geotechnical engineering 2

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

GEOTECHNICAL ENGINEERING – 2

UNIT – 1

1. What is soil exploration?

The field and laboratory studies carried out for obtaining the

necessary information about the surface and sub-surface features of the

proposed area including the position of the ground water table, are termed as

soil exploration or site investigation.

2. What is the object of soil exploration?

Determination of the nature of the deposits of soil,

Determination of the depth and thickness of the various soil strata and

their extent in the horizontal direction,

The location of groundwater and fluctuations in GWT,

Obtaining soil and rock samples from the various strata,

The determination of the engineering properties of the soil and rock

strata that affect the performance of the structure, and

Determination of the in-situ properties by performing field tests.

3. State the need of soil exploration.

To determine the basic properties of soil which affect the design and

safety of structure i.e., compressibility, strength and hydrological

conditions.

To determine the extent and properties of the material to be used for

construction.

To determine the condition of groundwater.

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To analyze the causes of failure of existing works.

4. What are the uses of soil exploration?

To select type and depth of foundation for a given structure

To determine the bearing capacity of the soil of the selected

foundation

To investigate the safety of the existing structure

To establish ground water level

5. What are the objectives of site exploration?

The object of site exploration is to provide reliable, specific and

detailed information about the soil and ground water conditions of the site

which may be required for a safe and economic design and execution of the

engineering work.

6. What is the equivalent penetration resistance given by Terzaghi’s and

pech?

𝑁𝑒 = 15 + 1

2 ( 𝑁 − 15 )

Where,

Ne = corrected value for overburden effect

N = actual value of blows

7. Define chunk sample.

Hand carved samples can be obtained if the soil is exposed, as in a test

pit, shaft or tunnel. Hand carved samples is also known as chunk samples.

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8. Define inside, outside clearance in sampling tube.

The inside clearance allows the elastic expansion of the sample when

it enters the sampling tube. It helps in reducing the frictional drag on the

sample, and also helps to retain the core. For an undisturbed sample, the

inside clearance should be between 0.5 to 3%.

𝐶𝑖 = 𝐷3 − 𝐷1

𝐷1 × 100

Outside clearance facilitates the withdrawal of the sample from the

ground. For reducing the driving force, the outside clearance should be as

small as possible. Normally it lies between 0 to 2%. Co should not be more

than Ci.

𝐶𝑜 = 𝐷2 − 𝐷4

𝐷4 × 100

9. What are the guidelines in terms of inside clearance and outside

clearance for obtaining undisturbed sample?

For an undisturbed sample, the inside clearance should be 0.5 to 3%

The wall of the sample should be smooth and should be kept properly

oiled

The outside clearance should be between 0 to 2%

10. List the various parameters affecting the sampling disturbance.

Area ratio

Inside clearance

Outside clearance

Inside wall friction

Design of non-return valve

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Method of applying force

11. How will you reduce the area ratio of a sampler?

Generally samples will area ratio less than 15% are said to be

undisturbed. So area ratio is reduced by reducing the thickness of the cutting

shoe.

12. When thin walled sampler is used for sampling?

Thin walled sampler is used for those soils which are very sensitive to

disturbances such as soft clays and plastic silts.

13. Name the samplers used to collect undisturbed soil samples.

Open drive sampler

Piston sampler

14. List the different types of samplers.

Open drive sampler

Denison sampler

Shelby tube and Thin walled sampler

Split spoon sampler

Scraper bucket sampler

Piston sampler

15. What is significant depth?

Exploration in general should be carried out to a depth up to which the

increase in pressure due to structural loading is likely to cause perceptible

settlement or shear failure. Such a depth is known as significant depth.

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16. What is sub – surface profile?

A Sub surface profile is a vertical section through the ground along

the line of exploration which indicates the boundaries of different strata,

along with their classification.

17. What are the methods available for site explorations?

Open excavations

Test pits

Trenches

Borings

Auger boring

Auger and shell boring

Wash boring

Percussion boring

Rotary boring

Sub-surface soundings

Geophysical methods

Seismic refraction method

Electrical resistivity method

18. List the various methods of soil exploration techniques.

Direct methods

Test pits, trial pits or trenches

Semi-direct methods

Borings

Indirect methods

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Soundings or penetration tests and geophysical methods

19. What are the methods of boring available for site exploration?

Auger boring

Auger and shell boring

Wash boring

Percussion boring

Rotary boring

20. Define CPT.

The cone Penetrometer is a device by which a 60o cone with a base

area of 10 cm2 is pushed into the soil, and the cone end resistance qc, to

penetration is measured. Most cone Penetrometers that are used commonly

have friction sleeves that follow the point. This allows independent

determination of the cone resistance (qc) and the frictional resistance (fc) of

the soil above it. The friction sleeves have an exposed surface area of about

150 cm2.

21. Write the advantages of SCPT over SPT.

There is no need of hammering action, just pushing into the ground

No need of bore holes, it is carried out on the ground

Engineering properties of the soil like permeability, shear strength,

compressibility can be evaluated.

22. What are the uses of standard penetration test?

The ultimate bearing capacity of cohesion less soil is determined from

the standard penetration number N. The standard penetration test is

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conducted at a number of selected points in the vertical direction below the

foundation level at intervals of 75cm or at a point where there is a change of

strata. An average value of N is obtained between the level of the base of

footing and the depth equal to 1.5 to 2 times the width of the foundation.

23. Write short notes on spacing of bore holes.

The spacing of bore holes depends upon the variation of subsurface

soil in the horizontal direction. The factors influencing the spacing of bore

holes are,

Type of soil

Fluctuation of water table

Load coming from structure

Importance of the structure

Economic feasibility

24. What are the limitations of hand augers in soil exploration?

Hand augers are not suitable for sands and gravels above the water

table

The sample is disturbed and suitable for identification purposes only

25. Write short notes on auger boring.

Augers are used in cohesive and other soft soils above water table.

Hand augers are used for depths upto about 6m. Mechanically operated

augers are used for greater depths and they can also be used in gravely soils.

Sample recovered from the soil brought up by augers are badly disturbed

and are useful for identification purposes only. Auger boring is fairly

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satisfactory for highway explorations at shallow depths and for exploring

borrow pits.

26. Define penetration resistance.

The total blows required for the second and third 15cm of penetration

is termed as the penetration resistance N.

27. Define standard penetration number.

The standard penetration number is equal to the number of blows

required for 300mm of penetration beyond a seating drive of 150mm.

28. List the various corrections to be carried out in SPT test.

Correction for overburden

Correction due to dilatancy/submergence

29. Write the expression for correction due to dilatancy / submergence.

The values N0 obtained after applying overburden correction is

corrected further for dilatancy if the stratum consists of fine sand and silt

below water table, for values of N greater than 15, using the following

expression.

𝑁𝑒 = 15 + ( 𝑁0 − 15 )

Where,

N0 = corrected value for overburden effect

30. Write the uses of bore log report.

Used to record the change of layer’s depth

Used to record the water level

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Used to record the water quality in deeper levels

31. Define the term dewatering.

The removal of excess water from the saturated soil mass is termed as

drainage or dewatering. The term dewatering is also used for lowering the

water table to obtain a dry area in the vicinity of the excavation.

32. How depth of borehole decided in soil exploration?

The depth of boring depends upon the type of proposed structure, its

total weight and the type of sub-soil encountered. Though normally the

depth of boring may be taken as 1.5 times the width of largest footing, but at

least on boring should be made deep enough to ascertain the existence of any

exceptionally compressible stratum, preferably up to rock.

33. How is the depth of exploration decided?

The depth of exploration required, depends on the type of proposed

structure, its total weight, the size, shape and disposition of the loaded areas,

soil profile and the physical properties of the soil that constitutes each

individual stratum.

34. What is meant by a non-representative sample? Name the laboratory

tests that could be conducted on this sample.

These samples comprise mixtures of materials from various soil or

rock layers. Here, there are possibilities of removal or exchange of some

mineral constituents by washing and sedimentation. Such examples, also

called wash or wet samples, are not a true representation of the mineral

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found at the bottom of the borehole. These samples are unsuitable for

identification and laboratory tests.

35. Define detailed exploration.

Detailed exploration follows as a supplement to general exploration

when large engineering works, heavy loads, complex and costly foundations

are involved. A detailed exploration is meant to furnish information about

soil properties such as shear strength, compressibility, density index and

permeability.

36. What is site reconnaissance?

An inspection of the site and study of topographical features is often

helpful in getting useful information about the soil and ground water

conditions and in deciding the future programme of exploration.

UNIT – 2

1. What are the causes of slope failure?

Gravitational force

Force due to seepage water

Erosion of the surface of slopes due to flowing water

The sudden lowering of water adjacent to a slope

Forces due to earthquakes

Rainfall

Construction activities at the toe of the slope

External loading

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2. What is called slope?

An exposed ground surface that stands at an angle (i) with the

horizontal is called slope. Slopes are required in the construction of

highway and railway embankments, earth dams, levees and canals. These

are constructed by sloping the lateral faces of the soil because slopes are

generally less expensive than constructing walls. Slopes can be natural or

manmade. When the ground surface is not horizontal a component of

gravity will try to move the sloping soil mass downwards.

3. What are the types of slopes?

Finite slopes

Infinite slopes

4. Draw the translational and compound failure.

5. What are the factors of safety used in stability analysis of slopes?

Factor of safety with respect to cohesion assuming to be fully

mobilized

Factor of safety with respect to friction assuming to be fully mobilized

Factor of safety with respect to shear strength

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Factor of safety with respect to height

6. Draw the face, toe and base failure of finite slopes.

7. Define factor of safety.

Factor of safety of a slope is defined as the ratio of average shear

strength ( 𝜏𝑓 ) of soil to the average shear stress ( 𝜏) developed along the

potential failure surface.

8. What is stability number?

In a slope, the component of the self-weight causes instability and the

cohesion contributes to stability. The maximum height ( 𝐻𝑐 ) of a slope is

directly proportional to unit cohesion (Cu) and inversely proportional to unit

weight (γ). In addition, Hc is also related to friction angle (Φ) and slope ‘i’.

9. Draw the total stress analysis for pure cohesive soil.

Analysis based on total stresses, also called ‘φ = 0 analysis’, gives the

stability of an embankment immediately after its construction. It is assumed

that the soil has had no time to drain and the shear strength parameters used

relate to the undrained strength with respect to total stresses. These may be

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obtained from either unconfined compression test or an undrained tri-axial

test without pore pressure measurements.

10. Draw the sketch for infinite slope in cohesive soil.

11. Draw the sketch for infinite slope in cohesion less soil.

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12. Draw the sketch for infinite slope in cohesive frictional soil.

13. What are the effects on tension crack?

It modifies the slip surface and reduces the length of the slip surface

It is usually filled with water and produces hydrostatic pressure along

the depth

It acts as channel for water to flow into underlying soil layers,

inducing seepage forces

It reduces the factor of safety

If a tension crack of depth 𝑧0 = 2 𝑐

𝛾 develops, water will enter in the

crack, exerting a hydrostatic pressure force Pw acting on the portion DE at a

height z0/3 from E. Hence the arc portion DE will be ineffective in resisting

the slide.

14. Draw the effect of tension cracks on stability.

In case of cohesive soil when the slope is on the verge slippage there

develops a tension crack at the top of the slope.

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15. Define slope angle.

It is the angle of inclination of a slope to the horizontal. The slope

angle is sometimes referred to as a ratio. For example, 2:1 (horizontal:

vertical).

16. What is meant by face, toe and base failure?

Face failure

This type of failure occurs when the slope angle is large and when the

soil at the toe portion is strong

Toe failure

In this case the failure surface passes through the toe. This occurs

when the slope is steep and homogeneous.

Base failure

In this case the failure surface passes below the toe. This generally

occurs when the soil below the toe is relatively weak and soft.

17. List the different types of slope failure.

Rotational failure

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Slope failure

o Face failure

o Toe failure

Base failure

Translational failure

Compound failure

Wedge failure

Miscellaneous failure

18. What is meant by rotational failure?

This type of failure occurs by rotation along a slip surface by

downward and outward movement of the soil mass. The slip surface is

generally circular for homogeneous soil conditions and non – circular in case

of non – homogeneous conditions.

19. What is meant by compound failure?

A compound failure is a combination of the rotational slips and

translational slip. A compound failure surface is curved at the two ends and

plane in the middle portion. A compound failure generally occurs when a

hard stratum exists at considerable depth below the toe.

20. Write down the assumption made in the analysis of slopes.

The stress is assumed to be two dimensional.

Coulomb equation for shear strength is applicable and parameters C

and Ф are known.

Seepage pressure was estimated from the assumed seepage conditions

and water levels.

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The conditions of plastic failure are assumed to be satisfied along the

critical surface.

21. Define slope stability analysis.

Slope stability analysis consists of determining and comparing the

shear stress developed along the potential rupture surface with the shear

strength of the soil. Attention has to be paid to geology, surface drainage,

groundwater and the shear strength of soils in assessing slope stability.

22. What are the types of stability analysis?

Stability analysis of infinite slopes

Infinites slopes in dry sand

Infinite slopes in c – Φ soil

Stability analysis of finite slopes

23. Define slip or failure zone.

It is a thin zone of soil that reaches the critical state or residual state

and results in movement of the upper soil mass.

24. What is critical failure plane?

Critical failure plane is defined as the plane along which the failure

occurs in which the shear stress on the plane is less than the maximum shear

stress.

25. What are the types of slip surface in a finite slope?

Planar failure surface

Circular failure surface

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Non – circular failure surface

26. What do you mean by planar failure surface?

Planar failure surface may commonly occur in a soil deposit or

embankment with a specific plane of weakness. It is common in stratified

deposit and the failure plane is parallel to the strata.

27. Where does a non – circular (composite) slip surface occur in a

homogeneous dam?

Foundation of infinite depth

Rigid boundary planes of maximum or zero Shear

Presence of relatively stronger or weaker layers.

28. What is landslip?

A landslide, also known as a landslip, is a form of mass wasting that

includes a wide range of ground movements, such as rock falls, deep failure

of slopes, and shallow debris flows. Landslides can occur in underwater,

called a submarine landslide, coastal and onshore environments.

29. Write about the uses of friction circle method.

This method uses total stress based limit equilibrium approach. In this

method the equilibrium of the resultant weight (W), the reaction (p) due to

frictional resistance and the cohesive force (c) are considered. The

magnitude direction and line of action of resultant weight (W), the line of

action of the reaction force (p) and the cohesive force (c) being known the

magnitude of p and c are determined by considering the triangle of forces.

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The friction circle method is useful for the stability analysis of slopes

made of homogeneous soils. In this method, the slip surface is assumed to

be an arc of a circle.

30. Write about the critical slip circle by fellenius direction angles.

In case of slopes in homogeneous cohesive soil deposits, the centre of

a critical circle can be directly located by using Fellenius direction angles.

Fellenius (1936) has given direction angles α and β for various slopes as

shown below.

For any given slope the corresponding direction angles α and γ are set

out from the base and the top as shown in Figure. The point of intersection

of these two lines is the centre of critical circle. After locating the centre of

critical circle the method of slices can be adopted to obtain minimum F.S.

31. Sketch the critical slip circle in c – Φ soils.

In case of c-Φ soils the procedure for locating critical slip surface is

slightly different.

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32. What is the Taylors stability number?

The dimensionless quantity 𝑐

𝐹𝑐 𝛾 𝐻 is called Taylor’s stability number

Sn.

33. Define finite slopes.

If the slope is of limited extent, it is called a finite slope.

34. What is the wedge failure?

A failure along an inclined plane is known as plane failure or wedge

failure or block failure. It occurs when distinct blocks and wedges of the

soil mass become separated.

35. What is translation failure?

A constant slope of unlimited extent and having uniform soil

properties at the same depth below the free surface is known as an infinite

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slope. In practice, the slopes which are of considerable extent and in which

the conditions on all verticals are adequately represented by average

conditions are designated as infinite slopes. Translation failure occurs in an

infinite slope along a long failure surface parallel to the slope. The shape of

the failure surface is influenced by the presence of any hard stratum at a

shallow depth below the slope surface. Translational failures may also occur

along slopes of layered materials.

36. What are the cases of infinite slope in layered soils?

Three cases of stability analysis of infinite slopes are considered

Cohesion less soil

Cohesive soil

Cohesive – frictional soil

37. Define infinite slopes.

If a slope represents the boundary surface of a semi-infinite soil mass,

and the soil properties for all identical depths below the surface are constant,

it is called an infinite slope.

38. What are the methods of finite slopes?

Culmann’s method of planar failure surface

The Swedish circle method (slip circle method)

The friction circle method

Bishop’s method

39. Where do we use the manmade slopes?

Manmade slopes were used in,

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Highways

Railways

Earth dams

River training works

40. Define landslide.

It is a relatively rapid lateral and downhill movement of a well-

defined earth mass (or land form). It occurs due to gravitational and seepage

forces.

41. What is meant by drawdown?

As soon as the pumping is done from a well, the water table is

lowered in its vicinity. This drop in water level in the well is called

drawdown.

42. Write about the failures on landslides.

Failure of natural slopes (landslides) and manmade slopes has resulted

in much death and destruction. Some failures are sudden and catastrophic;

others are widespread: some are localized. Civil engineers are expected to

check the safety of natural slopes and excavation.

43. Write about the condition of stability during steady seepage.

When seepage occurs at a steady rate through an earth dam or

embankment it represents critical condition for the stability of slope.

When seepage occurs, Pore water pressure (u) develops and this will

reduce the effective stress which in turn decreases the shear strength along

the failure surface.

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The following procedure is adopted to obtain stability,

Draw the cross section of the slope

Draw the potential failure surface

Divide the soil mass into slices

Calculate the weight (W) and the corresponding normal and tangential

components for all the slices in the usual way.

44. Write the types of retaining wall.

Gravity wall

Mass concrete or masonry wall

Wall on wells

Precast block wall

Two row sheet pile wall

Crib wall

Reinforced concrete wall

Cantilever type – ‘T’ wall or ‘L’ wall

Counterfort or buttressed wall

Sheet pile wall

Cantilever sheet pile wall

Anchored sheet pile wall or anchored bulkhead

45. What are the three forces acting in circular failure while analyses

through friction circle method?

Weight (W) of the sliding wedge

Cohesive force (C) developed along the slip surface

Reaction (R) on the slip surface

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46. Define sliding.

A retaining structure has a tendency to move away from the backfill

surface because of the horizontal driving forces resulting from the soil

backfill and other forces such as surcharge. Generally, the wall resists

sliding by the frictional resistance developed between the foundation of the

wall and foundation soil.

47. Define topple.

This is a forward rotation of soil and/or rock mass about an axis below

the center of gravity of mass being displaced.

48. Define effective stress.

It is the nominal stress transmitted through the particle to particle

contact in soil. 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. The effective stress controls the

shear strength and compressibility of the soil.

49. Define effective stress analysis.

When the pore water pressure exist in the embankment due to

seepage, sudden drawdown or due to any other reason, then stability should

be computed based on effective stress analysis.

𝜎′ = 𝜎 − 𝑢

50. Write the relation between strength envelope and angle of repose.

If the normal and shear stress corresponding to failure are plotted,

then a curve is obtained. The plot or the curve is called strength envelope.

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Angle of repose is defined as the maximum inclination of a plane at

which a body remains in equilibrium over the inclined plane by the

assistance of friction only. The earth particles lack in cohesion and have a

definite angle of repose and angle of repose is equal to angle of friction (Φ).

51. Mention the slope protection measures.

Covering the slope by a layer of broken stones or coarse gravel.

Using vegetation,

Covering the slope by pitching in mortar over a suitable filter.

Using reinforced concrete lining over a suitable filter.

52. Draw the Swedish method of slices for a cohesive – frictional soil.

For a cohesive frictional soils the undrained strength envelope shows

both c and Φ values. The total stress analysis can be adopted.

53. Define depth factor.

The ratio of total depth (H + D) to depth H is called depth factor (Df).

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For toe failure, Df = 1

For base failure, Df > 1

UNIT – 3

1. What is known as coefficient of earth pressure?

When the retaining wall is moving away from the backfill then the

ratio between lateral earth pressure and vertical earth pressure is called

coefficient of active earth pressure.

𝐾𝑎 = 𝑙𝑎𝑡𝑒𝑟𝑎𝑙 𝑒𝑎𝑟𝑡ℎ 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒

𝑣𝑒𝑟𝑡𝑖𝑐𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒=

1 − sin 𝛷

1 + sin 𝛷

When the retaining wall is moving towards the backfill, then the ratio

between the lateral earth pressure and the vertical earth pressure is called the

coefficient of passive earth pressure.

𝐾𝑝 = 𝑙𝑎𝑡𝑒𝑟𝑎𝑙 𝑒𝑎𝑟𝑡ℎ 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒

𝑣𝑒𝑟𝑡𝑖𝑐𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒=

1 + sin 𝛷

1 − sin 𝛷

2. What is earth pressure at rest?

The earth pressure at rest, exerted on the back of a rigid, unyielding

retaining structure, can be calculated using theory of elasticity, assuming the

soil the semi-infinite, homogeneous, elastic and isotropic.

3. What are the methods for calculating lateral earth pressure

coefficients?

Rankine’s theory

Coulomb’s wedge theory

Rebhann’s method

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Culmann’s method

4. What is surcharge?

The position of backfill lying above a horizontal plane at the elevation

of the top of the wall is called the surcharge, and its inclination to the

horizontal is called surcharge angle β.

5. What is the angle of wall friction?

The resultant pressure acts on the back of the wall at one third the

height of the wall from the base and is inclined at an angle δ is called the

angle of wall friction.

6. Define surcharge load.

A surcharge load results from forces that are applied along the surface

of the backfill behind the wall. These forces apply an additional lateral force

along the back of the wall. Surcharge pressures result from loads such as a

line load, strip load, embankment load, traffic (such as a parking lot), floor

loads and temporary loads such as construction traffic and stockpiles of

material.

7. What is meant by conjugate plane and conjugate stresses?

In finding out the active earth pressure for the backfill with sloping

surface by Rankine’s theory, an additional assumption that the vertical and

lateral stresses are conjugate is made. It can be shown that if the stress on a

given plane at a given point is parallel to another plane, the stress on the

latter plane at the same point must be parallel to the first plane. Such planes

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are called the conjugate planes and the stresses acting on them are called

conjugate stresses.

8. State the assumptions made in Rankines theory of earth pressure.

The soil mass is homogeneous and semi – infinite

The soil is dry and cohesion less

The ground surface is plane, which may be horizontal or inclined

The back of the retaining wall is smooth and vertical

The soil element is in a state of plastic equilibrium

9. Write the assumption in Terzaghi’s bearing capacity theory.

The soil is homogeneous and isotropic and its shear strength is

represented by Coulomb’s equation.

The strip footing has a rough base, and the problem is essentially two

dimensional.

The elastic zone has straight boundaries inclined at ψ = Φ to the

horizontal and the plastic zones fully develop.

Pp consists of three components which can be calculated separately

and added, although the critical surfaces for these components are not

identical.

Failure zones do not extend above the horizontal plane through the

base of the footing.

10. State the assumptions made in coulombs theory.

The backfill is dry, cohesion less, homogeneous, isotropic and

elastically undeformable but breakable.

The slip surface is plane which passes through the heel of the wall.

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The sliding wedge itself acts as a rigid body and the value of earth

pressure is obtained by considering the limiting equilibrium of the

sliding wedge as a whole.

The position and direction of the resultant earth pressure are known.

11. Compare Rankine’s and Coulomb’s theory.

RANKINE’S THEORY COULOMB’S THEORY

The intensity of earth pressure at

each depth is known. So point of

application of the earth pressure is

known at any depth.

Only the total earth pressure value

acting on the retaining structures

can be calculated. The point of

application of earth pressure can be

calculated from Coulomb’s

assumption that all points on the

back of the retaining wall are

essentially considered as feet of

failure surface.

Wall is smooth and vertical. Wall is rough and sloped.

Wall moved sufficiently so soil is in

plastic failure mass.

Wall is rigid, straight failure plane

and rigid failure wedge.

12. Define coulomb theory.

Coulomb, in 1776, developed an earth pressure theory which includes

the effect of friction between the backfill and the wall. The theory considers

a dry, non – cohesive inclined backfill, and the lateral earth pressure

required to maintain the equilibrium of a sliding wedge with a plane slip

surface is calculated.

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13. Write the coulomb’s equations to account for wall friction.

𝐾𝑎 = cos2( 𝛷 − 𝜃 )

cos2 𝜃 cos( 𝛿 + 𝜃 ) ( 1 + √sin( 𝛿+ 𝛷 ) sin( 𝛷− 𝛽 )

cos( 𝛿+ 𝜃 ) cos( 𝛽− 𝜃 ) )

2

𝐾𝑝 = cos2( 𝛷 + 𝜃 )

cos2 𝜃 cos( 𝛿 − 𝜃 ) ( 1 − √sin( 𝛿+ 𝛷 ) sin( 𝛷+ 𝛽 )

cos( 𝛿− 𝜃 ) cos( 𝛽− 𝜃 ) )

2

14. What are the points to consider coulomb case with no soil-wall friction?

For the coulomb case with no soil wall friction (i.e. δ = 0) and a

horizontal backfill surface, both the Coulomb and Rankine method

yield equal results.

As the soil friction angle Φ increases (i.e. soil becomes stronger), the

active pressure coefficient decreases, resulting in a decrease in the

active force while the passive pressure coefficient increases, resulting

in an increase in the passive force.

15. What are the limitations of Terzaghi’s analysis?

The theory is applicable to shallow foundations

As the soil compresses, increases which is not considered. Hence fully

plastic zone may not develop at the assumed.

All points need not experience limit equilibrium condition at different

loads.

Method of superstition is not acceptable in plastic conditions as the

ground is near failure zone.

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16. What is depth of tension crack?

The tensile stress decreases with an increase in depth, and it becomes

zero at a depth z, given by,

𝑧 = 2 𝑐

𝛾 tan 𝛼 =

2 𝑐

𝛾 √𝑘𝑎

The depth z is known as the depth of tension crack.

17. What do you understand by the term coefficient of earth pressure at

rest?

When the retaining wall is at rest then the ratio between the lateral

earth pressure and the vertical pressure is called the co-efficient of the earth

pressure at rest,

𝐾0 = 𝑙𝑎𝑡𝑒𝑟𝑎𝑙 𝑒𝑎𝑟𝑡ℎ 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒

𝑣𝑒𝑟𝑡𝑖𝑐𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒

18. What are the different types of earth pressure?

There are three types of earth pressures on the basis of the movement

of the wall.

Earth Pressure at rest

Active Earth Pressure

Passive Earth Pressure

19. Define lateral earth pressure.

Lateral earth pressure is the pressure that soil exerts in the horizontal

direction. The lateral earth pressure is important because it affects the

consolidation behavior and strength of the soil and because it is considered

https://en.wikipedia.org/wiki/Pressure

https://en.wikipedia.org/wiki/Soil

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in the design of geotechnical engineering structures such as retaining walls,

basements, tunnels, deep foundations and braced excavations.

20. Define passive earth pressure.

When the wall moves towards the backfill, there is an increase in the

pressure on the wall and this increase continues until a maximum value is

reaches after which there is no increase in the pressure and the value will

become constant. This kind of pressure is known as passive earth pressure.

21. Define active earth pressure.

When the wall moves away from the backfill, there is a decrease in

the pressure on the wall and this decrease continues until a minimum value

is reaches after which there is no reduction in the pressure and the value will

become constant. This kind of pressure is known as active earth pressure.

22. Define earth pressure.

It is the lateral pressure exerted by a soil mass against an earth

retaining structure (or on a fictitious vertical plane located within a soil

mass).

23. What is a sheet pile wall?

A sheet pile wall consists of a series of sheet piles driven side by side

into the ground, thus forming a continuous vertical wall for the purpose of

retaining an earth bank. They are commonly used for water front structures,

temporary construction and light weight construction where sub – soil is

poor for supporting a retaining wall.

https://en.wikipedia.org/wiki/Geotechnical_engineering

https://en.wikipedia.org/wiki/Retaining_wall

https://en.wikipedia.org/wiki/Basement

https://en.wikipedia.org/wiki/Tunnel

https://en.wikipedia.org/wiki/Deep_foundation

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24. What are the other forces acting on the wall?

Other forces acting on the wall aside from the earth pressure force

acting on the wall, other forces might also act on the wall and these are

superimposed onto the earth pressure force. For example, these forces might

include,

Surcharge load

Earthquake load

Water pressure

25. What do you understand by plastic equilibrium of soils?

A body of soil is said to be in plastic equilibrium, if every point of it is

on the verge of failure.

26. Why retaining walls are usually designed for active pressure?

From Rankine’s assumption, no existence of frictional forces at the

wall face. The resultant pressure must be parallel to the surface of the

backfill. The existence of the friction makes the resultant pressure inclined

to the normal to the wall at an angle that approaches the friction angle

between the soil and the wall.

UNIT – 4

1. Define foundation.

A foundation is that part of the structure which is in direct contact

with and transmits loads to the ground.

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2. Define shallow foundation.

According to Terzaghi, a foundation is shallow if its depth is equal to

or less than its width. It can be adopted for the light structures and the soil

having high bearing capacity.

3. Define deep foundation.

According to Terzaghi, foundation is deep if its depth is equal to or

greater than its width. It can be adopted for the multi storey building.

4. Define floating foundation.

It is defined as a foundation in which the weight of the building is

approximately equal to the full weight of the soil including water excavated

from the site of the building.

5. In which situation is raft foundation is used?

Raft foundation is adopted for medium heavy structures where the soil

is having low bearing capacity. Raft foundation is used to reduce settlement

above highly compressible soils, by making the weight of structure and raft

approximately equal to the weight of the soil excavated.

6. Where do you use mat foundation?

It is used when the area of isolated footing is more than fifty

percentage of whole area or the soil bearing capacity is very poor. When the

allowable soil pressure is low, or the building loads are heavy, the use of

spread footings would cover more than one half of the area and it may prove

more economical to use mat or raft foundation.

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7. When trapezoidal combined footings are provided?

When the projection parallel to the length of the footing is restricted

on both the sides.

When the length of the footing is restricted.

8. Under what circumstances, a strap footing is adopted?

When the distance between the two columns is so great, so that

trapezoidal footing is very narrow and so it is uneconomical. It transfers the

heavy load of one column to other column.

9. What is the function of strap beam in a strap footing?

The strap connects the two isolated footing such that they behave as

one unit. The strap simply acts as a connecting beam.

10. What is meant by foundation soil?

It is the upper part of the earth mass carrying the load of the structure.

11. Sketch the contact pressure distribution diagram for a rigid footing on

sand and clay.

12. Write the equation for shear strength of soil.

𝑆 = 𝑐 + 𝜎 𝑡𝑎𝑛𝛷

Where,

S = shear stress

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σ = normal stress

c = cohesion

Φ = angle of internal friction

13. What are the three components of total settlement of footings?

Immediate elastic settlement

Consolidation settlement

Settlement due to secondary consolidation

14. Define immediate settlement.

Immediate or elastic settlement takes place during immediately after

the construction of the structure. It is also known as the distortion settlement,

as it is due to distortions within the foundation soil.

15. Define consolidation settlement.

This component of settlement occurs due to gradual expulsion of

water from the voids of soil. This component is determined using Terzaghi’s

theory of consolidation.

16. Define footing.

A footing is a portion of the foundation of a structure that transmits

loads directly to the soil.

17. Define spread footing.

A spread footing or simply footing is a type of shallow foundation

used to transmit the load of an isolated column, or that of a wall to the

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subsoil. This is most common type of foundation. The base of the column

or wall is enlarged or spread to provide individual support for the load.

18. What is a mat foundation?

It is a combined footing that covers the entire area beneath a structure

and supports all the walls and columns.

19. List the various factors affecting the bearing capacity of soil.

Nature of soil and its physical and engineering properties

Nature of the foundation and other details such as the size, shape,

depth below the ground surface and rigidity of the structure

Total and differential settlements that the structure can withstand

without functional failure

Location of the ground water table relative to the level of the

foundation

Initial stresses, if any

20. A footing was designed based on ultimate bearing capacity arrived for

the condition of water table at the ground surface. If there is a chance

for rise in water level much above the ground level do you expect any

change in the bearing capacity, why?

The raise in water level above the ground level would not change the

bearing capacity of the soil. Because the soil under submerged condition

when the water reaches the ground surface. Therefore the water above the

ground level does not affect the unit weight of soil.

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21. What is the effect of rise of water table on the bearing capacity and the

settlement of a footing on sand?

The pressure of water affects the unit weight of soil. Hence bearing

capacity is affected due to the effect of water table. For practical purpose it

is more sensitive when the water table rises above depth 13m from footing.

22. List the types of bearing capacity failure (or) modes of failure.

General shear failure

Local shear failure

Punching shear failure

23. Define general shear failure.

General shear failure results in a clearly defined plastic yield slip

surface beneath the footing and spreads out one or both sides, eventually to

the ground surface. Failure is sudden and will often be accompanied by

severe tilting generally associated with heaving. This type of failure occurs

in dense sand or stiff clay.

24. Define local shear failure.

Local shear failure results in considerable vertical displacement prior

to the development of noticeable shear planes. These shear planes do not

generally extend to the soil surface, but some adjacent bulging may be

observed, but little tilting of the structure results. This shear failure occurs

for loose sand and soft clay.

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25. Define punching shear failure.

Punching shear failure occurs where there is relatively high

compression of soil under the footing, accompanied by shearing in the

vertical direction around the edges of the footing. Punching shear may occur

in relatively loose sand with relative density less than 35%.

26. What is proportioning of footing?

Proportioning of footing is defined as the arrangement of footing in

the combined footing system, in which, it is arranged in such a way that, the

centroid of the area in contact with the soil lies on the line of action of

resultant of the loads.

27. List the various ground improvement techniques.

Compaction pile

Blasting

Pre – compression

Stone column

Vibrofloatation

Terraprobe method

Lime piles

Geotextiles

Grouting

Electro osmosis

Thermal treatment

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28. Compare general and local shear failure.

S. NO GENERAL SHEAR

FAILURE LOCAL SHEAR FAILURE

1. Well defined failure pattern. Well defined wedge and slip

surfaces only beneath the

foundation.

2. A sudden Catastrophic failure

accompanies by tilting of

foundation.

There is no tilting of foundation.

Slip surface not visible beyond

the edges of the foundation.

3. Bulging of ground surface

adjacent to the foundation.

Slight bulging of ground surface

adjacent to the foundation.

4. The load settlement curve

indicates the ultimate load

clearly.

The load settlement curve does

not indicate the ultimate load

clearly.

29. Define the term bearing capacity of soil.

The supporting power of a soil is referred to as its bearing capacity.

30. What is meant by stabilization?

Stabilization, in a broad sense, incorporates the various methods

employed for modifying the properties of a soil to improve its engineering

performance. Stabilization is being used for a variety of engineering works,

the most common application being in the construction of road and air –

field pavements, where the main objective is to increase the strength or

stability of soil and to reduce the construction cost by making best use of the

locally available materials.

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31. What are the factors affecting lime stabilization?

Type of soil

Amount of lime

Ratio of fly ash to lime

Type of lime

Workability of soil

Compressive strength of soil

32. What are the advantages of lime stabilization?

Soil becomes more workable.

Strength is generally improved.

Lime stabilization increases the compressive strength sometimes as

high as 60 times.

It is effective for soils.

33. What are the disadvantages of lime stabilization?

Lime is produced by burning of lime stone in kilns, so that it is

harmful for environment.

It needs more cost to burnt lime stone.

It is not effective for sandy soils.

There is limited percentage of amount of lime required about 2 to 10%

of the soil.

34. What are the different types of shallow foundation?

Spread footing

Strap footing

Combined footing

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Mat or raft footing

Strip footing

Wall footing

35. List the different types of raft foundation.

Flat plate type

Flat plate thickened under columns

Beam and slab construction

Box structures

Mats placed on piles

36. Write down the formula used to find the minimum depth of foundation

by Rankine’s formula.

According to Rankine’s formula, depth of foundation is given by,

𝐷 = 𝑞

𝛾 (

1 − sin 𝛷′

1 + sin 𝛷′ )

2

Where,

q = intensity of loading

Φ’ = effective angle of shearing

γ = density of soil solids

37. Write about bearing capacity of square and circular footings.

If the soil support of a continuous footing yields due to the imposed

loads on the footings, all the soil particles move parallel to the plane which

is perpendicular to the centre line of the footing. Therefore the problem of

computing the bearing capacity of such footing is a plane strain deformation

problem. On the other hand if the soil support of the square and circular

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footing yields, the soil particles move in the radial and not in parallel planes.

Terzaghi has proposed certain shape factors to take care of the effect of the

shape on the bearing capacity.

38. What is stone column?

Stone columns refer to columns of compacted, gravel size stone

particles constructed vertically in the ground to improve the performance of

soft or loose soils. The stone can be compacted with impact methods, such

as with a falling weight or an impact compactor or with a vibroflot, the more

common method. The method is used to increase bearing capacity, reduce

foundation settlements, improve slope stability, reduce seismic subsidence,

reduce lateral spreading and liquefaction potential, permit construction on

loose/soft fills, and pre collapse sinkholes prior to construction in Karst

regions.

39. Define gross pressure intensity.

The gross pressure intensity q is the total pressure at the base of the

footing due to the weight of the superstructure, self – weight of the footing

and the weight of the earth fill, if any.

40. What is net pressure intensity?

It is defined as the excess pressure, or the difference in intensities of

the gross pressure after the construction of the structure and the original

overburden pressure.

𝑞𝑛 = 𝑞 − �̅� = 𝑞 − 𝛾𝐷

Where,

γ is the average unit weight of soil above the foundation base.

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41. What is ultimate bearing capacity?

The ultimate bearing capacity is defined as the minimum gross

pressure intensity at the base of the foundation at which the soil fails in

shear. It is denoted by the symbol qf.

42. Define net ultimate bearing capacity.

It is the minimum net pressure intensity causing shear failure of soil.

The ultimate bearing capacity qf and the net ultimate capacity are evidently

connected by the following relation.

𝑞𝑛𝑓 = 𝑞𝑓 − �̅�

Where,

�̅�, is the effective surcharge at the base level of the foundation.

43. What is the effective surcharge at the base level of foundation?

It is the intensity of vertical pressure at the base level of foundation,

computed assuming total unit weight for the portion of the soil above the

water table and submerged unit weight for the portion below the water table.

It is denoted by the symbol �̅�.

44. Define net safe bearing capacity.

The net safe bearing capacity is the net ultimate bearing capacity

divided by a factor of safety F.

𝑞𝑛𝑠 = 𝑞𝑛𝑓

𝐹

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45. Define safe bearing capacity.

The maximum pressure which the soil can carry safely without risk of

shear failure is called the safe bearing capacity. It is equal to the net safe

bearing capacity plus original overburden pressure.

𝑞𝑠 = 𝑞𝑛𝑠 + 𝛾𝐷 = 𝑞𝑛𝑓

𝐹+ 𝛾𝐷

Sometimes, the safe bearing capacity is also referred to as the ultimate

bearing capacity qf divided by a shape factor F.

46. What is safe bearing pressure?

It is the intensity of loading that will cause a permissible settlement or

specified settlement for the structure. It is also called as net soil pressure.

47. List out the methods of computing elastic settlements.

Based on the theory of elasticity

Pressure meter method

Janhu –Bjerram method

Schmentmann’s method

48. What is allowable bearing pressure or capacity?

It is the net load intensity at which neither the soil fails in shear nor

there is excessive settlement detrimental to the structure. It is denoted by the

symbol qa.

49. What is meant by swelling potential?

Swelling potential is defined as the percentage of swell of a laterally

confined sample in an odometer test which is soaked under a surcharge load

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of 7kpa after being compacted to maximum dry density at optimum moisture

content according to the AASHTO compaction test.

50. What is total settlement of a footing?

Total settlement is defined as the settlement due to immediate elastic

settlement, consolidation settlement and settlement due to secondary

consolidation.

𝑆 = 𝑆𝑖 + 𝑆𝑐 + 𝑆𝑠

51. What is meant by allowable settlement?

It is the maximum settlement beyond which the foundation fails due

to excessive settlement

Permits a maximum allowable settlement of 40 mm for isolated

foundation on sand and 65 mm for those on clay.

For raft foundations on sand 40 mm to 65 mm and that on clay 65 mm

to 100 mm.

52. Define differential settlement.

The differential settlement Sd is the difference in total settlement

between two foundations or between two points on a single foundation.

Differential settlements are generally more troublesome than total

settlements because they distort the structure. This causes cracking in walls

and other members.

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UNIT – 5

1. What are under-reamed piles?

Under – reamed pile is special type of bored pile having an increased

diameter or bulb at some point in its length, to anchor the foundation in

expansive soil subjected to alternate expansion and contraction.

2. Define friction pile.

Friction piles are used to transfer loads to a depth of a friction load

carrying material by means of skin friction along the length of the piles.

3. What are anchor piles?

Anchor piles provide anchorage against horizontal pull from sheet

piling or other pulling forces.

4. What are fender piles?

Fender piles and dolphins are used to protect water front structures

against the impact from ships or other floating objects.

5. Define negative skin friction.

When the soil layer surrounding the portion of the pile shaft settles

more than the pile, a downward drag occurs on the pile. The drag is known

as negative skin friction.

6. Define block skin resistance.

When piles are placed close to each other, then negative skin

resistance may act effectively on block perimeter of pile group.

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𝑄𝑛 = 𝑆𝐿𝑝 + 𝛾𝐿𝐴

Where,

S = shear resistance of soil

L = depth of fill

p = perimeter of pile group

γ = unit weight of soil

A = area of pile group enclosed in perimeter p

7. Define negative skin pressure.

Negative skin friction force for a single pile is equal to the sharing

resistance times the surface area of the pile. Therefore the negative skin

friction on a pile group is

𝑄𝑢𝑔 = 𝑐 × 𝑝𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟 × 𝑙𝑒𝑛𝑔ℎ𝑡 (For group)

𝑄𝑢𝑔 = 𝑛 𝑚 𝑐 𝐴𝑝 (For individual)

8. How are piles classified as per load transfer?

End bearing pile

Friction pile

Combined end bearing and friction pile

9. Define ultimate load carrying capacity of pile.

The ultimate load carrying capacity, or ultimate bearing capacity, or

the ultimate bearing resistance Qup of a pile is defined as the maximum load

which can be carried by a pile, and at which the pile continues to sink

without further increase of load.

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10. Write down Danish formula.

𝑄𝑢 = 𝑊 × ℎ × 𝜂ℎ

𝑆 + 1

2 𝑆𝑜

11. What are the factors governing the selection of piles?

Soil condition

Type of structure or building

Adjacent site condition

Construction techniques availability

Location of ground water table

Durability

12. What are the factors consider while selecting the type of pile?

The loads

Time available for completion of the job

The characteristics of the soil strata involved

The ground water conditions

The availability of equipment

The statutory requirements of building codes

13. What are the conditions where a pile foundation is more suitable than a

shallow foundation?

Huge vertical load with respect to capacity

Very weak soil

Huge lateral loads

For fills having very large depth

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Uplift situation

Urban areas for future and huge construction near the existing

building

14. How piles are classified based on method of installation?

Driven pile

Driven and cast – in – situ pile

Bored and cast – in – situ pile

Screw pile

Jacked pile

15. What is grouting?

It is a process in which the holes are drilled in soil (or rock) and a

grout (usually cement and water mixture) is injected into the holes. It

improves the bearing capacity and also reduces the permeability and

seepage.

16. What are the limitations of the dynamic pile load formula?

For clays, the dynamic formulae are valueless because the skin

friction developed in clay during driving is very much less than which

occurs after a period of time. Also, the point resistance is much more

at the time of driving because of pore pressure developed in clay,

which reduces later on when the pore pressure dissipate.

Dynamic formulae give no reduction about probable future settlement

or temporary changes in soil structure.

The formulae do no taken into account the reduced bearing capacity

of pile when in a group.

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Law of impact used for determining energy loss is not strictly valid

for piles subjected to restraining influence of the surrounding soil.

In Hiley’s formula, a number of constants are involved, which are

difficult to determine.

17. What are the limitations of the dynamic pile load test?

It is largely depends on the nature of the ground through which the

pile was driven to get down to finished level.

It takes very little account of the effect of friction on sides of pile, and

this friction tends only to develop later.

18. List the limitations of plate load test.

Size effect

Scale effect

Time effect

Interpretation of failure load

Reaction load

Water table

19. List out the type of pile based on material used.

Concrete pile

Pre – cast

Cast – in – situ

o Driven piles: case or uncased

o Bored piles: pressure piles and under reamed piles

Timber pile

Steel pile

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H – pile

Pipe pile

Sheet pile

Composite pile

Concrete and timber

Concrete and steel

20. How is the selection of pile carried out?

The selection of the type, length and capacity is usually made from

estimation based on the soil condition and magnitude of the load.

21. What is mean by group settlement ratio?

The settlement of pile group is found to be many times that of a single

pile. The ratio of the settlement of the pile group to that of a single pile is

known as the group settlement ratio.

22. Define group efficiency of pile.

The group efficiency of a pile is defined as the ratio of the ultimate

load of the group to the sum of individual ultimate loads.

𝜂𝑔 = 𝑄𝑢𝑔

𝑛 𝑄𝑢𝑝

Where,

Qug = load carried by group of friction piles

Qup = load carried by each friction pile

n = number of piles

ηg = efficiency of pile group

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23. What are the types of hammer?

Drop hammer

Single acting hammer

Double acting hammer

Diesel hammer

Vibratory hammer

24. What is pile driver?

Piles are commonly driven by means of a hammer supported by a

crane or by a special device known as a pile driver.

25. What are the methods to determine the load carrying capacity of a pile?

Dynamic formulae

Static formulae

Pile load test

Penetration test

26. What are the two types of dynamic formulae?

Engineering news formula

Hiley’s formula

27. What is meant by single-under reamed pile?

When the pile has only one bulb, it is known as single under – reamed

pile.

28. Write down the static formulae.

𝑄𝑢𝑝 = 𝐴𝑠 . 𝑟𝑓 + 𝐴𝑝 . 𝑟𝑝

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Where,

As = surface area of pile

Ap = area of cross section of pile

rf = average skin friction

rp = unit point or toe resistance

29. Define modulus of sub-grade reaction.

The ratio of the soil reaction (p) to the deflection (y) at any point is

defined as the modulus of sub-grade reaction Es or soil modulus or the

coefficient of sub-grade reaction.

𝐸𝑠 = − 𝑝

𝑦

The minus sign indicates that as p increases, y decreases.

30. What are the points to be considered on installation of pile?

Rate of penetration

Continuity of penetration

Installation methods (i.e. driving, jacking or vibration)

Mode of penetration (i.e. weather it is plugged or not)

31. For identical soil conditions, the load permitted on bored pile is lesser

than driven pile of identical shape and dimensions, why?

The load carrying capacity of bored cast in situ pile will be much

smaller than that of a driven pile in sand. The angle of shearing resistance of

the soil is reduced by 30, to account for the loosening of the sand due to the

drilling of the hole.

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32. How will you determine the capacity of a pile?

The allowable load Qa is the safe load which the pile can carry safely

and is determined on the basis of,

Ultimate bearing resistance divided by suitable factor of safety

The permissible settlement

Overall stability of the pile foundation

33. What is the use of batter pile?

The batter piles are used to resist large horizontal forces or inclined

forces.

34. Write about the plate bearing test.

Plate bearing test is a field test to determine the ultimate bearing

capacity of soil, and the probable settlement under a given loading. The test

essentially consists in loading a rigid plate at the foundation level, and

determining the settlements corresponding to each load increment. The

ultimate bearing capacity is then taken as the load at which the plate starts

sinking at a rapid rate. The method assumes that down to the depth of

influence of stresses, the soil stratum is reasonably uniform.