CE2305 FOUNDATION ENGINEERING L T P C

3 0 0 3

OBJECTIVE

At the end of this course student acquires the capacity to assess the soil

condition at a given location in order to sugest suitable foundation and also gains

the knowledge to design various foundations.

UNIT I SITE INVESTIGATION AND SELECTION OF FOUNDATION 9

Scope and objectives – Methods of exploration-auguring and boring – Water boring and

rotatory drilling – Depth of boring – Spacing of bore hole - Sampling – Representative

and undisturbed sampling – sampling techniques – Split spoon sampler, Thin tube

sampler, Stationary piston sampler – Bore log report – Penetration tests (SPT and

SCPT) – Data interpretation (Strength parameters and Liquefaction potential) –

Selection of foundation based on soil condition.

UNIT II SHALLOW FOUNDATION 9

Introduction – Location and depth of foundation – codal provisions – bearing capacity of

shallow foundation on homogeneous deposits – Terzaghi’s formula and BIS formula –

factors affecting bearing capacity – problems - Bearing Capacity from insitu tests (SPT,

SCPT and plate load) – Allowable bearing pressure, Settlement – Components of

settlement – Determination of settlement of foundations on granular and clay deposits –

Allowable settlements – Codal provision – Methods of minimising settlement, differential

settlement.

UNIT III FOOTINGS AND RAFTS 9

Types of foundation – Contact pressure distribution below footings and raft - Isolated

and combined footings – Types and proportioning - Mat foundation– Types, applications

uses and proportioning-- floating foundation.

UNIT IV PILES 9

Types of piles and their function – Factors influencing the selection of pile – Carrying

capacity of single pile in granular and cohesive soil - Static formula - dynamic formulae

(Engineering news and Hiley’s) – Capacity from insitu tests (SPT and SCPT) – Negative

skin friction – uplift capacity – Group capacity by different methods (Feld’s rule,

Converse Labarra formula and block failure criterion) – Settlement of pile groups –

Interpretation of pile load test – Forces on pile caps – under reamed piles – Capacity

under compression and uplift.

UNIT V RETAINING WALLS 9

Plastic equilibrium in soils – active and passive states – Rankine’s theory – cohesionless

and cohesive soil - Coloumb’s wedge theory – condition for critical failure plane - Earth

pressure on retaining walls of simple configurations – Graphical methods (Rebhann and

Culmann) - pressure on the wall due to line load – Stability of retaining walls.

TOTAL: 45 PERIODS

TEXT BOOKS

1. Murthy, V.N.S, “Soil Mechanics and Foundation Engineering”, UBS Publishers

Distribution Ltd, New Delhi, 1999.

2. Gopal Ranjan and Rao, A.S.R. ”Basic and Applied Soil Mechanics”, Wiley Eastern

Ltd., New Delhi (India), 2003.

REFERENCES

1. Das, B.M. “Principles of Foundation Engineering (Fifth edition), Thomson Books /

COLE, 2003

2. Bowles J.E, “Foundation analysis and design”, McGraw-Hill, 1994

3. Punmia, B.C., “Soil Mechanics and Foundations”, Laxmi publications pvt. Ltd., New

Delhi, 1995.

4. Venkatramaiah,C.”Geotechnical Engineering”, New Age International Publishers,New Delhi, 1995

Chapter -1

SITE INVESTIGATION AND SELECTION OF FOUNDATION

Scope:

Site investigations or subsurface explorations are done for obtaining the subsurface conditions at the site of proposed construction.

It includes determination of soil profile at the site, taking samples and determining the engineering properties of the soils, and in-situ testing of the soil.

Site investigation is generally required for every big engineering projects.

Objectives:

Selection of the type and the depth of foundation suitable for a given structure.

Evaluation of the load-bearing capacity of the soil.

Estimation of the probable settlement of a structure.

Establishment of ground water table.

Prediction of lateral earth pressure for structures like retaining walls, sheet pile bulkheads, and braced cuts.

Establishment of construction methods for changing subsoil conditions.

EXPLORATION PROGRAM:

The purpose of the exploration program is to determine, within practical limits, the stratification and engineering properties of the soils underlying the site.

The principal properties of interest will be the strength, deformation, and hydraulic characteristics.

The program should be planned so that the maximum amount of information can be obtained at minimum cost.

STAGES IN EXPLORATION:

Sub – surface explorations are generally carried out in three stages. They are:

Reconnaissance.         

Preliminary exploration.         

Detailed exploration.

Reconnaissance:

Site reconnaissance is the first step in the sub- surface exploration program.

It includes a visit to the site and to study the maps and other relevant records available.

It helps in deciding the method of exploration to be adopted, types of samples to be taken

and in-situ testing of soils.

Preliminary exploration:

The aim of the Preliminary Exploration is to determine the composition of each soil

stratum at the site. The depth of bed rock and ground water table is also determined.

It is done in the form of few borings or test bit to collect the soil.

Test are conducted with cone penetrometers and sounding rods to obtain information

about strength and compressibility of soils.

Geophysical method also used in preliminary exploration for locating the boundaries of

different strata.

DETAILED EXPLORATION:

The purpose of detailed exploration is to determine the properties of soil in different

strata.

It includes an extensive boring program sampling and testing of samples in laboratory.

Field test such as vane shear test, plate load test are conducted for heavy structures such

as bridges, dams, and multi storey buildings it is conducted to find the natural properties

of soil.

For small projects especially at sites where the soil structure is uniform detailed

investigation is not required.

METHODS OF EXPLORATION:

The available methods of exploration can be broadly classified into two categories:

Direct method.

Indirect method.

Direct Methods:

Open Excavation.

Boring Methods.

Indirect Methods:

Seismic Method.

Electric Resistivity Method.

METHODS OF BORING:

Auger boring.

Wash boring .

Rotary drilling.

Auger boring:

Soil auger’ is a device that is useful for advancing a bore hole into the ground.

Augers may be hand-operated or power-driven; the former are used for relatively small

depths (less than 3 to 5 m), while the latter are used for greater depths.

The soil auger is advanced by rotating it while pressing it into the soil at the same time.

It is used primarily in soils in which the bore hole can be kept dry and unsupported.

As soon as the auger gets filled with soil, it is taken out and the soil sample collected.

Two common types of augers, the post hole auger and the helical auger.

If the sides of the hole cannot remain unsupported, the soil is prevented from falling in by

means of a pipe known as ‘shell’ or ‘casing’.

The casing is to be driven first and then the auger; whenever the casing is to be extended,

the auger has to be withdrawn, this being an impediment to quick progress of the work.

An equipment called a ‘boring rig’ is employed for power-driven augers, which may be

used up to 50 m depth (A hand rig may be sufficient for borings up to 25 m in depth).

Casings may be used for sands or stiff clays. Soft rock or gravel can be broken by chisel

bits attached to drill rods. Sand pumps are used in the case of sandy soils.

Water boring:

Wash boring is commonly used for exploration below ground water table for which the

auger method is unsuitable.

This method may be used in all kinds of soils except those mixed with gravel and

boulders.

Initially, the hole is advanced for a short depth by using an auger. A casing pipe is pushed

in and driven with a drop weight.

A hollow drill bit is screwed to a hollow drill rod connected to a rope passing over a

pulley and supportedby a tripod.

Water jet under pressure is forced through the rod and the bit into the hole.

This loosens the soil at the lower end and forces the soil-water suspension upwards along

the annular surface between the rod and the side of the hole.

This suspension is led to a settling tank where the soil particles settle while the water

overflows into a sump. The water collected in the sump is used for circulation again.

The soil particles collected represent a very disturbed sample and is not very useful for

the evaluation of the engineering properties.

Wash borings are primarily used for advancing bore holes; whenever a soil sample is

required, the chopping bit is to be replaced by a sampler.

The change of the rate of progress and change of color of wash water indicate changes in

soil strata.

Rotatory drilling:

This method is fast in rock formations.

A drill bit, fixed to the lower end of a drill rod, is rotated by power while being kept in

firm contact with the hole.

Drilling fluid or bentonite slurry is forced under pressure through the drill rod and it

comes up bringing the cuttings to the surface.

Even rock cores may be obtained by using suitable diamond drill bits.

This method is not used in porous deposits as the consumption of drilling fluid would be

prohibitively high.

        

Depth of Borings:

In order to furnish adequate information for settlement predictions, the borings should

penetrate all strata that could consolidate significantly under the load of the structure.

This necessarily means that, for important and heavy structures such as bridges and tall

buildings, the borings should extend to rock.

For smaller structures, however, the depth of boring may be estimated from the results of

previous investigations in the vicinity of the site, and from geologic evidence.

Experience indicates that damaging settlement is unlikely to occur when the additional

stress imposed on the soil due to the weight of the structure is less than 10% of the initial

stress in the soil due to self-weight.

Based on this, recommended depths of borings for buildings are about 3.5 m and 6.5 m

for single- and two-storey buildings.

For dams and embankments, the depth ranges between half the height to twice the height

depending upon the foundation soil.

According to IS: 1892-1979: “The depth of exploration required depends upon the type

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

area, soil profile and the physical properties of the soil stratum.

Spacing of bore hole:

The lateral extent of exploration and the spacing of bore holes depend mainly on the variation of the strata in the horizontal direction.

The exploration should be extensive so as to reveal major changes in the properties of the sub-surface strata.

For small and less important buildings, even one bore hole or trial pit in the centre may suffice.

But for compact buildings, covering an area of about 0.4 hectares, there should be at least 5 bore holes, one at the centre and four near the corners. As shown in the fig.

For large, multistoried buildings the bore holes should be drilled at all the corners and also at important locations.

The spacing between the bore holes is generally kept between 10 to 30 m, depending upon the variation in the subsurface conditions and loading. As shown in the fig.

For highways, subsurface explorations are usually carried out along the proposed centre line or along the propose ditch line.

The spacing of bore holes usually varies between 150 and 300 m. if the sub-strata is erratic the spacing may be reduced to even 30 m.

In case of concrete dams, the spacing of bore holes generally varies between 40 and 80 m.

Sampling:

Soil Sampling’ is the process of obtaining samples of soil from the desired depth at the

desired location in a natural soil deposit, with a view to assessing the engineering

properties of the soil for ensuring a proper design of the foundation.

The ultimate aim of the exploration methods described earlier, it must be remembered, is

to obtain soil samples besides obtaining all relevant information regarding the strata. The

devices used for the purpose of sampling are known as ‘soil samplers’.

Determination of ground water level is also considered part of the process of soil

sampling.

Types of Samples

Broadly speaking, samples of soil taken out of natural deposits for testing may be classified as:

Disturbed sample

undisturbed samples,

depending upon the degree of disturbance caused during sampling operations.

Disturbed sample:

A disturbed sample is that in which the natural structure of the soil gets modified partly

or fully during sampling.

Disturbed samples may be further subdivided as:

(i) Non-representative samples

(ii) Representative samples.

Non-representative samples

Non-representative samples consist of mixture of materials from various soil or rock

strata or are samples from which some mineral constituents have been lost or got mixed

up.

Soil samples obtained from auger borings and wash borings are non-representative

samples. These are suitable only for providing qualitative information such as major

changes in subsurface strata.

Representative samples.

Representative samples contain all the mineral constituents of the soil, but the structure of

the soil may be significantly disturbed.

The water content may also have changed. They are suitable for identification and for the

determination of certain physical properties such as Atterberg limits and grain specific

gravity.

UNDISTURBED SAMPLES:

Undisturbed sample is that in which the natural structure and other physical properties

remain preserved.

Undisturbed’, in this context, is a purely relative term, since a truly undisturbed sample

can perhaps be never obtained as some little degree of disturbance is absolutely inevitable

even in the best method of sampling devised till date.

Undisturbed samples may be defined as those in which the material has been subjected

to minimum disturbance so that the samples are suitable for strength tests and

consolidation tests.

Tube samples and chunk samples are considered to fall in this category.

Besides using a suitable tube sampler for the purpose, undisturbed samples may be

obtained as ‘chunks’ from the bottom of test pits, provided the soil possesses at least

some cohesion.

Sample Disturbance

The design features of a sampler, governing the degree of disturbance of a soil sample are

the dimensions of the cutting edge and those of the sampling tube, the characteristics of

the non-return valve and the wall friction. In addition, the method of sampling also

affects the sample disturbance.

The walls of the sampler should be kept smooth and properly oiled to reduce wall friction

in order that sample disturbance be minimized. The non-return valve should have a large

orifice to allow the air and water to escape quickly and easily when driving the sampler.

Area ratio is the most critical factor which affects sample disturbance; it indicates the

ratio of displaced volume of soil to that of the soil sample collected.

If Ar is less than 10%, the sample disturbance is supposed to be small. A r may be as high

as 30% for a thick wall sampler like split spoon and may be as low as 6 to 9% for thin

wall samplers like shelby tubes.

The inside clearance, CI, should not be more than 1 to 3%, the outside clearance Co

should also not be much greater than CI.

Inside clearance allows for elastic expansion of the soil as it enters the tube, reduces

frictional drag on the sample from the wall of the tube, and helps to retain the core.

Outside clearance facilitates the withdrawal of the sample from the ground.

The recovery ratio Rr = L/H

Where, L = length of the sample within the tube, and

H = depth of penetration of the sampling tube.

This value should be 96 to 98% for a satisfactory undisturbed sample. This concept is

more commonly used in the case of rock cores.

Sampling techniques:

Split spoon sampler,

Thin tube sampler,

Stationary piston sampler.

Split-Spoon Sampler

The split spoon sampler is basically a thick-walled steel tube, split length wise. The

sampler as Standardized by the I.S.I. (IS: 2131-1986—Standard Penetration Test for

soils) is shown in Fig.

A drive shoe attached to the lower end serves as the cutting edge. A sample head may be

screwed at the upper end of split spoon.

The standard size of the spoon sampler is of 35 mm internal and 50.8 mm external

diameter.

The sampler is lowered to the bottom of the bore hole by attaching it to the drill rod.

The sampler is then driven by forcing it into the soil by blows from a hammer.

The assembly of the sampler is then extracted from the hole and the cutting edge and

coupling at the top are unscrewed. The two halves of the barrel are separated and the

sample is thus exposed. The sample may be placed in a glass jar and sealed, after visual

examination.

If samples need not be examined in the field, a liner is inserted inside the split spoon.

After separating the two halves, the liner with the sample is sealed with wax.

Thin tube sampler:

Thin-walled sampler, as standardised by the ISI (I.S.: 2132-1986 Code of Practice for

Thin-walled Tube Sampling of Soils), is shown in Fig.

The length of the tube is 5 to 10 times the diameter for sandy soils and 10 – 15 times the

diameter for clayey soils.

The diameter generally varies between 40 and 125 mm, and the thickness varies from

1.25 to 3.15 mm.

The sampler tube is attached to the drilling rod and lowered to the bottom of the bore

hole.

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.

The tube should be pushed to the length provided for the sample. At least 5 minutes after

the tube into its final position, the tube is turned 2 revolutions to shear off the sample at

the bottom before it is withdrawn.

The tube is taken out and its ends are sealed before the transportation. These are used for

obtaining undisturbed samples of clay.

Thin walled sampler.

Stationary piston sampler:

A piston sampler 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 desired depth, the piston is prevented from

moving downward suitable arrangement, which differs in different types of piston

samplers.

The thin tube sampler is pushed past the piston to obtain the sample. The piston remains

in close contact with the top of the sample.

The presence of the piston prevents rapid squeezing of the soft soils into the tube and

reduces the disturbance of the sample.

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

During the withdrawal of the sampler, the piston provides protection against the water

pressure which otherwise would have occurred on the top of the sample.

These are used for obtaining undisturbed samples of soft and sensitive clays.

Bore log report:

A detailed record of boring operation and other tests carried out in the field is an essential

part of the field work.

The bore hole log is made during the boring operations. The soil is classified based on the

visual examination of the disturbed samples collected.

A typical example of a bore hole log is given below. The log should include the

difficulties faced during boring operations including the occurrence of sand boils, and the

presence of artesian water conditions if any, etc.

PENETRATION TESTS:

Subsurface soundings are used for exploring soil strata of an erratic nature. They are

useful to determine the presence of any soft pockets between drill holes and also to

determine the density index of cohesionless soils and the consistency of cohesive soils at

various desired depths from the ground surface.

Methods of sounding normally consist of driving or pushing a standard sampling tube or

a cone. The devices involved are also termed ‘penetrometers’,

since they are made to penetrate the subsoil with a view to measuring the resistance to

penetration of the soil strata, and thereby try to identify the soil and some of its

engineering characteristics. The necessary field tests are also called ’penetration tests’.

If a sampling tube is used to penetrate the soil, the test is referred to as the Standard

Penetration Test (SPT, for brevity).

If a cone is used to penetrate the soil, the test is called a ‘Cone penetration test’. Static

and dynamic cone penetration tests are used depending upon the mode of penetration

static or dynamic.

Standard Penetration Test (SPT)

The Standard Penetration Test (SPT) is widely used to determine the parameters of the

soil

in-situ. The test is especially suited for cohesionless soils as a correlation has been

established between the SPT value and the angle of internal friction of the soil.

The test consists of driving a split-spoon sampler into the soil through a bore hole 55 to

150 mm in diameter at the desired depth.

A hammer of 640 N (65 kg) weight with afree fall of 750 mm is used to drive the

sampler. The number of blows for a penetration of 300 mm is designated as the “Standard

Penetration Value” or “Number” N.

The test is usually performed in three stages. The blow count is found for every 150 mm

penetration. If full penetration is obtained, the blows for the first 150 mm are ignored as

those required for the seating drive.

The number of blows required for the next 300 mm of penetration is recorded as the SPT

value. The test procedure is standardized by ISI and set out in “IS: 2131-1986 Standard

Penetration Test”.

Usually SPT is conducted at every 2 m depth or at the change of stratum. If refusal is

noticed at any stage, it should be recorded.

In the case of fine sand or silt below water-table, apparently high values may be noted

for N. In such cases, the following correction is recommended (Terzaghi and Peck,

1948):

where N′ = observed SPT value,

and N = corrected SPT value.

For SPT made at shallow levels, the values are usually too low. At a greater depth, the

same soil, at the same density index, would give higher penetration resistance.

The effect of the overburden pressure on SPT value may be approximated by the equation:

where N′ = observed SPT value,

N = corrected SPT value, and

σ = effective overburden pressure in kN/m2

Static Cone Penetration Test (Dutch Cone Test)

The Static cone penetration test, which is also known as Dutch Cone test, has been

standardized by the ISI and given in “IS: 4968 (Part-III)-1976—Method for subsurface

sounding for

Soils Part III Static cone penetration test”.

Among the field sounding tests the static cone tests in a valuable method of recording

variation in the in-situ penetration resistance of soils, in cases where the in-situ density is

disturbed by boring operations, thus making the standard penetration test unreliable

especially under water. The results of the test are also useful in determining the bearing

capacity of the soil at various depths below the ground level. In addition to bearing

capacity values, it is also possible to determine by this test the skin friction values used

for the determination of the required lengths of piles in a given situation.

The static cone test is most successful in soft or loose soils like silty sands, loose sands,

layered deposits of sands, silts and clays as well as in clayey deposits.

In areas where some information regarding the foundation strata is already available, the

use of test piles and loading tests thereof can be avoided by conducting static cone

penetration tests.

Experience indicates that a complete static cone penetration test up to depths of 15 to 20

m can be completed in a day with manual operations of the equipment, making it one of

the inexpensive and fast methods of sounding available for investigation.

The equipment consists of a steel cone, a friction jacket, sounding rod, mantle tube, a

driving mechanism and measuring equipment.

The steel cone shall be of steel with tip hardened. It shall have an apex angle of 60° ± 15′

and overall base diameter of 35.7 mm giving a cross-sectional area of 10 cm2. The

friction jacket shall be of high carbon steel. These are shown in Fig. 18.9.

The sounding rod is a steel rod of 15 mm diameter which can be extended with additional

rods of 1 m each in length. The mantle tube is a steel tube meant for guiding the sounding

rod which goes through it. It should be of one metre in length with flush coupling.

The driving mechanism should have a capacity of 20 to 30 kN for manually operated

equipment and 100 kN for the mechanically operated equipment. The mechanism

essentially consists of a rack and pinion arrangement operated by a winch. The reaction

for the thrust may be obtained by suitable devices capable of taking loads greater than the

capacity of the equipment.

The hand operated winch may be provided with handles on both sides of the frame to

facilitate driving by four persons for loads greater than 20 kN.

For the engine driven equipment the rate of travel should be such that the penetration

obtained in the soil during the test is 10 to 15 mm/s.

Hydraulic pressure gauges should be used for indicating the pressure developed.

Alternatively, a proving ring may also be used to record the cone resistance. Suitable

capacities should be fixed for the gauges.

Basically, the test procedure for determining the static cone and frictional resistances

consists of pushing the cone alone through the soil strata to be tested, then the cone and

the friction jacket, and finally the entire assembly in sequence and noting the respective

resistance in the first two cases.

The process is repeated at predetermined intervals. After reaching the deepest point of

investigation the entire assembly should be extracted out of the soil.

The results of the test shall be presented graphically, in two graphs, one showing the cone

resistance in kN/m2 with depth in metres and the other showing the friction resistance in

kN/m2 with depth in metres, together with a bore hole log.

The cone resistance shall be corrected for the dead weight of the cone and sounding rods

in use. The combined cone and friction resistance shall be corrected for the dead weight

of the cone, friction jacket and sounding rods.

These values shall also be corrected for the ratio of the ram area to the base area of the

cone.

The test is unsuitable for gravelly soils and for soils with standard penetration value N

greater than 50. Also, in dense sands the anchorage becomes too cumbersome and

expensive and for such cases dynamic cone penetration tests may be carried out.