geotechnical engineering 1

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Prepared by R.Vijayakumar, B.Tech (CIVIL), CCET, Puducherry

GEOTECHNICAL ENGINEERING – 1

UNIT – 1

1. Define porosity.

Soils usually consist of particles, water and air. The porosity ‘n’ of a

given soil sample is the ratio of the volume of voids to the total volume of

the given soil mass.

𝑛 = 𝑉𝑣

𝑉

2. Define density index.

It is defined as the ratio of the difference between the void ratio of the

soil in its loosest state and its natural void ratio to the difference between the

void ratios in the loosest and densest state. It is also called relative density

or degree of density.

𝐼𝐷 = 𝑒𝑚𝑎𝑥 − 𝑒

𝑒𝑚𝑎𝑥 − 𝑒𝑚𝑖𝑛

Where,

e = Natural void ratio of the deposit

emax = Void ratio in the loosest state

emin = Void ratio in the densest state

3. What are the various types of transported soil?

Aeolian deposit

Alluvial deposit

Glacial deposit

Lacustrine deposit

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Marine deposit

4. What is absolute specific gravity?

All the internal voids of soil particles (permeable and impermeable)

are excluded for determining the true volume of voids, the specific gravity

obtained is called absolute or true specific gravity.

5. What is dry mass density?

The dry mass density is the mass of solids per unit of total volume

(prior to drying) of the soil mass.

𝜌𝑑 = 𝑀𝑑

𝑉

6. What is specific gravity?

The specific gravity is defined as the ratio of the weight of a given

volume of soil solids at a given temperature to the weight of an equal

volume of distilled water at that same temperature.

𝐺 = 𝛾𝑠

𝛾𝑤

7. How do you classify the soil?

Residual soil

Transported soil

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8. Distinguish between porosity and void ratio.

S. No POROSITY VOID RATIO

1.

2.

Soils usually consist of

particles, water and air. The

porosity ‘n’ of a given soil

sample is the ratio of the

volume of voids to the total

volume of the given soil mass.

𝑛 = 𝑉𝑣

𝑉

It is defined as the ratio of volume

of voids to the volume of soil

solids in the given soil mass.

𝑒 = 𝑉𝑣

𝑉𝑠

9. Sketch the phase diagram for a soil and indicate the volume and weights

of the phase on it.

10. What is saturated density of soil?

When soil mass is saturated its bulk density is called saturated density.

It is defined as the ratio of soil mass of saturated sample to its total volume.

It is denoted by the symbol ρsat.

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𝜌𝑠𝑎𝑡 = 𝜌𝑤 ( 𝐺 + 𝑒 )

1 + 𝑒

11. Define soil mechanics.

Soil mechanics is defined as the application of the laws and principles

of mechanics and hydraulics to engineering problems dealing with soil as an

engineering material.

12. Define the term residual soil.

Residual soils are those which have remained over the parent rock

from which they have been formed. They are relatively shallow in depth.

They are characterized by a gradual transition from soil through partially

weathered rocks, fractured and fissured rock to bedrock.

13. How do you distinguish between clay and silt?

S. No CLAY SILT

1.

2.

3.

4.

It is an aggregate of mineral

particles of microscopic and sub-

microscopic range.

The soil may be organic or

inorganic.

Organic clays are more

compressible because of the

presence of finely divided organic

matter.

Inorganic clays are generally more

plastic than the organic clays.

Particles become dusty when

dry and are easily brushed off

hands.

The soil may be inorganic.

Potentially large expansion

upon wetting.

Difficult to compaction.

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14. What are the different types of soil structures?

Following are the types of soil structures which have been recognized

in various soil deposits.

Coarse grained skeleton structure

Cohesive matrix structure

Dispersed structure

Flocculent structure

Honeycomb structure

Single grained structure

15. What are the minerals present in clay soil?

Attapulgite

Calcium smectite

Chlorite

Illite

Kaolinite

Smectite

Sodium smectite

Vermiculite

16. Define submerged unit weight.

Where the in-situ soil is saturated with water, the particles are

subjected to an all-round thrust. The submerged unit weight is given by

difference in the unit weight of a saturated soil and the unit weight of water.

The submerged unit weight 𝛾′ is the submerged weight of soil solids

( 𝑊𝑑 )𝑠𝑢𝑏 per unit of total volume (V) of the soil mass. It is also called

buoyant unit weight.

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𝛾′ = ( 𝑊𝑑 )𝑠𝑢𝑏

𝑉 = 𝛾𝑠𝑎𝑡 − 𝛾𝑤

17. Define state of consistency.

The term consistency refers to the relative ease with which a soil mass

can be deformed and is used to describe the degree of firmness of fine-

grained soils for which consistency relates to a large extent to water content.

The four states of consistency suggested by Atterberg are indicated in this

figure.

18. What is degree of saturation?

In a given volume of voids of a sample, some space is occupied by

water and the rest by air. In a fully saturated sample, the voids get

completely filled with water. The degree of saturation ‘S’ is defined as the

ratio of the volume of water present in a given soil mass to the total volume

of voids in it.

𝑆 = 𝑉𝑤

𝑉𝑣

The degree of saturation is usually expressed as a percentage and is

also known as percent saturation.

S = 1 (for fully saturated sample)

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S = 0 (for perfectly dry sample)

19. Define water content.

The water content ‘w’, also called the moisture content, is defined as

the ratio of weight of water 𝑊𝑤 to the weight of solids ( 𝑊𝑠 𝑜𝑟 𝑊𝑑) in a

given mass of soil.

𝑤 = 𝑊𝑤

𝑊𝑑 × 100

20. Define plastic limit.

It is defined as the minimum water content at which a soil will just

begin to crumble when rolled into a thread approximately 3 mm in diameter.

Plastic limit is the water content corresponding to an arbitrary limit between

the plastic and the semi-solid states of consistency of a soil. It is denoted by

the symbol 𝑤𝑃.

21. Define liquid limit.

It is defined as the minimum water content at which the soil starts to

flow. Liquid limit is the water content corresponding to the arbitrary limit

between liquid and plastic state of consistency of a soil. It is denoted by the

symbol 𝑤𝐿.

22. Define shrinkage limit.

It is defined as the maximum water content at which a reduction in

water content will not cause a decrease in the volume of a soil mass. It is

lowest water content at which a soil can still be completely saturated. It is

denoted by the symbol 𝑤𝑆.

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23. Define toughness index, consistency index, liquidity index, flow index

and plasticity index.

Toughness index is defined as the ratio of the plasticity index to the

flow index. It is denoted by the symbol 𝐼𝑇.

𝐼𝑇 = 𝐼𝑃

𝐼𝑓

Consistency index or the relative consistency is defined as the ratio of

the difference between liquid limit and natural water content to the plasticity

index. It is denoted by the symbol 𝐼𝐶.

𝐼𝐶 = 𝑤𝐿 − 𝑤

𝐼𝑃

Liquidity index or the water-plasticity ratio is the ratio expressed as a

percentage, of the difference between the natural water content of a soil and

its plastic limit, to its plasticity index. It is denoted by the symbol 𝐼𝐿.

𝐼𝐿 = 𝑤 – 𝑤𝑃

𝐼𝑃

Flow index is the slope of flow curve obtained by plotting water

content as ordinate or natural scale against number of blows as abscissa on

logarithmic scale.

𝐼𝑓 = 𝑤1 − 𝑤2

log10𝑛2

𝑛1

Plasticity index is defined as the numerical difference between the

liquid limit and the plastic limit of a soil.

𝐼𝑃 = 𝑤𝐿 − 𝑤𝑃

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24. What is the use of consistency limits?

The consistency limits and related indices are very useful for soil

identification and classification. The limits are often in specification for soil

compaction and in semi empirical methods of design. The liquid limit and

plastic limit depend on both the type and amount of clay.

25. Define co-efficient of uniformity.

It gives the range of grain sizes in a given sample. CU must be greater

than 4 for gravels and 6 for sands. It is denoted by the symbol CU.

𝐶𝑈 = 𝐷60

𝐷10

26. Define co-efficient of curvature.

Coefficient of Curvature is a measure of the smoothness of the

gradation curve. It is denoted by the symbol CC. For well graded soil, CC

must be between 1 and 3.

𝐶𝑐 = ( 𝐷30 )2

𝐷10 × 𝐷60

UNIT – 2

1. What is quick sand condition?

Sand is said to be quick sand condition when the flow is upward under

a hydraulic gradient, which reduces the effective stress to zero.

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2. State the properties of flow nets.

The flow lines and equipotential potential lines meet at right angles to

one another.

Moving parallel is not possible.

The quantity of water flowing through each flow channel is the same.

Similarly, the same potential drop occurs between two successive

equipotential lines.

Smaller the dimensions of the field, greater will be hydraulic gradient

and velocity of flow through it.

In a homogeneous soil, every transition in the shape of the curves is

smooth, being either elliptical or parabolic in shape.

3. State Darcy’s law.

The law of flow of water through soil was first studied by Darcy

(1856) who demonstrated experimentally that for laminar flow conditions in

a saturated soil, the rate of flow or the discharge per unit time is proportional

to the hydraulic gradient.

𝑣 = 𝑞

𝐴= 𝑘 𝑖

Where,

q = discharge per unit time

A = total cross-sectional area of soil mass, perpendicular to the direction of

flow

i = hydraulic gradient

k = Darcy’s co-efficient of permeability

v = velocity of flow or average discharge velocity

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4. What are the various uses of flow nets?

A flow net can be utilized for the following purposes:

Determination of seepage

Determination of hydrostatic pressure

Determination of seepage pressure

Determination of exit gradient

5. Define the term Geostatic stresses.

The vertical stress in a soil to its self-weight is termed as Geostatic

stress.

6. Define effective stress.

Effective stress equals the total stress minus the pore water pressure,

or the total force in the soil grains divided by the gross cross-sectional area

over which the force acts.

7. What is permeability?

It is defined as the property of a porous material which permits the

passage or seepage of water (or other fluids) through its interconnecting

voids. A material having continuous voids is called permeable.

8. What are the methods for measurement of permeability?

The co-efficient of permeability can be determined by the following

methods:

Laboratory methods

Constant head permeability test

Falling head permeability test

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Field methods

Pumping – out tests

Pumping – in tests

Indirect methods

Computation from grain size or specific surface

Consolidation test data

Horizontal capillarity test

9. Define seepage velocity.

The actual or true velocity of water flowing through the voids is

termed as seepage velocity.

10. Define discharge velocity.

Quantity of water flowing in unit time through a unit gross cross

sectional area of soil at right angles to the direction of flow. It does not

account for flow through soil voids.

11. What is ground water table?

Ground water is that water which fills the voids in the soil up to the

ground water table and Translocate through them. It tries to fill completely

all the voids.

12. Define the term capillary tension.

The tensile stress caused in water is called the capillary tension or the

capillary potential. It is the pressure deficiency, pressure reduction or

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negative pressure in the pore water (or the pressure below atmospheric) by

which water is retained in a soil mass.

13. Define the term coefficient of permeability.

The co-efficient of permeability or simply permeability, is defined as

the average velocity of flow that will occur through the total cross-sectional

area of soil under unit hydraulic gradient. The dimensions of the co-efficient

of permeability ‘k’ are the same as those of velocity. It is usually expressed

as cm/sec.

Water can flow through the pore spaces in the soil and the soil is

considered to be a permeable. Thus the property of porous medium such as

soil, water or other fluids can flow through it. It is named as permeability or

co-efficient of permeability. The percolating power of the soil mass is

named as permeability.

14. What are the factors affecting permeability of soil?

Some of the factors, which affect permeability, are,

Adsorbed water

Degree of saturation

Grain / particle size

Impurities in water

Properties of water

Shape of particles

Structure of soil mass

Temperature

Viscosity

Void ratio

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15. List the assumptions made in the Laplace’s equation.

The following assumptions are made in the derivation of the Laplace

equation.

The flow is laminar

Water and soil are incompressible

Soil is isotropic and homogeneous

The soil is fully saturated

The flow is steady (i.e.) flow condition does not change with time

Darcy’s law is valid

16. What is laminar flow?

Flow of fluids is described as laminar, if a fluid particles flow follows

a definite path and does not cross the path of other particles.

UNIT – 3

1. What is meant by contact pressure?

Contact pressure is defined as the vertical pressure acting at the

surface of contact between the base of a footing and the underlying soil

mass. The actual contact pressure distribution, however, depends upon the

flexural rigidity of the footing and the elastic properties of the sub-grade.

2. What are the approximate methods of determination of vertical stress

under loaded areas?

Equivalent point load method

Sixty degree distribution

Two to one load distribution method

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3. State any two assumptions made in Westergaard’s analysis.

Westergaard’s (1938) analysis is used to solve the problems on the

pressure distribution in soil under point load, assuming the soil to be an

elastic medium of semi-infinite extent but containing numerous, closely

spaced, horizontal sheets of negligible thickness of an infinite rigid material

which permits only downward deformation on the mass as a whole without

allowing it to undergo any lateral strain.

4. Write down Boussinesq’s equation for stresses due to a point load.

𝜎𝑧 = 3𝑄

2𝜋𝑧2 [

1

1 + ( 𝑟

𝑧 ) 2

]

5

2

5. Define the term pressure bulb or pressure bulb concept in soils.

The zone in a loaded soil mass bounded by an isobar of given vertical

pressure intensity is called a pressure bulb.

6. Define Influence diagram.

The vertical stress distribution diagram, due to a concentrated load, at

a depth z if such a diagram is plotted for unit load (Q = 1), it is called the

influence diagram.

7. What is an Isobar or stress isobar or isobar diagram?

An isobar is a curve or contour connecting all points below the ground

surface of equal vertical pressure. An isobar is a spatial, curved surface of

the shape of a bulb, because the vertical pressure on a given horizontal plane

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is the same in all directions at points located at equal radial distances around

the axis of loading.

8. Write the assumptions involved in Boussinesq’s equation.

The following assumptions are made in the solutions by the theory of

elasticity.

The soil mass is an elastic medium, for which the modulus of

elasticity ‘E’ is constant.

The soil mass is homogeneous (i.e.) all its constituent parts or

elements are similar and it has identical properties at every point in it

in identical directions.

The soil mass is isotropic (i.e.) it has identical elastic properties in all

directions through any point of it.

The soil mass is semi-infinite (i.e.) it extends infinitely in all

directions below a level surface.

9. What is the use of Newmarks Influence chart?

A more accurate method of determining the vertical stress at any point

under a uniform loaded area of any shape is with the help of influence chart

or influence diagram originally suggested by Newmarks (1942).

10. What are the factors affecting the contact pressure distribution?

Elastic properties of the sub-grade

Flexural rigidity of the footing

Type of soil

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11. What are the pressure distribution diagrams?

The following pressure distribution diagrams can be adopted.

Stress isobar or isobar diagram

Vertical pressure distribution on a horizontal plane

Vertical pressure distribution on a vertical plane

UNIT – 4

1. State the assumptions in the derivation of Terzaghi’s consolidation

equation.

The soil mass is homogeneous.

The void spaces are completely filled with water.

The soil solids and water are incompressible.

Darcy’s law is valid.

The seepage flow and deformation are in one-dimensional direction.

Strains are small.

The permeability is constant over the range of effective stresses.

There is a unique pressure void ratio relationship (i.e.) the co-efficient

of compressibility is constant.

The time lap in consolidation is entirely due to low permeability of

soil.

2. Define compressibility.

When a compressive load is applied to soil mass, a decrease in its

volume takes place. The decrease in the volume of soil mass under stress is

known as compression and the property of soil mass pertaining to its

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susceptibility to decrease in volume under pressure is known as

compressibility.

3. Define coefficient of compressibility.

The co-efficient of compressibility is defined as the decrease in void

ratio per unit increase of pressure.

4. What do you mean by optimum moisture content?

The dry density goes on increasing as the water content is increased,

till maximum density is reached. The water content corresponding to the

maximum density is called the optimum moisture content w0.

5. Define hydrodynamic lag.

The delay caused in consolidation by the slow drainage of water out

of a saturated soil mass is called hydrodynamic lag.

6. What is Frost heave?

Water migrates upward from the water table to the capillary fringe.

When the atmospheric temperature falls to the freezing point and the ice is

formed. This results in an increase in the volume of the soil. This is known

as frost heave.

7. List the various factors influencing compaction of soil.

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

Addition of admixtures

Amount and type of compaction

Method of compaction

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Type of soil

Water content

8. What is consolidation?

In a saturated soil mass having its voids filled with incompressible

water, decrease in volume or compression can take place when water is

expelled out from the voids. Such a compression resulting from a long term

static load and the consequent escape of pore water is termed as

consolidation.

9. Define the term relative compaction.

Degree of compaction is also sometimes expressed in terms of an

index called relative compaction (Rc) defined as the ratio of dry density to

the maximum dry density from compaction test.

𝑅𝑐 = 𝛾𝑑

𝛾𝑑𝑚𝑎𝑥

10. State any two assumptions made in Terzaghi’s theory of one

dimensional consolidation.

Compression and flow are one-dimensional

Darcy’s law is valid

The soil grains and water are both incompressible

The soil is completely saturated

The soil is homogeneous

11. What is the use of consolidation data?

The consolidation test data are then used to determine the following:

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Co-efficient of consolidation

Co-efficient of permeability

Void ratio and co-efficient of volume change

12. Differentiate compaction and consolidation.

S. No COMPACTION CONSOLIDATION

1.

2.

3.

4.

5.

Compaction is the process by

which soil particles are packed

more closely together by

mechanical means.

It is achieved through reduction

of air voids.

It is a rapid process.

It is an artificial process.

Proper compaction of soil is

achieved at optimum moisture

content.

Consolidation is the process by

which soil particles are packed

more closely together under the

application of static loading.

It is achieved through gradual

drainage of water from soil pores.

It is a gradual process. In some

soils it takes many years.

It is a natural process.

Consolidation is strictly

applicable for saturated or nearly

saturated clays or soils with low

permeability.

13. What do you mean by Initial consolidation?

When a load is applied to a partially saturated soil, a decrease in

volume occurs due to expulsion and compression of air in the voids. A

small decrease in volume occurs due to compression of solid particles. The

reduction in volume of the soil just after the application of the load is known

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as initial consolidation or initial compression. For saturated soils, the initial

consolidation is mainly due to compression of solid particles.

14. What are the factors influenced by the compressibility of the clay?

The compressibility of clays may also be caused by three factors are,

The expulsion of double layer water from between the grains

Slipping of the particles to new positions of greater density

Bending of the particles as elastic sheets

15. Define hydrodynamic pressure.

The pressure that builds up in pore water due to load increment on the

soil is termed as excess pore pressure or excess hydrostatic pressure or

hydrodynamic pressure.

16. Define primary consolidation.

The reduction in volume of soil which is due principally to a

squeezing out of water from the voids is termed as primary consolidation or

primary compression or primary time effect.

17. Define secondary consolidation.

Even after the reduction of all excess hydrostatic pressure to zero,

some compression of soil takes place at a very slow rate. This is known as

secondary consolidation or secondary compression or secondary time effect.

18. What is immediate settlement?

The settlement which is caused by the elastic deformation of dry,

moist and saturated soils without any change in moisture content.

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UNIT – 5

1. Define the term stress path.

A stress path is a curve or a straight line which is the locus of a series

of stress points depicting the changes in stress in a test specimen or in a soil

element in-situ, during loading or unloading.

2. What are the shear parameters of a soil?

c and 𝛷 are the shear parameters of a soil

Where,

c is known as apparent cohesion

𝛷 is known as angle of internal friction.

3. Write down the Skempton’s equation for increase in pore pressure due

to increase in principal stresses.

∆𝑢 = 𝐵 [ ∆𝜎3 + 𝐴 ( ∆𝜎1 − ∆𝜎3 ) ]

4. Under what circumstances is vane shear test used?

Vane shear test is a quick test, used either in the laboratory or in the

field, to determine the undrained shear strength of cohesive soil. The vane

shear test consists of four thin steel plates, called vanes, welded orthogonally

to a steel rod.

5. Mention the advantages of Tri-axial shear test as compared to direct

shear test.

The shear tests under all the three drainage conditions can be

performed with complete control.

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Precise measurements of the pore pressure and volume change during

the test are possible.

The stress distribution on the failure plane is uniform.

The state of stress within the specimen during any stage of the test, as

well as at failure is completely determinate.

6. What are the advantages of direct shear test?

It is easy and quick test for sands and gravels.

Large deformations can be achieved by reversing shear direction. This

is useful for determining the residual strength of a soil.

Large samples may be tested in large shear boxes. Small samples

may give misleading results due to imperfections (fractures and

fissures) or the lack of them.

Samples may be sheared along predetermined planes. This is useful

when the shear strengths along fissures or other selected planes are

required.

7. What are the disadvantages of direct shear test?

Uniform deformations and stresses in the specimen.

The stress-strain behavior cannot be determined.

The estimated stresses may not be those acting on the shear plane.

There is no means of estimating pore pressures so effective stresses

cannot be determined from undrained tests.

Undrained strengths are unreliable because it is impossible to prevent

localized drainage without high shearing rates.

In practice shear box tests are used to get quick and crude estimates of

failure parameters.

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8. Name the test used to determine shear strength of soil in the field.

Cone Penetration test

Dynamic cone penetration test

Static cone penetration test

Vane shear test

9. What is shear strength equation?

The shear strength of a soil is a basic geotechnical engineering

parameter and is required for the analysis of foundations, earthwork, and

slope stability problems. This is because of the nature of soil, which is

composed of individual soil particles that slide (i.e., shear past each other)

when the soil is loaded. The Coulomb’s shear strength equation is given by,

𝑆 = 𝑐 + 𝜎 𝑡𝑎𝑛𝛷

10. Define the term principle planes.

A principal plane is defined as the plane on which the stress is wholly

normal on one. This does not carry shearing stress.

From mechanics it is known that there exist three principal planes at

any point in a stressed material. The normal stress acting on the principal

plane is known as principal stresses.

11. State Mohr’s coulomb theory.

The Mohr’s coulomb theory of securing strength of a soil, first

propounded by coulomb (1976) and later generalized by Mohr’s, is the most

commonly used concept. The functional relationship between the normal

stress on any plane and the shearing strength available on that plane was

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assumed to be linear by coulomb. Thus the following is usually known as

coulomb’s law.

𝑆 = 𝐶 + 𝜎𝑡

𝐶 = 𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑐𝑜ℎ𝑒𝑠𝑖𝑜𝑛

𝛷 = 𝑎𝑛𝑔𝑙𝑒 𝑖𝑛𝑡𝑒𝑟𝑛𝑎𝑙 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛

12. Write the four laboratory methods of determining the shearing

resistance.

The shear resistance can be determined in the laboratory by the following

four methods:

Direct shear test

Tri-axial shear test

Unconfined compression test

Vane shear test

13. What are the methods to determine shear strength?

Direct shear test

Tri-axial shear test

Unconfined compression test

Vane shear test

Consolidated undrained test

Drained test

Undrained test or quick test

14. What are the factors affecting shear strength?

Resistance due to interlocking of particle.

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Frictional resistance between the individual soil grains which may be

sliding friction, rolling friction.

Adhesion between soil particle and cohesion.

15. Classify soils based on shear strength.

On the basis of shear strength, soil can be classified as:

Cohesion less soils

Purely cohesive soils

Cohesive-frictional soils

Cohesion less soils:

These soils derive the shear strength from inter granular

friction. It has no cohesion i.e. c′ = 0. These soils are also called

frictional soil. Example: sand and gravels.

Purely cohesive soils:

These are the soils which exhibit cohesion but the angle of

shearing resistance φ = 0. These soils are also called φu = 0 soil.

Example: saturated clays and silts under undrained conditions.

Cohesive – frictional soils:

These are composite soils having both c′ and φ′. These are also

called C – φ soil. Example: clayey sand, silty sand, sandy clay etc.

16. What do you mean by C – 𝛷 soil?

These are composite soils having both c′ and φ′. These are also called

cohesive - frictional soil. Example: clayey sand, silty sand, sandy clay etc.

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17. What are the factors that affect shear strength of cohesion less soils?

Shape of particles, gradation, confining pressure, deviator stress,

vibration, repeated loading and type of minerals are the factors that affect

shear strength of cohesion less soils.

18. What are the factors that affect shear strength of cohesive soils?

Structure of clay, clay content, drainage condition, rate of strain,

repeated loading, confining pressure, plasticity index, disturbance are the

factors that affect shear strength of cohesive soils.

19. What are the different types of failure of a tri-axial compression test

specimen?

Brittle failure

Plastic failure

Semi plastic failure

20. What is Mohr’s circle? What are the characteristics of Mohr’s circle?

The co-ordinates of points on the circle represent the normal and

shearing stress on inclined planes at a given point. This circle is known as

Mohr’s circle of stress (Mohr, 1870). The graphical method for the

determination of stresses on a plane inclined to the principal stress is called

Mohr’s wide.

The characteristics are,

The maximum angle of obliquity β max is obtained by drawing a

tangent to the circle from the origin O.

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Shear stresses plane at right angle to each other are numerically equal

but are of opposite signs.

21. What do you know about undrained and drained test?

In the undrained test, no drainage of water is permitted. Hence there

is no dissipation of pore pressure during the entire test. In the drained test,

drainage is permitted throughout the test during the application of both

normal and shear stress.

22. Define shear strength of soil.

The shear strength of soil is the resistance to deformation by

continuous shear displacement of soil particles or on masses upon the action

of a shear stress. The failure conditions for a soil may be expressed in terms

of limiting shear stress, called shear strength or as a function of the principal

stresses.

23. What are the limitations of coulomb’s theory?

It neglects the effect of the intermediate principal stress.

It approximates the curved failure envelope by a straight line which

may not give correct results.

24. What is the main cause of slope failure?

Slope failures occur when the rupturing force exceeds resisting force.

geotechnical engineering 1

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