 tan(φ)2. If φ=0, As Per Terzaghi

– Nc = 5.7– Nq = 1– Nɣ = 0

3. For purely cohesive soil, net ultimate capacity is dependent only on cohesion

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2. If φ=0, As Per Pradatl

• Nc = 5.14

• Nq = 1

• Nɣ = 0

Skempton Method

• Skempton Bearing capacity analysis is for clay soil that is saturated (φ=0)

• Nc = Skempton Bearing capacity Factor which depends upon Df/Bf ratio

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Bearing Capacity Based on Field Test Data

Standard Penetration Test

Plate load test

Static Cone Penetration Test

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Bearing Capacity Based on Field Test

Data

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1. Standard Penetration Test (SPT)

Penetration value N is calculated

Suitable for granular soils Split spoon sampler is

used to make borehole Load of 65 kg and having

free fall 75cm is used N value is found out and

corrected


Data

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2. Plate Load Test Plates of size 30, 45,

65, 75 cm are used Plate is placed at

proposed level of foundation and increment loading is done

For each increment settlement is noted

Short duration test hence can not be done for clays

Settlement =>


Data

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3. Static Cone Penetration Test (SCPT)

Difference between SPT and SCPT is that we obtain continuous readings in SCPT and in SPT, we get a discontinuous record

Cone is pushed into the soil at the rate of 20mm/sec and upto a depth of 100mm

Used for soft clay, silt, fine to medium sand

EARTH PRESSURE

• Lateral force exerted by soil on any structure retaining that soil

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Earth Pressure

Earth Pressure at Rest

Active Earth Pressure

Passive Earth Pressure

Coefficient of Earth Pressure• Earth Pressure at

Rest

• Active Earth Pressure

• Passive Earth Pressure

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• Failure plane makes an angle 45+φ/2 with horizontal plane in case of Active Earth Pressure

• Failure plane makes an angle 45-φ/2 with horizontal plane in case of Passive Earth Pressure

Earth Pressure For Cohesive Soil

1. When tension cracks are not developed

2. When Tension Cracks are developed

Critical Height for Unsupported Vertical cutoff:

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