M.A.M.College of Engineering, Trichy

Department of Civil Engineering

CE2305 – Foundation Engineering.

UNIT – 1 Site Investigation and selection of foundation.

Question Bank

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PART A

1. List the various methods of soil exploration techniques.

1. Pits and trenches

2. Boring a) augur boring b) wash or water boring c) rotary boring d) percussion boring

3. Geophysical methods a) seismic refraction b) electrical resistivity

4. Standard penetration test

5. Static cone penetration test

2. Write short notes on Augur boring.

An augur is a type of tool which is used for understanding the characteristics of the subsurface

soil. Generally there are two types of augurs,

a) Manually operated augur

b) Mechanically operated augur

3. Define standard penetration number.

The number of blows required to penetrate 300 mm of the split spoon sampler beyond a seating

drive of 150mm is known as penetration number (N).

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

The two corrections are

a) Dilatancy correction ( Silty sand)

b) Over burden pressure correction ( Granular soil)

5. What are the uses of soil exploration?

a) To select type and depth of foundation for a given structure

b) To determine the bearing capacity of the soil of the selected foundation

c) To investigate the safety of the existing structure

d) To establish ground water level

6. What is soil exploration?

The process of collection subsoil sample by an appropriate method to a needed depth and check

those samples for knowing the properties is called soil exploration.

7. List the different types of samplers.

a) Standard split spoon sampler

b) Shelby and thin walled tube sampler

c) Denison sampler

d) Piston sampler

e) Scraper bucket sampler

8. List the various parameters affecting the sampling disturbance.

a) Area ratio

b) Inside clearance

c) Outside clearance

d) Inside wall friction

e) Position of non return wall

f) Recovery ratio

g) Methods of applying force

9. Write the advantages of SCPT over SPT.

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

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

c) Engineering properties of the soil like permeability, Shear strength, Compressibility can be

evaluated.

10. 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,

a) Type of soil

b) Fluctuation of water table

c) Load coming from structure

d) Importance of the structure.

e) Economical feasibility.

PART B

1. Write a detailed note on various samplers.

a) Standard split spoon sampler

This is a most commonly used sampler for obtaining disturbed soil samples. It is mostly used

in SPT test.

The sampler essentially consists of three parts:

1. Driving shoe of 75mm length with internal dia 38mm and external dia 50mm.

2. Split steel tube of 450mm length which can be split into two halves.

3. Coupling at top which is 150mm length. The coupling head is provided with a ball check

and 4 venting part of 10mm dia to make easy of sample recovery.

After the bore hole of sufficient depth is made, the sampler is lowered into the bottom of the

hole. It is driven into the soil by hammering action. Suppose if there is any presents of fine

sand below water table, the sampling become difficult and use of spring catcher is necessary.

This type of sampling gives highly disturbed soil and it is possible to determine only the

index properties of the soil sample.

b) Shelby and thin walled tube sampler

It is made up of steel. The outer dia of the tube may be between 40mm and 125mm.

The commonly used sampler has the outer dia of either 50.8mm or 76.2mm. The bottom of

the tube acts as a cutting edge. The area ratio will be less than 15% and the inside clearance

varies between 0.5 and 3%.

The length of the thin walled tube sampler is 5 to 10 times the dia for sandy soil and 10 to 15

times the dia for clayey soil. The dia generally varies between 40 to 125mm. the thickness

varies between 1.25 to 3.15mm. The sampler tube is attached to the drilling rod and lowered

to the bottom of tube. It is then pushed into the soil.

Care shall be taken to push the tube into the soil by a continuous rapid motion without impact

or twisting. At least 5minutes after pushing the tube into the final position, it turned two

revolutions and to shear the soil sample at the bottom of the hole, before the sampler is

withdrawn.

c) Piston sampler

It consists of a thin walled tube with a piston inside. The piston keeps the lower end of the

sampling tube closed when the sampler is lowered to the bottom of the hole. After the

sampler has been lowered to the desire depth the piston is prevented from moving downwards

by suitable arrangements. The thin tube sampler is pushed downwards to obtain the sample.

The piston remains in close contact with the top of the sample. The presence of piston

prevents the rapid squeezing of the soft soil into the tube and reduced the disturbance of the

sample. A vacuum is created on the top of the sample which helps in retaining the sample.

They are used for obtaining undisturbed soil sample.

2. Write a detailed on various types of boring.

a) Augur boring

b) Wash (or) water boring

c) Percussion boring

a) Augur boring :

An augur is a type tool which is used for understanding the characteristics of the sub surface

soil.

Generally there are two types of Augurs,

i) Manually operated Augurs (upto 6 m)

ii) Mechanically operated Augurs (upto 12m)

Augur boring is highly useful to understand the subsurface soil profile for constructing

highways, railways, Airport etc., where the depth of exploration is small.

It is best suited for partially saturated soil. It is highly unsuitable if Boulders are present

below the ground.

The soil sample obtained will be highly disturbed.

b) Wash (or) Water Boring :

Soil exploration by wash boring is done by the following ways:

1. A casing pipe of 2 to 3 m length is inserted into soil.

2. A hollow drill rod is inserted inside the casting.

3. Water is allowed inside the drilling rod from water pipe and the water emerges out at the

bottom of drilling rod through chopping bit with pressure.

4. The water pressure lubricates the soil below and the soil is transformed into soil slurry

paste and it rises through the annular space between the casing and the drilling rod.

5. The disintegrated soil with water will be collected separately in a tub.

6. The continuous chopping and jetting action will be done till sufficient depth of boring is

reached.

c) Percussion boring :

This type of boring is highly suitable at the places where Boulders, rocks are present.

A vertical hole is made is heavy chisel is allowed to drop inside the vertical hole continuously

and boring of sufficient depth will be reached.

3. Explain SPT test in detail.

SPT is inside test especially for cohesion less soil which cannot be easily sampled. They

are used to find the relative density & angle of shearing resistance of cohesion less soil. The SPT

is conducted in a box hole using Standard Split Spoon samplers. When the bore hole has been

drilled to the desired depth, the drilling tools are removed & the sample is lowered to the bottom

of the hole. The sample is driven into the soil by drop hammer of weight 63.75kg mass falling

from height of 750mm at the rate of 30 blows per minute. The no of hammer blows necessary to

driven 150mm of the sample is counted, the sample is further driven by 150mm & the no of

blow is recorded likewise the sampler is once again driven by 150mm & the no of blows is

recorded. The number of blows recorded for the first 150mm will not be taken for the

calculations.

The number of blows recorded for the last 150 intervals all added to give the Standard

Penetration Number. The Standard Penetration Number is equal to the number of blows

necessary for 300mm penetration beyond sealing drive of 150mm.Its notation is "N". If no of

blows for 150mm driven exceeds so the test is discontinued.

The SPT is corrected for:

1) Dilatancy correction

2) Overburden pressure correction

1) Dilatancy Correction

Silty, fine sand & fine sand present below water table develops pore pressure which

cannot be easily dissipated. The pore pressure increases the resistance of soil & hence SSN.

Terzaghi & Peck recommended the following correction in the case of silty sand

Where,

NC=corrected value

NR=observed value

If NR less than or equal to 15 then NC = NR

2) Overburden pressure Correction

In granular soil the Overburden Pressure affects the penetration resistance. If two soils

having the same relative density but with different confining pressure are tested, the with high

confining pressure gives a higher penetration number for soil .as a confining pressure in

cohesion less soil increases with depth, the penetration number for soil at shallow depth is under

estimate & that of deeper depth is over estimated for uniformly the N value obtained from field

test under different effective Overburden Pressure are corrected to a standard effective

Overburden Pressure.

Gibbs & Holts recommended the use of the following equation for dry or moist clean

sand,

NC =NR * 350 / σ+70

Where,

NC = corrected value

NR = recorded values

σ = effective O.P. in KN/mm2

The above equation is applicable if σ less than or = 280 KN/mm2

The ratio of NC/NR should lie between 0.45 and 2. If ratio NC/NR is greater than 2,

NC should be divided by 2 to obtain the design value used in finding the bearing capacity of soil.

The corrected given by Bazaara & also by Peck & Bazaara is one of the commonly

used correction.

1) N = NR

If σ = 71.8kN/m2

2) If σ < 71.8KN/m2

Then,

N = 4NR / (1+0.0418 σ)

3) If σ > 71.8 KN/m2

Then,

N = 4NR / (3.25+0.0104 σ)

4. Explain the various parameters which affect the sampling in detail.

a) Area ratio (Ar)

It is defined as the ratio of maximum cross section area of cutting edge to the area of the soil

sample.

Ar = Maximum cross sectional area of the cutting edge x 100

Area of soil sample

Ar = (D22-D1

2) x 100 / D2

2

For good quality undisturbed sample the area ratio should be 10 % or less than 10 %.

b) Inside clearance (Ci)

Ci = (D3-D1) X 100 / D1

The inside clearance allow the elastic expansions of soil when enter the tube.

For obtaining good quality of undisturbed soil sample, the inside clearance should be between

0.5 – 3%.

This is also reduces the frictional drag on the soil sample.

c) Outside clearance (Co)

C0 = (D2-D4) x 100 / D4

The outside clearance reduces the force of application on the sample while driving. For

obtaining good quality undisturbed soil sample it’s the value be between 0 – 2%.

d) Inside wall friction:

Greater the inside wall friction, the lesser will be the amount of undisturbed soil sample

obtained. The inside wall friction can be reduced by applying oil inside the sampling tube

before driving.

e) Position of non return wall

The non return wall present in the sample should contain a larger orifice which permits the

escape of air, water, slurry to escape the sampling tube during driving and orifice should be

close as soon as the sampler is withdrawn.

f) Methods of applying force

Good quantity of undisturbed soil sample depends upon the methods of applying force. The

sampler should be pushed and not driven.

g) Recovery ratio (Lr)

It is the ratio of recovered length of sample to the penetration length of sample.

Lr = recovered length of sample

Penetration length of sample

When, Lr = 1 indicates good recovery

Lr < 1 indicates soil is compressed

Lr > 1 indicates soil is swelled.

5. Explain the Geophysical methods.

Geophysical method is one of the methods for exploration of soil. In which way, electricity are

used for exploration. Based on above, it classified into two types, they are,

a) Seismic refraction

b) Electrical resistivity

a) Seismic refraction method :

This method is devised based on fact that seismic wave have different velocity in different type

of soil & rock stratums. Further, the waves are refracted when they are cross the boundary

between different types of soil. The method enables the determination of general soil type & the

approximate depth of boundaries of strata.

The method consists of inducing impact or generating shock by exploding a small charge at or

near the ground surface. The radiating shock waves are recorded by device called geophone

which records the time of travel of the waves. The geophones are installed at suitable known

distance on the ground in line from the source of shock or the same is moved away from the

geophone to produce shock waves at given intervals.

As the distance between the shock source & the geophone increases the refracted waves reach

the geophone earlier than the direct waves. The arrival time is plotted against the distance

between the source & the geophone .the depth of boundary between the two strata can be

estimated from the equation:

D = (d/2). ((v2-v1)/ (v2+v1))^ (1/2)

b) Electrical resistivity method :

The electrical resistivity method is based on the measurement & recording of changes in the

mean resistivity or apparent specific resistance of various soils. The resistivity is usually defined

as the resistant between the opposite forces of a unit cube of the material. Significant variations

in resistivity can be detected between different type of soil strata, above & below the water table

between unissued rocks & soils between voids & soil or rocks.

The test is carried out by driving 4 metal spikes to serve as electrodes into the ground along a

straight line into equal distances current from a battery , flows though a soil between the outer

electrodes , producing an electrical field within the soil. The potential difference (E) between the

two inner electrodes is then measured .the apparent resistivity is given by the equation:

Resistivity, p = 6.28DE / I

Where,

D in cm, E in volts, I in amperes & p in ohm-cm.

CE2305 – Foundation Engineering.

UNIT – 2 Shallow Foundation.

Question Bank

1. Safe Bearing Capacity:

Maximum intensity of loading that the foundation will safely carry without the risk of

shear failure of soil irrespective of any settlement that may occur.

As per Rankine’s formula

D min= P/W[(1-𝑠𝑖𝑛∅)/(1+𝑠𝑖𝑛∅)]2

Where,

P-safe bearing capacity in N/m2

W = Unit weight of soil in KN / m3

∅-Angle of repose of soil in degrees

2. List the types of bearing capacity failure:

Vesic (1973) classified the bearing capacity failure into 3 categories

1. General shear failure

2. Local shear failure

3. Punching shear failure

3. Write the components of settlement:

There are three types of settlements are

1. Immediate (or)elastic settlement(Si)

2. Consolidation settlement(Sc)

3. Secondary consolidation settlement(Ss)

4. Write the assumptions in Terzaghi’s bearing capacity theory:

1. The base of the footing is rough

2. The footing is laid at a shallow depth (Df<B) or (Df=B)

3. The load on footing is vertical and uniformly distributed

4. The shear strength of soil is governed by mohr’scolumb equations

5. Z=C+𝜎tanϕ

The ratio of L/B is infinite.where L is the the length and B is the width of footing.

5. List the limitations of plate load test:

1. Size effect

2. Scale effect

3. Time effect

4. Interpretation of failure load

5. Reaction load

6. Water table

6. List the various methods of computing elastic settlement:

1. Based on the theory of elasticity

2. Pressure meter method

3. Janhu –Bjerram method

4. Schmentmann’s method

7. Define general shear failure:

At a certain load intensity equal to qu (ultimate bearing capacity), the settlement increases

suddenly .A shear failure occurs in the soil at the load and failure extend to the ground

surface. This type failure is known as general shear failure. A heave on the sides is always

observed in general shear failure. This type of failure occur when a strip footing rest on stiff

clay or a dense sand.

8. Immediate settlement (Si):

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.

9. Consolidation settlement (SC):

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

10. Write the limitations of Terzaghi theory:

1. Terzaghi theory is valid only for shallow fountations (Df<B or Df=B) .

2. The load on the footing won’t be vertical and uniformly distributed as assumed by

Terzaghi.

Types of shear failure:

Vesic (1973) classified bearing capacity failures into three categories:

General shear failure:

Fig (a) shows a strip footing resting on the surface of a dense sand or a stiff

clay.

The figure also shows the load settlement curve for the footing, where “q” is

the load per unit area and “s” is the settlement. At a certain load intensity

equal to qu(ultimate bearing capacity),the settlement increases suddenly.

A shear failure occurs in the soil at the load and the failure surfaces extent to

the ground surfaces.

This type of failure is known as general shear failure.

A heave on the sides is always observed in general shear failure.

Local shear failure:

Fig(b) shows strip footing resting on a medium dense sand or on a clay of

medium consistency.

The figure also shows the load settlement curve.

When the load is equal to certain value qu, the foundation movement is

accompanied by sudden jerks.

The failure surfaces gradually extend outwards from the foundation

However a considerable movement of the foundation is required for the

failure surfaces to extend to the ground surface.

The load at which this happens is equal qu. Beyond this point, An increase of

load is accompanied by a large increase in settlement.

This type of failure is known as local shear failure.

A heave is observed only when there is substantial vertical settlement.

Punching Shear failure:

Fig(c) shows a strip footing resting on a loose sand or a soft clay.

In this case, the failure surfaces do not extend up to the ground surface.

There are jerks in foundation at a load of qu the footing fails at load of qu at

which stage load-settlement curve becomes steep and practically linear.

This type failure is known as punching shear failure.

No heave is observed. There is only vertical movement of footing

Vesic proposed a relationship for the mode of failure based on the relative

density Dr and Df/ B*, where B* = 2B (L/(B+L)), in which B is the width of

the footing L is the length of footing.It is worth nothing that even for the same

relative density (Dr), the mode of failure may change with a change in Df/B*

ratio.

It is generally observed that for shallow foundation, the ultimate load occurs

at a foundation settlement of 4 to 10% of B in the case of general shear

failure, and at a settlement of 15 of 25% of B in local or punching shear

failure.

PLATE LOAD TEST:

The allowable bearing pressure can be determined by conducting a plate load test

at the at the site. The conduct a plate load test a pit of size 5Bp *5Bp where, Bp is

the size of the plate, is excavated to a depth equal to depth of foundation ( Df).

The size of plate is usually 0.3m square.

It is made of steel and is 25mm thick occasionally circular plates are used.

A centre hole of the size Bp* Bpis excavated in the pit. The length of the centre

hole is obtained from the following relation. (Dp/Bp)=(Df/Bf)

Dp=(Dp/Bp) *(Df)

where,

Bp=width of plate

Bf=width of foundation

PROCEDURE:

For conducting plate load test. The plate is placed in the centre hole and the load

is apply by hydraulic jack.

The reaction to the hydraulic jack is provided by the reaction beam.

The setting load of 7KN/m2 is first applied, which is released after some time.

The load is then applied in increment of about 20% of a safe load or 1/10th

of the

ultimate load.

The settlement is recorded after 1,5,10,20,40,60 minutes and further after can be

interval of 1 hour.

These hourly observation are contained for days soil until the rate of settlement is

less then 0.2mm/hour.

The test conducted until failure are atleast the settlement of 25mm.

The ultimate for the plate qu (p) is indicated by a break on the log-log plat

between the load intensity (q) and the settlement(s).

The ultimate bearing capacity of the proposed foundation qu (f) can be obtained

from the following relation.

(a) For clays soils qu (f ) = qu (p)

(b) For sandy soils qu (f ) = qu (p) *(Bf/Bp)

The place load is also used determine the settlement for a given intensity of

loading (q0).

The relation between settlement of the plate (Sp) and that of foundation (Sf) for

the same load intensity are given below

(i) For clays soils Sf =Sp*(Bf/Bp) (ii) Sp can be obtained from load intensity

settlement curve q0.

(iii) For sandy soils Sf= Sp [ (Bf*(Bp=0.3)) / (Bp*(Bf=0.3)) ]

q0=(Q/Af)

Q= loss

Af = Area of footing

S = [ (1-u2) / Es] * I * qB

Plot between the settlement and the load qb. The slope of the line is equal to

(1-u2) * (I / Es).

Plate Load Test

`Limitations of plate load test:

Size effect

Scale effect

Time effect

Interpretation of failure load

Reaction load

Water table