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E4-E5 Civil (Technical) Rev date: 20-04-11

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Chapter-3

AN OVERVIEW

OF

SOIL MECHANICS

AND

FOUNDATION DESIGN

R.N. Yadav, SDE (BS-C), 9412739253(M) E-mail – [email protected]



An Overview of Soil Mechanics and Foundation Design

1.0 Introduction Geotechnica l Engineering is a re lat ively modern branch of

civil engineer ing. As a disc ipline, it is academica lly as excit ing as

practically cha llenging ‘Geotechnica l engineer ing’ is actually the new

name o f a sub ject known earlier as ‘Soil Mechanics and Foundation

Engineer ing’. Of this, foundation engineering, at least as an art, is as

anc ient as c ivil engineering whereas the roots o f So il Mechanics, which

fo rms its scient if ic base, can be traced only from 1773 with Coulomb’s

law for shear strength of soil given in tha t year. Subsequent

contr ibutions were few up to the year 1925, which was the b irth of

modern So il Mechanics with the publication of Terzaghi’s celebrated

book ‘Erdbaumechanik . Professor Kar l von Terzaghi, who is r ight ly,

regarded as the father of modern Soil Mechanics.

Befo re designing a foundation for a structure it is

essent ial to know the behavior of so ils under loads. For study o f

behavior of soils in depth knowledge o f soil mechanics is required. It

is essent ial to associate the structu ral engineer in drawing up the so il

invest igation programme and interpretat ion of the report. He must vis it

the site to facilitate proper scrutiny of the so il investigat ion report by

comparing the resu lts and the recommendat ion with the information

ava ilab le from similar sites and constructed p ro jects.

2.0 Field Identification Of Soils

Soil gra ins consist of inert rock minera ls (cobb le, grave l, sand ,

silt) , o ften combined with s ignificant amounts of cla y (say, more than

5 percent). While inert silt grains ma y be angu lar or rounded (thus

contr ibuting to greater or less angle o f interna l fr ict ion, Ø), partic les

of clay are small plate lets with negat ive charges on both faces which

attract the posit ively charged ends of water molecules. This bond is

responsible for the cohesion ends of water molecules. This bond is

responsible for the cohesion ‘C’ of clay. Silt or sand with appreciable

amounts of clay (say, more than 15 percent) behaves like clayey so il

since the permeab ilit y of c lay is o f the o rder of 10-7

cent imeters/second compared to 10cms/ second for sand. This capacit y

of the clay to ho ld the water molecules for long even when p ressure is



applied on the so il, great ly influences it s behavior i.e. shears strength,

compressibilit y and permeabilit y.

2.1 Simple and Quick Methods of Field Identifica tion of Soils:

(i) Fine sand is differentia ted from silt by placing a spoonful of so il

in a glass jar or test tube, mixing with water and shaking it to a

suspension. Sand sett les f irst, fo llowed by s ilt which may take

abou t five minutes. This test may also be used fo r cla y which

takes more than 10 minutes to start sett ling. The percentages of

clay, s ilt and sand are assessed by observing the depths of the

sediments.

(ii) Silt is different iated from clay as follows:-

(a) Cla y lumps are more d iff icult to crush with f ingers than

silt . When moistened , the soil lump surface textu re is fe lt

with the f inger. If it is smooth, it is clay; if rough, it is

silt .

(b) A ball o f the so il is formed and shaken horizonta lly on the

palm of the hand. If the materia l becomes shiny from water

coming to the su rface, it is silt .

(c) If soil conta ining appreciab le percent c la y is cut with a

knife, the cut surface appears lustrous. In case o f s ilt, the

surface appears du ll.

(iii) Field : ind icat ion for the consistency of cohesive soils are as

fo llows:-

Stiff : Cannot be moulded with in the figure –

Medium: Can be moulded by the f ingers on strong pressure.

Readily indented with thumb nail.

Soft : Easily moulded with the fingers.

(iv) Co lo r of the so il ind icates it s origin and the condit ion under

which it was deposited.Sand and gravel deposits ma y contain

lenses of s ilt , cla y or even organic deposits. If so, the

presumptive bearing capacit y is reduced.Based on the f ield

ident if icat ion of the soil, the presumptive bearing capacit y of the

soil can be guessed b y referr ing to table 2 of IS 1904 –1986. The

object ives o f preliminar y soil invest igat ion are to drawn up an

appropriate p rogram for detailed so il invest igat ion and to

examine the sketch p lans and preliminary drawings prepared by

the Architect from the point of suitabilit y o f the proposed

structu re.



TABLE 1 : SAFE BEARING CAPACITY

S. No.

TYPE OF ROCKS/ SOILS

SAFE BEARING

CAPACITY

REMARKS

(1) (2) (3) (4)

a) Rocks kN/ m2

1. Rocks (hard) wi thout laminat i on defects, for example, gr ani t e, t rap and d iori t e

3 240 -

2. Laminat ed rocks, for example, st one and l imest one in sound condi t ion

1 620 -

3. Residual deposi ts of sh at ter ed and broken bed rock and hard shale, cemented mate rial

880 -

4. Soft r ock 440 -

b) Non-cohesi ve soi ls

5. Gravel , s and and gravel , compact and offer in g high resi stan ce t o penetr at ion wh en excavated by tools

440 (See Note 2)

6. Coarse sand, compact and dry 440 Dry means that the ground water level is at a depth n ot less than the width of foundat i on below the base of the found at ion

7. Medium sand, compact and dry 245 -

8. Fine sand, si te(dry lumps easi l y pul veri zed by the fin ger s)

150 -

9. Loose gravel or sand gravel mi xtures, loose coar se t o medium sand, dry

245 (See Note 2)

10 . Fine sand, loose and dry 100 -

c) Cohesi ve so i ls

11 . Soft shale, or st i f f clay in deep bed , dry 440 This group is susceptible to

long term consolidation

settlement

12 . Medium cl ay, r eadi l y ind ented with a thumb nai l

245 -

13 . Moist cl ay and sand -cl ay mi xture which can be indented with st ron g thumb pressure

150 -

14 . Soft cl ay ind ented with moderate thu mb pressure

100 -

15 . Very soft clay whi ch can be pen etrated several cent i meter s wi th the thumb

50 -

NOTE : – Values ar e ver y much rough fo r the following reasons: a) Effect o f characterist ics of foundations ( that is, effect of dep th,

width, shape, roughness, etc) has not been considered. b) Effect of range o f soil properties ( that is, angle of fr ict iona l

resis tance, cohesion, water table, density, etc) has no t been considered.

c) Effect of eccentr icit y and indicat ion of l oads has not been considered.



3.0 Soil Mechanics –Basic Concepts 3.1 Soil Mass –Represented By 3-Phase System: - Soil so lids, water and air are co nst ituents of soil mass are represented diagrammatically as three phase system shown below.

Vs =Volume of soil solids. W s =Weight o f so il solids.

Va =Volume of air. Wa =Weight of a ir considered as negligib le.

Vw =Volume of water. Ww =Weight of water. V=Total volume of soil mass = Vs+ Va + Vw

W=Total Weight of soil mass = W s+

Ww

1) Water content. : The water content w, also called the moisture content, is

defined as ratio of weight of water & weight of soil solids.

w = Weight of water x 100 Weight of soil so lids The water content is genera lly expressed as a percentage.

2) Unit Weights: The weight o f soil per unit volume is defined as

unit weight or specific weight. In SI units is expressed as N/m3

or kN/m3. In so il Engineer ing five different f ive unit weights are

used in var ious computations.

i) Bulk Unit Weight (γ) .

The bulk unit weight is the total mass W of the soil per unit of its total

volume.

Thus, γ = W V

ii) Dry Unit Weight (γd) . : The dry u nit weight is the weight of

soil solids per units total volume of the soil mass.



γd = Ws

V

The dry u nit weight is used to express the denseness of the soil. ii i) Unit Weight o f Soil Solids (γ s) : The unit weight of so il solids

is the mass of soil solids (w s) per units of volume of so lids (Vs) :

γ s = W s

Vs

iv) Saturated Unit Weight (γ s at) : When the soil mass is saturated,

its bulk unit weight (γ) is ca lled satu rated unit weight. The

saturated unit weight is the rat io of the to tal soil mass of

saturated sample to its tota l volume.

γs at = W s (saturated)

V

v) Submerged Unit Weight (γ’): When the so il exits below water it

is in submerged condit ion. The submerged unit weight (γ’) of

soil is defined as the submerged weight per unit tota l vo lume.

γ’ = W s ub = γ s at - γw

V

3. Specific gravity G : is defined as the ra t io o f the unit weight of

soil solids to that o f water:

G = γ s / γw 4. Voids ra tio . (e) Voids ratio e o f a given soil sample is the

ratio o f the volume o f vo ids to the vo lume of so il solids in the

given soil mass.

Thus, e = V v/V s = n / 1-n 5. Porosity. (n) The porosit y n of a given so il sample is the ra t io

of the vo lume of vo ids to the total vo lume of the given soil mass.

Vv e

n = = V e +1



The vo ids rat io e is genera lly expressed as a fract ion, while the

porosity n is expressed as a percentage and is, therefore a lso referred

to as percentage vo ids.

6 Degree of Saturation . The degree of satu ration Sr is defined as

the rat io of the volume of water p resent in a given so il mass to

the total volume of vo ids in it .

S r = Vw

Vv 6 . Various Inter-Relations i) e. S r = w.G ii) e = w.G (for Sr = 1 or fully saturated soil degree of

saturation 100% )

G . γw i i i) γd = 1 + e

iv) ( G + e.S r ) γw γ = 1 + e

v) For S r = 0 , G . γw

γ = γd = 1 + e

vi) For S r = 1 , γ = γ sa t = ( G + e)γw

1 + e

vi) γ γd = 1 + w

vii) γ’ = (G -1)γw 1 + e

7. Density Index : The term densit y index ID or relative densit y or

degree of densit y is used to express the re lat ive compactness o f a

natural soil deposit. The densit y index is defined as the rat io of

the difference between the vo ids ratio of the so il in it s loosest

state and its natural voids rat io (e) to the difference between the

voids rat ios in the loosest and densest states:

emax - e ID emax – emin



where emax = voids ratio in the loosest state

emin = voids ratio in the densest state

e = natural voids ratio of the deposit.

This term is used for cohesion less spoil only. When the natural state of the

cohesion less soil is in its loosest form e = emax and hence ID = 0. When the natural

deposit is in its densest state e = emin and hence ID = 1.

4.0 Plasticity Characteristics of Soils

Plastic it y of so il is its abilit y to undergo deformation without

cracking or fracturing. Plastic it y is an important index property of f ine

grained so ils, especially clayey so ils.

Fine grained soil may be mixed with water to form a plastic paste which can

be moulded into any form by pressure. The addition of water reduces the cohesion

making the soil still easier to mould. Further addition of water reduces the cohesion

until the material no longer retains its shape under its own weight, but flows as a

liquid. Enough water may be added until the soil grains are dispersed in a suspension.

If water is evaporated from such a soil suspension, the soil passes through various

stages or states of consistency. In 1911,the Swedish agriculturist Atterberg divided

the entire range from liquid to solid state into four stages : (i) the liquid state, (ii) the

plastic state, (iii) the semi –solid state and (iv) the solid state. He set arbitrary limits,

known as consistency limits or Atterberg limits. As shown in the fig. below.

a) Liquid limit (wl). Liquid limit is the water content corresponding to the

arbitrary limit between liquid and plastic state of consistency of a soil. It is

defined as the minimum water content at which the soil is still in the liquid

state, but has a small strength against flowing.



b) Plastic limit (wp). Plastic limit is the water content corresponding to an

arbitrary limit between the plastic and the semi solid states of consistency of a soil. It is defined as the minimum water content at which a soil will just begin to crumble when rolled into a thread approximately 3 mm in a diameter.

c) Shrinkage limit (ws). Shrinkage limit is defined as the maximum water

content at which a reduction in water content will not cause decrease in the

volume of soil mass. It is lowest water content at which a soil can still be

completely saturated.

d) Plasticity index (Ip). The range of consistency with in which a soil exhibits

plastic properties is called plastic range and is indicated by plasticity index.

The plasticity index is defined as the numerical difference between the liquid

limit and the plastic limit of soil:

Ip = wl - wp

5. Unified Soil Classification And Indian Standard Classification.

USC system and as adopted by the ISI (IS : 1498 –1970: Classification and

Identification of soils for general engineering purpose) is given below.

Soils are broadly divided into three divisions.

Coarse grained soil. In these soils, 50% or more of the total material by

weight is larger than 75 micron IS sieve size.

Fine grained soils. In these soils, 50% or more of the total material by

weight is smaller than 75 micron IS sieve size.

Highly organic soils and other miscellaneous soil materials. These soil

contain large percentage of fibrous organic matter, such as peat, and

the particles of decomposed vegetation. In addition, certain soils

containing shells, cinders and other non soil materials in sufficient

quantities are also grouped in this division.

1. Coarse grained soils. Coarse grained soils are further divided into

two sub – divisions:

(a) Gravels (G). In these soils more than 50% the coarse fraction (+ 75

micron) is larger than 4.75 mm sieve size. This sub division includes

gravels and gravelly soil, and is designated by symbol G.

(b) Sands (S). In these soils more 50% the coarse fraction is smaller than

4.75 mm IS sieve size. This sub division includes sands and sandy

soils.



Each of the above sub-divisions are further sub divided into four

groups depending upon grading and inclusion of other materials.

W : Well graded

C : Clay binder

P : Poorly graded

M : Containing fine materials not covered in

other groups.

These symbols used in combination to designate the type of coarse grained soils. For

example, GC means clayey gravels.

2. Fine grained soils. Fine grained soils are further divided into three sub

divisions. (a) Inorganic silts and very fine sands :M (b) Inorganic clays :C (c) Organic silts and clays and organic matter : O

The fine grained soils are further divided into the following groups on the

basis of the following arbitrarily selected values of liquid limit which is a

good index of compressibility:

(i) Silts and clays of low compressibility, having a liquid less than 35, and

represented by symbol L.

(ii) Silts and clays of high medium compressibility, having a liquid limit

greater than 35 and less than 50, and represented by symbol I .

(iii) Silts and clays of high compressibility, having liquid limit greater than

50, and represented by a symbol H.

Combination of these symbols indicates the type of fine grained soil. For

example, ML means inorganic silt with low to medium compressibility.





TABLE 2.0 BASIC SOIL COMPONENTS (IS CLASSIFICATION)

Soil

Soil Components

Symbol Particle size range and description

Coarse Grained

Boulder Cobble Gravel Sand

None None G S

Round to angular, bulky hard, rock particle,Average diameter more than 30 cm Round to angular, bulky hard, rock particle, Average diameter smaller than 30 cm but retained on 80 mm sieve. Rounded to angular, bulky, hard, rock particle, passing 80mm sieve but retained on 4.75 mm sieve Coarse : 80 mm to 20 mm sieve Fine : 20 mm to 4.75 mm sieve Rounded to angular bulky, hard, rocky Particle, passing 4.75 mm sieve retained on 75 micron sieve. Coarse : 4.75 mm to 2.0 mm sieve Medium : 2.0 mm to 4.25 micron sieve. Fine : 425 micron to 75 micron sieve.

Fine grained Components

Silt Clay

M C

Particle smaller than 75 micron sieve identified by behavior , that it is slightly plastic or non plastic regardless of moistureand exhibits little or no strength when air dried. Particles smaller than 75 micron sieve identified by behavior , that is, it can be made to exhibit plastic properties within a certain range of moisture and exhibits considerable strength when air dried.

Organic matter

O

Organic matter in various sizes and stages of decomposition.



Table 3.0 Classif ication o f Coarse-gra ined Soils (ISC System)

Divisio

n

Subdivisio

n

Grou

p

symb

ol

Typical

Names

Laboratory Criteria Remark

(1) Coarse-gra ined soils (More than half of materia l is larger than 75-micro

Gravel (G) (more than half of coarse fraction is larger than 4.75 mm IS sieve)

Clean gravels (Fines less than 5%)

(1) GW (2) GP

Well graded gravels Poorly graded gravels

Cu greater than 4 Cc between than 1 and 3 Not meet ing all gradat ion requirements fo r GW

When fines are between 5% to 12% border line cases requiring dual symbols such as GP-GM, SW-SC, etc.

Gravels with appreciable amount of fines (Fines more than 12%)

(3) GM

Silt y gravels Claye y gravels

Atterberg limits below A-line o r Ip less than 4 Atterberg limits below A-line o r Ip less than 7

Atterberg Limits plotting above A-line with Ip between 4 and 7 are border-line cases requiring use of dual symbol GM-GC

(4) GC

Table (Continued)

Divi

sion

Subdivision Grou

p

symb

ol

Typ ical

Names

Labora tory Criteria Remark

Sand

Clean

Well -

Cu greater than 6



(S) (more than half o f coarse fract ion is Smaller than 4 .75 mm IS s ieve)

Sand (Fines less than 5%)

(5) GW (6) SP

graded gravels Poorly - graded gravels

Cc between than 1 and 3 Not meeting al l gradat ion requ irements for SW

Sands with appreciable amount o f f ines (Fines more than 12%)

(7) SM (8) SC

Silt y Sands Cla ye y gravels

Atterberg limits below A-line or Ip less than 4 Atterberg limits below A-line or Ip less than 7

Atterberg’s Limits plo tting above A-line with Ip between 4 and 7 are border-line cases requiring use of double symbol SM-SC

(Continued)

Divi

sion

Subdi

visio

n

Group

Symbo

ls

Typic

al

name

s

Laboratory

Criteria

(see Fig

5.6)

Remarks

(2) Fine – gra ined soils

Low-compressibilit y (L) (Liquid

(1) GW (2) CL

Inorganic silt s with none to low

Atterberg limits plot below A-line or Ip less than 7 Atterberg limits plot below A-line or Ip less

Atterberg limits plotting above A-line with Ip between 4

(1) Organic and inorganic soils plotted in the same zone in plast icit y chart are dist inguished by odour and colour or



(more than 50% pass 75 IS Sieve)

Limit less than 35%) Intermediate compressibilit y

(I) (Liquid limit greater than 35 bu t less than 50%

(3) OL (4) MI (5) CI (6) OI

plasticit y Inorganic clays of low plasticit y Organic silt s of low plasticit y Inorganic s ilts of medium plasticit y Inorganic clays of medium plasticit y Organic s ilts of medium plasticit y

than 7 Atterberg limits plot below A- line Atterberg limits plot below A- line Atterberg limits plot above A- line Atterberg limits plot below A- line

to 7 (hatched zone) ML-CL

liquid limit test after oven-drying. A reduction in liquid limit after oven-drying to a va lue less than three-fourth of the liquid limit before oven-drying is posit ive ident ificat ion of organic so ils. (2) Black cotton soils o f Ind ia lie along a band partl y above the A- line and partly below the A line



(Continued)

Divisio

n

Subdivis

ion

Group

Symbols

Typica l names Labora tory

Criteria (see Fig

5.6)

Remar

ks

High compressibilit y (H) (Liqu id limit greater than 50%)

(7) MH

Inorganic silt s of high compressibilit y

Atterberg limits plo t below A-line

See plast icity chart (Fig. 56)

(8) CH

Inorganic c lays of high plast icit y


(9) OH

Organic clays of medium to high plastic ity


(3) Highly o rganic soil

PT

Peat and o ther highly organic soils

Readily identif ied by co lour, odour, spongy fee l and fibrous texture



6.0 Bearing Capacity Definitions 1. Footing: - A footing is a portion of the foundation of a structure that transmits

loads directly to the soil.

2. Foundation: - A foundation is that part of the structure which is in direct

contact with and transmits loads to the ground.

3. Foundation soil: - It is the upper part of the earth mass carrying the load of

the structure.

4. Bearing capacity: - The supporting power of a soil or rock is referred to as its

bearing capacity. The term bearing capacity is defined after attaching certain

qualifying prefixes, as defined below.

5. Gross pressure intensity (q):- 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.

6. Net pressure intensity (qn) :- 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.

Thus, if D is the depth of footing qn = q – γD

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

7. Ultimate bearing capacity (qu):-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.

8. Net ultimate bearing capacity (qnu):- It is the net increase in pressure at the

base of foundation that causes shear failure of soil.

qnu = qu– γD 9. Net safe bearing capacity (qns) :-The net safe bearing capacity is the net

ultimate bearing capacity divided by a factory of safety F.

qns = qnf

F 10. Gross Safe bearing capacity (qs) :-The maximum pressure which the soil can

carrying 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 + γ D.



11. Allowable bearing capacity or pressure. (qna) :- It is the net loading

intensity at which neither the soil fails in shear not there is excessive

settlement detrimental to the structure.

Failures in Soil 1. General Shear Failure: - An analysis of the condition of complete

bearing capacity failure, usually termed general shear failure, can be

made by assuming that the soil behaves like an ideally plastic material. In

such a failure, the soil properties are assumed to be such that a slight

downward movement of footing develops fully plastic zones and the soil

bulges out.

2. Local Shear Failure:-In the case of fairly soft or loose and compressible

soil, large deformation may occur below the footing before the failure

zones are fully developed. Such a failure is called a local shear failure.

I.S. Code Method for Computing Bearing Capacity:

General

IS Code (IS: 6403 – 1981) recognizes, depending upon the deformations

associated with the load and the extent of development of failure, three types

of failure of soil support beneath the foundations, they are (a) General Shear

Failure; (b) Local Shear Failure; and (c) Punching Shear Failure, occurs on

soils of high compressibility. In such a failure, there is vertical shear around

the footing, perimeter and compression of soil immediately under the footing,

with soil on the sides of the footing remaining practically uninvolved.

2. Bearing capacity equation for strip footing for c-Ø soils

The ultimate net bearing capacity of strip footing is given by the following

equations:

i) For the case of General shear failure:

qnu = cNc + γ D (Nq –1) + 0.5 B γ Nγ ---------------(1)

ii) For the case of local shear failure:

qnu = 2/3 cNc’ + γ D (Nq’ – 1) + 0.5 B γ Nγ’ ------------(2)

For obtaining Nc’, Nq’ , Nγ’ bearing capacity facotors corresponding to local shear failure, calculate(Φm) = tan-1 (0.67 Φ ) and read Nc, Nq , Nγ for general shear failure as given in table 4.0 below.



Table 4.0 Bearing Capacity Factors (Is : 6403 –1981)

Degree Nc Nq Nr

0 5.14 1.0 0.0 5 6.49 1.57 0.45

10 8.35 2.47 1.22 15 10.98 3.94 2.65

20 14.83 6.40 5.39

25 20.72 10.66 10.88 30 30.14 18.40 22.40

35 46.12 33.30 48.03 40 75.31 64.20 109.41

45 138.88 134.88 271.76

50 266.89 319.07 762.89

3.Shape factor, depth factor and inclination factor The above bearing capacity equations, applicable for strip footing, shall be

modified to take into account, the shape of the footing, inclination of loading, depth of

embedment and effect of water table. The modified bearing capacity formulate are

given below :

i) For general shear failure

qnu = cNc Sc dc ic + γ D (Nq-1) Sq dq iq +1/2 Bγ Nγ Sγ dγ iγ w’ -------(1)

ii) For local shear failure qnu = 2/3 cNc’ Sc dc ic + γ D(Nq’-1) Sq dq iq +1/2 Bγ Nγ’ Sγ dγ iγ w

’---(2)

The depth factors are given as under ;–

dc =1+ 0.2 (D/B ) NΦ 1/2 where NΦ = tan2 (45+ Φ /2)

dq = dγ =1 for Φ <100 and

dq = dγ =1 + 0.1 (D/B) NΦ 1/2 for Φ > 100

Shape Shape factors

Sc Sq S γ

1.Continous strip 1.0 1.0 1.0

2. Rectangle (1+ 0.2 B/L) (1+0.2 B/L) (1-0.4 B/L)

3. Square 1.3 1.2 0.8

4. Circle 1.3 1.2 0.6

The depth factors are to be applied only when the back filling is done with

proper compaction. The inclination factors are given as under

ic = iq =(1-α/90)2 and i γ = (1- α/ Φ )2

Where α = inclination of the load to the vertical, in degrees.



4. Effect of water table The effect of water table is taken into account in the form of a

correction factor w’. The value of w’ may be chosen as indicated below.

a) w’=1.0 If the water table is likely to permanently remain at or below at

a depth of (D+B) beneath the ground level surrounding the footing

below.

b) W’=0.5 If the water table is located at a depth D or likely to rise to the

base of footing or above,

If the water table is likely to permanently get located at depth Dw below the

G.L. such that D<Dw<(D+B), then w’ be obtained by linear Interpolation.

It may be noted that if the water table r ises above the base of footing, w’ will remain at it s minimum value of 0.5. 5 . Bearing capacity of Cohesion less soils (c=0)

For cohesion less soils having c=0, Indian Standard Code recommends

that the bearing capacity be calculated (a) based on relative density or (b)

based on standard penetration resistance value, and (c) based on static cone

penetration test.

(a) Based on relative density

In this method, bearing capacity may be calculated by Equations 1& 2

together with relevant shear strength parameter. In these formulate, c is taken

equal to zero.

(b) Based on standard penetration resistance value.

The standard penetration resistance is determined at a number of selected

points at intervals of 75 cm in the vertical direction or at change of strata and the

average value beneath each point is determined between the level of base of the

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

computing the value any individual value more than 50 percent of the average

calculated shall be neglected and average recalculated (the value for all loose

seams shall however be included).

Knowing the va lue of N, the va lue o f (Φ) is read from Fig. given in the IS Code. The ult imate net bear ing capacit y is then calculated from the formula. Where the shape facto rs, depth factors and inclinat ion facto rs are determined as described earlier , and the bearing capacit y factors Nq and Nγ are availab le. (c) Based on Static cone penetration test. The static cone point resistance qc is determined as per IS :4968 (Part III) 1976

at a number of selected points at intervals of 10 to 15 cm. the observed values are

corrected for the dead weight of sounding rods. Then the average value at each one of

the location is determined between the level of the base of the footing and the depth

equal to 1.5 to 2 times the width of footing. The average of static cone point resistance



value is determined for each one of the location and the minimum of the average

value is used in the design.

The net ult imate bearing capacit y o f shallow str ip foo ting on cohesion less soil deposit is then determined from Fig. given in the IS Code. 6. Bearing Capacity of Cohesive Soils (Φ = 0) The net ult imate bearing capac it y immediately a fter

construct ion on fair ly saturated homogenous cohesive soils can be

calculated from the expression.

qnu = c Nc Sc dc ic

Where Nc = 5.14 (for Φ =0) The va lue of c is obtained from unconfined compressive strength

test. Alternat ively, cohesion c may be determined from the stat ic cone

point res istance.

7.0 Planning for Soil Investigation

Soil invest igation must co nform to the provisions in I.S. 1892 –

1979. The scope of invest igat ion is indica ted in para 2 .1 and 2.2 of this

code. Engineer ing properties of soil depend on the soil structu re, i.e.

nature of soil grains and their arrangement, vo lume of air and water

(degree of satu ration and porosit y). Since these vary from one locat ion

to another, the program of soil invest igation needs to be evo lved for

each project. It should p rovide for adequate data and make appropriate

recommendation supported by proper calcu lat ions in respect of the

fo llowing:

1. The type o f foundation.

2. Allowable bearing capacit y fo r the foundation.

3. Total and d ifferent ial sett lements.

4. Highest groundwater level ever reached.

5. Anticipated construction problems and suggested solution

(sheep piling, dewater ing, boulders/rock excavat ion,

differentia l, sett lements, damage to adjacent property,

environment etc.)

A copy of the surveyed site plan and layout plan of buildings

indicating the type and sizes of the buildings are required. It is

essentia l that the location of bore holes together with the reduced

levels are marked on the site plan.



To determine the natu re and extent of detailed so il invest igat ion, a

preliminary invest igat ion is necessar y as st ipulated in para 3.1.1 of

I.S. 1892 - 1979. Knowing the t ype of superstructure, the firs t step is

to inspect the s ite and its ne ighborhood and collect the informat ion

abou t the soil p ro file, t ype of foundation generally adop ted and to

guess the presumptive allowable bear ing pressure for the soil. This is

done through reconnaissance and simple visual/manual tests. If soil

invest igation details are not ava ilable for nearb y s ites, a test pit or a

bore ho ld may be dug to examine the soil at foundation leve l.

Knowledge of regional soil deposits corresponding to the

loca lit y, preva lent p ract ices of subso il invest igat ion and foundation

design great ly facilitate drawing up an appropriate program of soil

invest igation. Major regional soil deposits of India are - Alluvia l soils,

Black co tton so ils, Later it ies, Desert so ils and Sub marine so ils

(Reference may be made to Indian contributions to Geotechnica l

Engineer ing published by Ind ian Geotechnical societ y for sources of

informat ion of the Regional deposits).

1. Deta iled so il investigation

Deg r ee s o f a pp l i ca b i l i ty o f va r i o us f i e l d a nd l a bo ra to ry

tes t s a re i nd i ca t ed i n Ta bl e s 1 a nd 2 . The s i tua t i o ns i n

wh i ch ea c h t e s t i s a ppl i ca b l e a nd t he l i mi ta t i o ns o f such

tes t s a re d i s cusse d i n the f o l l o wi ng pa rag ra phs.

In arriving at the a llowable bearing pressure on

foundations, bo th the ult imate bearing capacit y (based on shear

strength and the permissible settlement are taken into account.

Normally sett lement governs the design bu t for narrow strip

foundations on so ft at shallow depths, bearing capacit y based on

shear failure may govern.

1 .1 Characteristics of soil in foundation

a) Cohesion less so ils and soils with cohesion and angle of

internal friction ( c - Ø so ils )



Sand and s ilt are cohesion less so ils. Silt with even 5 to 8 percent of clay has s ignificant cohesion. Shear strength, ‘ s’ of soil is developed due to res istance to ro lling, s liding and defo rmat ion of soil particles/skeletal structure. Cohesion, c is due to inter – particle attract ion due to presence of clay and the angle of interna l fr ict ion ‘Ø’ is essent ially due to res istance to inter – partic le slip of coarser grains like s ilt and sand. Shear strength‘s’ is given b y s = c + σ tan ø

Where σ is normal stress on the shear plane.

Since water has no shear strength, the entire shear strength is due to

inter-granular pressure which is affected by the excess pore water

pressure developed in clayey so ils. The parameters ‘c’ and ‘ø’

correspond ing to maximum shear st rength are determined b y

considering effect ive pressu res which are equal to total pressu re

minus pore water pressure. These are determined b y conso lidated

drained test fo r cohesion less soils (and for c - ø soils if ins itu drainage

occurs as the load is applied). During testing, the excess pore water

pressure is diss ipated complete ly through a slow process o f

conso lidation and an equally s low process of shear. The t ime required

fo r gradual increment of load upto shear failure is determined as per

appendix A of I.S. 2720 (part 13) – 1986. soil in situ exists, genera lly,

in a consolidated state (ō3 ) . As construction proceeds, additiona l

loads come on to the so il. If the permeabilit y o f the soil is low, which

can occur if the f ine gra ined soil contains more than 15 percent cla y

and is classif ied as c lay with intermediate o r high compressibilit y, the

excess pore water pressu res developed in the cla yey so il can not

diss ipate as fast as the rate of applicat ion of load. Hence fo r cla ye y

soils with appreciable cla y content ( say more than 15 percent) , the soi l

parameters ‘C’ and ø are determined from conso lidated un-drained test

in which the soil is consolidated slowly but sheared quickly. If the

clay content is high ( sa y more than 30 percent) or very low ( say less

than 15 percent) , the tests are performed by Box shear as per I.S. 2720

(Part 13) – 1986. The resu lts are representat ive of f ield condit ions

under plane shear only (which is 15 to 20 percent higher than for

tr i-axia l shear). For semi pervious cohesive soils, the conso lidated

un-drained Test is performed by Tri-axia l Test (as per I.S. 2720 ( part

II ) since the inevitable ( though small) drainage of the soil during

shear ing in Box Shear Test introduces an element of error. Shear

strength o f st iff intact c lays such as boulder clays, c laye y s ilts are

better determined by drained tests s ince the soils are generally over

conso lidated.



Saturation reduces the shear strength and long term t ime dependant

conso lidation o f clay takes place during test ing, only if the so il is

saturated. It is thus necessar y to determine shear strength of the so il in

saturated condit ion if the soil in s itu is likely to be sa turated due to

ris ing of the ground water table. Hence it is essent ia l to ascertain the

highest ground water leve l ever reached. Due to the capac it y of clay to

absorb water by capillar y act ion and the very large var iat ion in shear

strength of unsaturated claye y so ils with moistu re content, resu lts of

Box Shear Test canno t reliab ly rep resent in situ shear strength of

unsaturated clay. Even while consider ing the results of conso lidated

un-drained Tri-axial Test or in s itu test on unsatu rated soils, the

effect of var iat ion of insitu shear strength due to possible change in

moisture content due to rain or r ise in water table needs to be

considered.

Satisfactory und isturbed samples of cohesion less so ils are

diff icult to obtain from bore holes. Soil obtained from the split

spoon sampler from standard penetrat ion test may possess large shear

strains due to distu rbance. Hence shear tests in the laboratory on

cohesion less so ils do not represent the true site condit ion. The most

common field test is the standard penet ration test (Ref. I.S. 2131 –

1981). This test, if carefully execu ted, in soil und isturbed by boring

operations, enab les to est imate sat isfactorily the bear ing capacit y as

per I.S. 6403 - 1981 and allowable bearing pressure on settlement

considerat ion as per I.S. 8009 (Part 1 ) – 1976. By using the same

equipment and with the same driller, ‘N’ va lues in the same soil can

be rep roduced with a coefficient o f var iation o f about 10 percent. Use

of defect ive equipments such as a damaged anvil, worn out driving

shoe, old/oily/poorly lubricated rope sheaves etc. can resu lt in

significant ly erroneous ‘N’ values. Pushing a boulder while driving

the sampler, rapid withdrawal of sugar or b it plug causing a quick

cond it ion at the bottom of the bore – ho le by too much d ifference in

the water levels between the ground water tab le and in the ho le are

other sources o f error.

The original standard penetrat ion Test was developed for sand.

However, at present it is commonly used fo r all t ypes of soils.

Alluvia l s ilt deposits are mixtures of med ium dense fine sand and silt

with a small percent of c lay. In some cases, layers o f st iff soil are

encountered at depth o f 6 to 10 meters. Delhi s ilt has abou t 20 – 35%

sand, 50-65% silt and upto 15 percent clay.



b) Cohesive soils

Due to very low permeability, highly cohesive soils in their natural state

posses shear strength due to cohesion only and are prone to time dependant

settlement. Particles of clay being very small in diameter (less than 0.002 mm),

grain size analysis of the soil fraction passing 75 micron is determined as per I.S.

2720 (Part IV) – 1985. Except when the soil is non – plastic (indicated by the

inability to perform the test to determine plastic limit), it is essential to determine the

percentage of clay and silt separately. Natural clay deposits may contain upto 70% or

even more of material belonging to sand and silt grades. Such clayey soils, when

saturated, behaves as if they are purely cohesive under normal loading conditions

from the building. Silt with even 25% clay behaves as clay. Apparent angle of

internal friction is low in the un-drained condition since no water is expelled from the

soil initially when the load is applied. This is the accepted basis for calculating

ultimate bearing capacity of saturated clays. Only in the case of very slow rate of

loading, or with very silty soils, drained condition persists during loading, producing

increase in effective pressure on soil due to decrease in pore water pressure.

Consequently shear strength is increased due to increase in the angle of internal

friction from apparent to true value.

In most cases, allowable bearing pressure is dependant on permissible total

settlement but in every case the foundation is checked against shear failure. Tri-axial

tests on undisturbed samples in the laboratory, in situ vane shear test to determine the

shear strength and static cone test for bearing capacity of predominantly cohesive

soils are reliable.

Shear strength of soft sensitive clays are measured by in-situ vane shear test

as per I.S. 4434 – 1978 since laboratory tests on disturbed samples of such soils are

not reliable.

In cohesive soils, apart from static tests, in situ compressive strength tests are

routinely made using a Pen/Pocket pentro-meter. It is usual practice to take thin

walled tube samples for laboratory testing and compare the field and laboratory test

results.

Alluvial clay deposits consist and clay deposited in river valleys and estuaries

(on the bed of the sea). They are normally consolidated. Stiff surface crust is due to

exposure to the effects of weather and vegetation. Load bearing structures with very

shallow and narrow foundation in the surface crust are constructed which do not

transmit stresses to the underlying soft and highly compressible deposits. In the case

of wide or deep foundations, it is necessary to adopt low bearing pressures or use a

raft or piles. Alluvial clays, especially marine clays, are ‘sensitive’ to disturbance. If

they are disturbed in sampling or in construction operations (such as in piling) they

show a marked loss in shear strength.



1.2 Anticipated problems in construction due to soils characteristics.

In sand y/alluvial soils, if ground water table is lowered, ground

subsidence in the area surrounding the construction s ite ma y occur due

to consolidat ion of underlying c laye y layers. In such a case, it ma y be

necessary to provide a water reta ining barrier around the s ite if

structu res exists ad jacent to the excavat ion (since pumping to dewater

may produce 30 to 50 mm settlement within a short period of time).

When pore water in the so il is just enough to moisten sand but not

saturate it, the su rface tension makes it possible to provide shallow

excavat ions with near vert ical sides. With cont inued drainage and

evaporation or vibrat ion, the s ides collapse. Near vertical excavat ion in

a cohesive so il ma y collapse due to rainfa ll so ftening the clay and

creating excess pore water pressure.

Excavat ion in sands be low the water table may result in a s lumping of

the s ides and bo iling of the bottom, unless a properly designed ground

water lowering system is adopted.

If excavat ion goes below the f irm surface crust of alluvial clay,

support b y t imbering or sheet pilling is and st iffened trenches are

prone to failu re by heaving of the bottom and bulging of the side

supports.

1.3 Programme of detailed soil investigation

In planning the Programme, full advantages shou ld be taken of

ava ilab le informat ion from preliminar y invest igat ion, geo – technica l

consu ltant’s data base and soil Investigation reports fo r the nearby

sites and their correlat ion with actual perfo rmance of build ings and

load tests on p iles. If rock is encountered in a bore hole, boring must

extend at least 2 meters to different iate a boulder from bed rock. If

rock is encountered in different bore holes near abou t the proposed

foundation leve l, adequate number o f bore ho les are required to p lot

the rock contour. On the basis of preliminar y borings or prior site

knowledge, details of in s itu tests and laborato ry tests are worked out

keeping in view the limitat ion of each.

Current methods o f subsoil exp loration are outlined in Append ix

‘A’ o f IS 1892 – 1979 and the tests generally required are indicated in

Table 3 and Append ix A o f this Code of Pract ice.

A.S.T.M. suggests that when more than 15% of gravel or sand is

present in any t ype of soil, the descr iption should inc lude “with”. For



fine grained soils (with more than 50% passing 75 micron s ieve )

“with” ‘ sand’ or ‘gravel’ is written fo r percentages between 15 and 29

and “gravelly” of “sandy” for larger percentages.

Sands or gravels may be class ified by the standard penetrat ion

tests into broad groups as fo llows:

No o f S.P.T. blows ‘N’

Lo o se Le s s t ha n 1 0

Medium Dense 10 to 30

De nse ( o r c o mp a ct ) M o r e t ha n 3 0

Based on un-drained shear strength, clayey soils may be classified as follows

Soft (0.2 to 0.4 kg/cm)

Medium (0.4 to 0.75 kg/cm)

Stiff (0.75 to 1.5 kg/cm)

After estab lishing correlat ion on the basis of other re liable tests,

standard penetrat ion test resu lts have been in use for many years for

relat ive densit y, angle of internal frict ion, un-drained compressive

strength, settlement and modules of sub grade react ion. Some o f these

are of questionab le value unless co rroborated by adequate calibration

data for the loca lit y since many were o riginally proposed without

extensive study of the large number of var iables affect ing the ‘N’

values.

A. Tests required for classification of so ils

1) Classif icat ion as per IS 1498 – 1970 based on partic le s ize

analysis as per IS 2720 (Part 4) – 1985 and index properties

of the soil as per IS 2720 (part 5 ) – 1985 . On the basis of

index properties, if the soil is c lassif ied as clay o f

intermediate or high compressibility, It is necessar y to

determine the c lay and silt percentages separately. Hence in

addition to sieving, p ipette o r hydrometer test is necessary to

determine the percentage of cla y.

2) In assessing the engineering behavior of a cohesive so il, it is

necessar y to determine in s itu water content in addition to

liquid limit and plast ic limit of re-moulded soil.



B. Tests required to determine safe bearing capacity of shallow foundations (

including raft)as per I.S. 6403 – 1981.

Apart from ascertaining the highest level ever reached by the ground water

table and tests for classification of soil as per I.S. 1498–1970 based on grain

size analysis as per I.S. 2720 (part iv) – 1985 index properties of the soil as

per IS 2720 (Part 5) – 1985, the following tests are required to determine

safe bearing capacity based on shear strength consideration:

1) Standard penetration test as per I.S. 2131 – 1981 for coarse grained /fine

grained cohesion less soils and semi – pervious clayey soils (i.e. c – ø

soils with clay upto about 30 percent).

2) Direct shear (controlled strain) test as per I.S. 2720 (Part 13) – 1986.

Consolidated un-drained test for cohesive and for C – ø soils and

consolidated drained test for cohesion less soils. The results may be

compared with standard penetration test/static cone penetration test results.

Since there is escape of pore water during box shear, partial drainage

vitiates the consolidated un-drained test. Hence this test is not exact for

semi pervious soils such as clayey sands/silts (i.e. with clay more than

15% but less than 30%). For such soils, Tri-axial Tests are required if

shear strength is the critical criterion.

3) Static cone penetration test as per I.S. 4968 (part 3) – 1976 for foundations

on non stiff clayey soils such as fine grained soils (i.e. more than 50%

passing 75 micron sieve). In fine and medium coarse sands such tests are

done for correlation with S.P.T. and to indicate soil profiles at intermediate

points.

4) Unconfined compressive strength test as per I.S. 2720 (part 10) – 1973 for

highly cohesive clays except soft/sensitive clays.

5) Vane shear test for impervious clayey soils except stiff or fissured clays.

6) Tri-axial shears tests for predominantly cohesive soils. If shear strength is

likely to be critical.

C. Tests required determining allowable bearing pressure for shallow

foundations on settlement consideration.

1) Standard penetration test as per I.S. 2131 – 1981 for cohesion less soils and

semi pervious clayey soils (i.e. c – ø soils with clay upto about 30 percent)

2) Consolidation test as per I.S. 2720 (part 15) if the settlement of clayey

layer/layers calculated on the basis of liquid limit and in-situ void ratio

indicates that settlement may be critical. Consolidation test is not required if

the superimposed load on foundation soil is likely to be less than pre-

consolidation pressure (assessed from Liquidity Index and sensitivity or from

unconfined compressive strength and plasticity index).

3) Plate load test as per I.S. 1888 – 1982 for cohesion less soils and c – ø soils

where neither standard penetration test now consolidation test is appropriate

such as for fissured clay/rock, clay with boulders etc.



D. Test specially required for raft foundations (Refer para 3 of I.S. 2950

(Part I ) – 1981.

Apart from other tests for shallow foundations, the following tests are required

especially for raft foundation:

1) Static cone penetration test as per I.S. 4968 (part 3) – 1976 for cohesion

less soil to determine modulus of elasticity as per I.S. 1888 –1982.

2) Standard penetration test as per I.S. 2131 – 1981 for cohesion less soils

and c – ø soils to determine modulus of sub grade reaction.

3) Unconfined compressive strength test as per I.S. 2720 (part 10) – 1973 for

saturated but no pre-consolidated cohesive soil to determine modulus of

sub grade reaction.

4) As specified in I.S. 2950 (part I) – 1981, plate load test as per I.S. 1888 –

1982 where tests at sl. 1 to 3 above are not appropriate such as for

fissured clays/ clays boulders.

5) In case of deep basements in pervious soils, permeability is determined

from pumping test. This is required to analyze stability of deep excavation

and to design appropriate dewatering system.

E. Tests specially required for deep foundations

1) While the composition and depth of the bearing layer for shallow

foundations may vary from one site to another, most pile foundations in a

locality encounter similar deposits. Since pile capacity based on soil

parameters is not as reliable as from load tests, as a first step it is essential

to obtain full information on the type, size, length and capacity of piles

(including details of load – settlement graph ) generally adopted in the

locality. Correlation of soil characteristics ( from soil investigation reports)

and corresponding load tests (from actual projects constructed) is essential

to decide the type of soil tests to be performed and to make a reasonable

recommendation for the type, ‘size’ length and capacity of piles since most

formulae are empirical.

2) If information about piles in the locality are not available or reliable, it

may be necessary to drive a test pile and correlate with soil data.

3) Standard penetration test to determine the cohesion (and consequently the

adhesion based on or methods) to determine the angle of friction (ø and

consequently the angle of friction & between soil and the pile and also the

point resistance) for each soil stratum of cohesion less soil or c- ø soil.

4) Static cone penetration test to determine the cohesion (and consequently

the adhesion based on or methods) for soft cohesive soils and to check

with S.P.T. result for fine to medium sands. Hence for strata encountering

both cohesive and cohesion less soils, both S.P.T. and C.P.T. are required.

5) Vane shear test for impervious clayey soils.



6) Un-drained Tri-axial shear strength of undisturbed soil samples (obtained

with thin walled tube samplers) to determine ‘c’ and ø for clayey soils

(since graphs for correlations were developed based on un-drained shear

parameter). In case of driven piles proposed for stiff clays, it is necessary

to check with the ‘c’ and ‘ø’ from remoulded samples also. Drained shear

strength parameters are also determined to represent in situ condition of

soil at end of construction phase.

7) Self boring pressure meter test to determine modulus of sub grade reaction

for horizontal deflection for granular soils, very stiff cohesive soils, soft

rock and weathered or jointed rock.

8) Ground water conditions and permeability of soil influence the choice of

pile type to be recommended. Hence the level at which water in the bore

hole and the level at which water in the bore hole remains are noted in the

bore logs. Since permeability of clay is very low, It takes several days for

water in the drill hole to rise upto the ground water table. Ground water

samples need to be tested to consider the possible chemical effects on

concrete and the reinforcement. Result of the cone penetration test for the

same soil show substantial scatter. Hence, they need to be checked with

supplementary information from other exploration methods. Pressure

meters are used to estimate the in situ modulus of elasticity for soil in

lateral direction. Unless the soil is isotropic, the same value cannot be

adopted for the vertical direction. A list of tests required for soil

investigation is given in Table 3.

2) Recommendation in the soil investigation Reports:

Due to the difficulty in assessing the contact pressure on the

foundation soil by individual columns/wall. And variation in soil properties, it

is common practice to provide an adequate factor of safety while making

recommendation for the foundation based on results of soil investigation.

However, we may have a problem if the investigating firm recommends, say,

a special type of foundation with a safe bearing pressure of 8 tones per sq.

meter and it turns out that the safe bearing pressure is 12 tones per sq. meter

which would permit spread footings resulting in substantial economy.

Similarly, suggestion of a pile foundation without considering other economic

types of foundation is inappropriate. Hence, it is necessary to examine the

report to ensure that the recommendations flow from the data which have been

correctly interpreted.



2.1 Bearing capacity For shallow foundation, the current practice is to use an average ‘N’ value in

the zone affecting soil behavior. For spread footings, the effective zone

extends to a depth equal to twice the width below the footing. For a square

footing, the effective zone extends to a depth equal to one and a half times the

width (if the effective zones of adjacent footings do not overlap). Weighted

average is used. For piles, average ‘N’ for each stratum is used.

It is undesirable to place a footings on soil with a relative

density less than 0.5 in such cases, the soil should be compacted by drainage

and / or preloading prior to placing footings on it.

The effect of ground water table on settlement is considered

as per I.S. 8009 (Part 1) – 1976 and I.S. 6403 –1981.

Recent geo – technical studies indicate that prediction of

consolidation settlements is satisfactory when compared with actual

measurements. The predictions are better for inorganic insensitive clays than

for others. The predictions require great care if ‘e’ Vs log ‘p’ curve is curved

throughout or the clay is very sensitive. Much care is also required if the clay

is highly organic as the creep component of settlement is substantial.

If required, settlements can be computed for various point such as

corner, centre or beneath lightest or the heaviest parts of a building.

Differential settlement can be computed as the difference between the

settlements of columns with maximum and minimum settlement.

Alternatively, it may be estimated at 3/4th of the computed maximum total

settlement for spread footings for columns /walls.

Limiting the total settlement and the differential settlement to that permissible

as per I.S. 1904 – 1986, the allowable bearing pressure on the foundation soil

is recommend for various sizes of footings, based on equal settlement

consideration.

If after applying the empirical rules, or computing settlements of the structure

at various points based on the assumption of a flexible foundation, it is shown

that the total and differential settlements exceed safe limits for spread/ strip

footings and the structure itself does not have sufficient rigidity (i.e. unlike a

well tied building with adequate cross walls and reinforced concrete bands at

intermediate levels) to prevent excessive differential movement with ordinary

spread foundations, provision of a rigid raft foundations either with a thick

slab or with deep beams in both directions may be considered.

If a tall building with basement is founded on clay, the base of the

excavation will initially heave to a convex shape. As superstructure is



constructed floor by floor, the soil will be consolidated and the bottom will

finally deform to a concave (bowl) shape.

The critical factor for framed buildings is the relative rotation (or

angular distortion) whereas the ratio of deflection to length is critical in load

bearing walls which fall by sagging or hogging of the centre length of the

wall.

In view of excessive cost of a raft foundation, adequate soil

investigation must be done and the report should clearly bring out by proper

analysis of results that it is not possible to provide spread footings including

combined footings.

In some cases of alluvial deposits, there may be a variation in

characteristics of soil deposit beneath a large raft. A stiff crust of variable

thickness and extent.

Precautions may be indicated to avoid the lateral yield of soil if loose

sand is encountered beneath the edges of raft at depths less than 2.5 to 3.0

meters below the ground level.

The immediate settlement calculated on the basis of theory of elasticity is

strictly applicable to flexible bases only and is used to determine the contact

pressure distribution under the raft. In practice most foundations are

intermediate between ‘rigid’ and ‘flexible’. Even very thick ones deflect when

loaded by the superstructure. If the base is rigid, the settlement is uniform (but

raft may tilt) and the settlement is about 7% less. In the equation for

settlement, the weighted average of the modulus elasticity is adopted in place

of a single value for the entire depth below foundation. If ‘N’ values are used

to calculate the modulus of elasticity, which generally increases with depth,

weighted average of the modulus is calculated and used in computing

immediate settlement.

3 . Shear strength

In some cases, consolidated Drained Test on cohesion less soils (i.e.

soils containing less than 5 percent clay) may give a small value of cohesion,

of the order of 0.10 to 0.15 kg/cm2. This is attributed to test inaccuracy and

surface tension. Hence this small value of ‘c’ being unreliable is neglected in

analyzing field conditions (such as stability of slope etc.). Generally, deep cuts

in clayey soils are designed for short term stability based on total stress

analysis in consolidated un-drained condition. These are analyzed for long

term stability if the cut slope is to exist even when consolidated drained

conditions may occur.



4.0 Pile Foundation

A pile foundation is recommended only when a raft foundation

cannot be recommended due to excessive settlement (which must be

calculated from consolidation test) when the shallow foundation is on a loose

filled up soil or is under lain by a highly compressible soil stratum. The base

level of the piles is determined considering the end resistance of the stratum

and settlement behavior of the soil under the pile groups.

A slip of 5 to 10 mm of the soil is enough to develop full skin

resistance along the pile whereas a displacement of the order of 10 percent of

diameter of pile tip is necessary to mobilize full end bearing resistance.

Driven piles compact loose and medium dense cohesion less soils and

hence are preferable. For such piles, pile driving formulate are more reliable

for cohesion less soils than for cohesive soils. Large surface cracks are

formed by driven piles in stiff clay. Hence the skin resistance may be

neglected upto about 1.8 meters at top. Capacity of driven cast in-situ concrete

piles is determined as per Appendix A of I.S. 2911 (part 1/Sec 1) – 1979.

Capacity of bored piles is more dependent on the construction

technique than for driven piles. Soil is loosened as a result of boring

operations. Shaft friction values for bored piles in sands may be only half of

that for driven piles. This ratio is about one third for end bearing resistance. If

concrete is placed (but not mechanically compacted) while withdrawing the

shell tube, the surrounding cohesion less soil may be considered to be in loose

condition. Capacity of bored cast in situ concrete piles is determined as per

Appendix B of I.S. 2911 (part 1/Sec 2) – 1979.

If piles encounter shrinkable clays near the ground, due allowable may

be made for loss of frictional resistance and also for uplift due to swelling.

In st iff fissured clays, bored cast in situ piles or low.

Displacement driven p iles are usually recommended. Dense silt s

cause high penetration res istance for driven piles but the

capacit y of the pile remains low due to d isturbance o f the soi l

during driving.

Normally consolidated c lays cause ‘down – drag on bored

cast – in – situ piles due to consolidated on account of drainage

occurring as a result of boring.

Point resistance and skin fr ict ion of pile in sand

increases as the length of the pile increases upto the crit ica l



depth equal to 10 times the pile diameter fo r loose sand and

20 times for dense sand, Beyond this length, the va lues remain

constant.

Point res istance o f piles longer than 15 to 20 times the

diameter, driven through weak strata into thick firm sand

deposit increases with depth of embedment in this s tratum upto

a maximum value corresponding to 8 to 12 times the diameter

of the pile.

Except fo r bored piles in sand capacity of a group of piles

equals the sum of the capacit ies of ind ividual piles in the group .

In case of bored piles in sand, the capacit y is about two thirds

the sum of capac it y. Check is necessar y for failure of the pile

group as a single block.

Pile capac it y ma y be calculated by several appropriate

methods so as to establish upper and lower bound va lues. Errors

are very high when resu lts from one t yp e of so il deposit in one

loca lit y or valid for one year o f pile are extrapolated to derive

the value for different deposits in another localit y or another

type o f p ile invo lving a different construction technique.

With a view to limit the number of piles in each group to

the minimum, the recommendat ion should ind icate the highest

possible capacit y o f the pile considering the soil parameters,

the bore log and the appropriate t ype of pile.

5 . Conclusion:

Technical sanction of a project is based on sound engineering practice.

It is thus of utmost importance to evolve and acceptable practice for

planning of soil investigation and appropriate recommendation for

foundation. Every soil investigation report should be examined at an

appropriate level before acceptance of the recommendation regarding the

type of foundation and the allowable bearing pressure. This is essential in

view of the high cost of foundation and that any error in foundation is difficult

to rectify or may have disastrous consequence.



LIST OF VARIOUS FOUNDATION ENGINEERING CODES

SP 36 : Part 1 : 1987 Compendium of Indian standards on soil engineering: Part 1 Laboratory test ing o f soils fo r civil engineering purposes

SP 36 : Part 2 : 1988 Compendium of Indian standards on soil engineering: Part 2 Field test ing

IS 1080 : 1985 Code of practice for design and construction of sha llow foundations in so ils (other than raft, r ing and shell)

IS 1498 : 1970 Classif icat ion and ident if icat ion of so ils for general engineering purposes

IS 1725 : 1982 Specificat ion for soil based blocks used in general build ing construction

IS 1888 : 1982 Method o f Load Test on Soils

IS 1892 : 1979 Code of practice for subsurface invest igat ions for foundations

IS 1904 : 1986 Code of practice for design and construction of foundations in so ils : general requirements

IS 2131 : 1981 Method for Standard Penetration Test for Soils

IS 2132 : 1986 Code of practice for thin walled tube sampling of soils

IS 2720 : Part 2 : 1973 Methods o f test for soils: Part 2 Determinat ion of water content

IS 2720 : Part 3 : Sec 1 : 1980 Methods o f test for soils: Part 3 Determinat ion of spec ific gravit y Sect ion 1 fine gra ined soils

IS 2720 : Part 1 : 1983 Methods o f Test for Soils - Part 1 : Preparation of Dry So il Samples for Various Tests

IS 2720 : Part III : Sec 2 : 1980 Test fo r So ils - Part III : Determinat ion of Specific Gravit y - Sect ion 2 : Fine, Medium and Coarse Grained So ils

IS 2720 : Part 4 : 1985 Methods o f Test for Soils - Part 4 : Grain Size Analysis

IS 2720 : Part 5 : 1985 Method of Test fo r So ils - Part 5 : Determinat ion of Liquid and Plast ic Limit

IS 2720 : Part 6 : 1972 Methods o f test for soils: Part 6 Determination of shr inkage factors

IS 2720 : Part 9 : 1992 Methods o f test for soils: Part 9 Determinat ion of dry densit y- moisture content re lat ion by constant weight of soil method

IS 2720 : Part 10 : 1991 Methods of test for soils: Part 10 Determinat ion of unconfined compressive strength

IS 2720 : Part 11 : 1993 Methods of test for soils: Part 11



Determinat ion of the Shear Strength Parameters of a spec imen tested in inconsolidated, indrained triaxial compression without the measurement of pore water pressure

IS 2720 : Part 12 : 1981 Methods of test for soils: Part 12 Determinat ion of shear strength parameters of soil from conso lidated undrained tr iaxia l compression test with measurement of pore water p ressure

IS 2720 : Part 13 : 1986 Methods of Test for Soils - Part 13 : Direct Shear Test

IS 2720 : Part 14 : 1983 Methods of Test for Soils - Part 14 : Determinat ion of Densit y Index (Relat ive Densit y) o f Cohesionless Soils

IS 2720 : Part XV : 1965 Methods of Test for Soils - Part XV : Determinat ion of Consolidation Properties

IS 2720 : Part VII : 1980 Methods of Test for Soils - Part VII : Determinat ion of Water Content-Dry Densit y Relat ion Using Light Compaction

IS 2720 : Part 8 : 1983 Methods o f Test for Soils - Part 8 : Determinat ion of Water Content-Dry Densit y Relat ion Using Heavy Compaction

IS 2720 : Part 20 : 1992 Methods of test for soils: Part 20 Determinat ion of linear shrinkage

IS 2720 : Part 22 : 1972 Methods of test for soils: Part 22 Determinat ion of organic matter

IS 2720 : Part 23 : 1976 Methods of test for soils : Part 23 Determinat ion of ca lcium carbonate

IS 2720 : Part 25 : 1982 Methods of test for soils: Part 25 Determinat ion s ilica sesquioxide rat io

IS 2720 : Part 16 : 1987 Methods of Test for Soil - Part 16 : Laborato ry Determinat ion of CBR

IS 2720 : Part 17 : 1986 Methods of Test for Soils - Part 17 : Laborato ry Determinat ion of Permeability

IS 2720 : Part 18 : 1992 Methods of test for Soils - Part 18 : Determinat ion of Field Moistu re Equiva lent

IS 2720 : Part 19 : 1992 Methods of Test for Soils - Part 19 : Determinat ion of Centr ifuge Moistu re Equiva lent

IS 2720 : Part XXI : 1977 Methods of Test for Soils - Part XXI : Determinat ion of Total So luble Solids

IS 2720 : Part XXIV : 1976 Methods of Test for Soils - Part XXIV : Determinat ion of Cat ion Exchange Capacity

IS 2720 : Part 27 : 1977 Methods of test for soils: Part 27 Determinat ion of total solub le su lphates

IS 2720 : Part 28 : 1974 Methods of test for soils: Part 28 Determinat ion of dry densit y of so ils inp lace, by the sand



replacement method

IS 2720 : Part 30 : 1980 Methods of test for soils: Part 30 Laborato ry vane shear test

IS 2720 : Part 33 : 1971 Methods of test for soils: Part 33 Determinat ion of the densit y in p lace by the r ing and water replacement method

IS 2720 : Part 35 : 1974 Methods of test for soils: Part 35 Measurement o f negat ive pore water pressure

IS 2720 : Part 26 : 1987 Method of Test for Soils - Part 26 : Determinat ion of pH Value

IS 2720 : Part XXIX : 1975 Methods of Test for Soils - Part XXIX : Determinat ion of Dry Densit y of Soils In-place b y the Core-cutter Method

IS 2720 : Part 31 : 1990 Methods of Test for Soils - Part 31 : Fie ld Determinat ion of California Bear ing Rat io

IS 2720 : Part XXXIV : 1972 Methods of Test for Soils - Part XXXIV : Determinat ion of Densit y of Soil In-place by Rubber-balloon Method

IS 2720 : Part 36 : 1987 Methods of test for soils: Part 36 Laborato ry determination of permeability of granu lar soils (constant head)

IS 2720 : Part 37 : 1976 Methods of test for soils: Part 37 Determinat ion of sand equivalent values o f soils and fine aggregates

IS 2720 : Part 38 : 1976 Methods of test for soils: Part 38 Compaction control test (hilf method)

IS 2720 : Part XL : 1977 Methods of Test for Soils - Part XL : Determinat ion of Free Swell Index of Soils

IS 2720 : Part XLI : 1977 Methods o f Test for Soils - Part XLI : Measurement o f Swelling Pressure of Soils

IS 2720 : Part XXXIX : Sec 1 : 1977 Methods of Test for Soils - Part XXXIX : Direct Shear Test for Soils Conta ining Gravel - Sect ion I : Laborato ry Test

IS 2720 : Part XXXIX : Sec 2 : 1979 Methods of Test for Soils - Part XXXIX : Direct Shear Test for Soils Conta ining Gravel - Sect ion 2 : In-Situ Shear Test

IS 2809 : 1972 Glossary of Terms and Symbols Relat ing to So il Engineer ing

IS 2810 : 1979 Glossary of terms re lat ing to soil d ynamics

IS 2911 : Part 1 : Sec 1 : 1979 Code o f p ract ice for design and construct ion of pile foundations: Part 1 Concrete piles, Sect ion 1 Driven cast in-s itu concrete p iles

IS 2911 : Part 1 : Sec 2 : 1979 Code o f p ract ice for design and construct ion of pile foundations: Part 1 Concrete piles, Sect ion 2



Bored cast- in- s itu piles

IS 2911 : Part 1 : Sec 3 : 1979 Code o f p ract ice for design and construct ion of pile foundations: Part 1 Concrete piles, Sect ion 3 Driven precast concrete p iles

IS 2911 : Part 1 : Sec 4 : 1984 Code o f p ract ice for design and construct ion of pile foundations: Part 1 concrete p iles, Section 4 Bored precast concrete piles

IS 2911 : Part 2 : 1980 Code of practice for desing and construction of pile foundations: Part 2 Timber piles

IS 2911 : Part 3 : 1980 Code of practice for design and construction of pile foundations: Part 3 Under reamed p iles

IS 2911 : Part 4 : 1985 Code of practice for design and construction of pile foundations: Part 4 Load test on piles

IS 2950 : Part I : 1981 Code of Practice for Design and Construction of Raft Foundations - Part I : Design

IS 2974 : Part 2 : 1980 Code of practice for design and construction of machine foundations: Part 2 Foundations fo r impact type machine (hammer foundations)

IS 2974 : Part 3 : 1992 Code of practice for design and construction of machine foundations: Part 3 Foundations fo r rotary type machines (Medium and high frequency)

IS 2974 : Part 4 : 1979 Code of practice for design and construction of machine foundations: Part 4 Foundations fo r rotary type machines of low frequency

IS 2974 : Part 5 : 1987 Code of practice for design and construction of machine :foundations Part 5 Foundations for impact machines other than hammers (forging and stamping press, pig b reakers, drop crusher and jo lter)

IS 2974 : Part I : 1982 Code of Practice for Design and Construction of Machine Foundations - Part I : Foundation fo r Reciprocating Type Machines

IS 4091 : 1979 Code of Practice for Design and Construction of Foundations for Transmission Line Towers and Poles

IS 4332 : Part 1 : 1967 Methods o f test for stabilized soils: Part 1 Methods of sampling and preparation of stabilized so ils for test ing

IS 4332 : Part 3 : 1967 Methods of t est for s tabil ized soils : Part 3 Test for determination of moisture content -dr y densit y relat ion for stablized soils mixtures

IS 4332 : Part 4 : 1968 Methods of t est for s tabil ized soils : Part 4 Wetting and drying, freezing and thawing t es ts for compacted soil-cement mixtures

IS 4332 : Part 5 : 1970 Methods of t est for s tabil izd soils: Part 5 Determination of unconfined compressive strength of s t ablized soils

IS 4332 : Part II : 1967 Methods of Test for Stabil ized Soils - Part I I : Determination of Moisture Content of Stabil i zed Soil Mixtures



IS 4332 : Part 8 : 1969 Methods of t est for s tablized soils : Part 8 Determination of l ime content of l ime stabli zed soils

IS 4332 : Part 10 : 1969 Methods of t est for stabil ized soils: Part 10 Test for soil /bituminous mixtures

IS 4332 : Part VI : 1972 Methods of Test for Stabil ized Soils - Part VI : Flexural S trength of Soil-cement Using Simple Beam With Thi rd-point Loading

IS 4332 : Part VII : 1973 Methods of Test for S tabil ized Soils - Part VII : Determination of Cement Content of Cemen t Stabil ized Soils

IS 4332 : Part IX : 1970 Methods of Test for Stabil ized Soils - Part IX : Determination of the Bituminous S tabil izer Content of Bitumen and Tar Stabil ized Soils

IS 4434 : 1978 Code of p racti ce for in-si tu vane shear t est for soils

IS 4968 : Part 1 : 1976 Method for subsurface sounding for soils : Part 1 Dyna mic method using 50 mm cone without betonite s lurr y

IS 4968 : Part 3 : 1976 Method for subsurface sounding for soils : Part 3 Static cone penetration t est

IS 4968 : Part II : 1976 Method for S ubsurface Sounding for S oils - Part II : Dynamic Method Using Cone and Bentonite Slurr y

IS 5249 : 1992 Method of t est for determination of dynamic properties of soil

IS 6403 : 1981 Code of p racti ce for determination of bearing capacit y of shallow foundations

IS 8009 : Part II : 1980 Code of Practi ce for Calculation of Sett l ement of Foundations - Part II : Deep Foundations Sub jected to Symmet rical Static Verti cal Loading

IS 8009 : Part I : 1976 Code of Practi ce for Calculation of Sett l ements of Foundations - Part I : Shallow Foundations Subjected to S ymmetrical Static Verti cal Loads

IS 8763 : 1978 Guide for undist rubed sampling of sands and sandy soils

IS 9198 : 1979 Specifi cation for compaction rammer for soil t est ing

IS 9214 : 1979 Method for determination of modulus of sub-grade reaction (k-value) of soils in the fi eld

IS 9259 : 1979 Specifi cation for l iquid l imit apparatus for soils

IS 9456 : 1980 Code of p racti ce for design and construction of conical and hyperbolic paraboloidal types of shell foundations

IS 9556 : 1980 Code of practice for design and construction of diaphragm walls

IS 9640 : 1980 Specificat ion for split spoon sampler

IS 9669 : 1980 Specificat ion for CBR moulds and it s accessories

IS 9716 : 1981 Guide fo r lateral dynamic load test on p iles

IS 9759 : 1981 Guidelines for de-watering during construct ion

IS 10042 : 1981 Code of practice for s ite- investigat ions for foundation in gravel boulder deposits

IS 10074 : 1982 Specificat ion for compaction mould assembly fo r



light and heavy compaction test fo r soils

IS 10077 : 1982 Specificat ion for equipment for determinat ion of shr inkage factors

IS 10108 : 1982 Code of practice for sampling of so ils by thin wall sampler with stat ionery piston

IS 10270 : 1982 Guidelines for design and construction of prestressed rock anchors

IS 10379 : 1982 Code of practic for f ield control of moisture and compaction of soils o f embankment and subgrade

IS 10442 : 1983 Specificat ion for earth augers (spiral t ype)

IS 10589 : 1983 Specificat ion for equipment for determinat ion of subsurface sounding of soils

IS 10837 : 1984 Specificat ion for moulds and accessories fo r determinat ion of densit y index (relative density) o f cohesionless soils

IS 11089 : 1984 Code of practice for design and construction o f ring foundation

IS 11196 : 1985 Specificat ion for equipment for determinat ion of liquid limit of soils cone penetrat ion method

IS 11209 : 1985 Specificat ion for mould assembly for determinat ion of permeabilit y o f soils

IS 11229 : 1985 Specificat ion fo r shear box for testing of soils

IS 11233 : 1985 Code of practice for design and construction o f radar antenna, microwave and TV tower foundations

IS 11550 : 1985 Code of practice for f ie ld instrumentat ion of swelling p ressure in expansive soils

IS 11593 : 1986 Specificat ion for shear box ( large) fo r testing of soils

IS 11594 : 1985 Specificat ion for thin walled sampling tubes and sampler heads

IS 11629 : 1986 Code of practice for insta lla t ion and operation of single point hydrau lic over-flow sett ing gauge

IS 12023 : 1987 Code of practice for f ie ld monitoring of movement of structu res using tape extensometer

IS 12175 : 1987 Specificat ion for rap id moisture meter for rapid determinat ion of water content for soil

IS 12208 : 1987 Method fo r measurement of earth p ressure b y hydrau lic pressure cell

IS 12287 : 1988 Specificat ion for conso lidometer for determinat ion of consolidat ion properties

IS 12979 : 1990 Specificat ion for mould for determinat ion of linear shr inkage



IS 13094 : 1992 Guidelines for selectio n of ground improvement techniques for foundation in weak soils

IS 13301 : 1992 Guidelines for vibration isolat ion for machine foundations

IS 13468 : 1992 Specificat ion for apparatus for determinat ion o f dry densit y o f so ils by core cu tter method

IS 14893 : 2001 Non-Destructive Integr it y Testing of Piles (NDT) – Guidelines

IS 15284 : Part 1 : 2003 Design and Const ruction for Ground Improvement - Guidelines - Part 1 : Stone Columns


Questions

1. Explain Density index of soil?

2. Explain the different divisions in which the soil is broadly divided in Indian

standard of soil classification system?

3. Explain in brief sub division of soil on the basis of arbitrarily selected liquid limit

of fine grained soils?

4. Define Void ratio, Porosity and Degree of saturation of soil?

5. Explain in brief the different types of failure in soil?

6. Define Liquid Limit, Plastic Limit and Shrinkage Limit in Plasticity Characteristics

of Soils?

7. List the different Tests which are specially required for deep foundations?

8. Explain the effect of water table on bearing capacity of soil?

9. Define Ultimate bearing capacity and Gross safe bearing capacity of soil?

10. When Pile foundation is recommended?

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Documents

CH-3 An Overview of Soil Mechanics and Foundation Designtraining.bsnl.co.in/DIGITAL_LIBRARY_SOURCE... · An Overview of Soil Mechanics and Foundation Design 1.0 Introduction Geotechnical