Chapter02.Oerview of Soil Mechanics & Foundation Design

8/12/2019 Chapter02.Oerview of Soil Mechanics & Foundation Design

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E1-E2 Technical (Civil) Vol.-1 Rev date: 01-04-11

BSNL India For Internal Circulation Only Page: 1

E1-E2: TECHNICAL(CIVIL) VOL-1

CHAPTER-2

OVERVIEW OF SOILMECHANICS

&

FOUNDATION DESIGN


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Overview of Soil Mechanics & Foundation Design

1.0 Introduction

Geotechnical Engineering is a relatively modern branch of civilengineering. As a discipline, it is academically as exciting as

practically challenging Geotechnical engineering is actually the new

name of a subject known earlier as Soil Mechanics and Foundation

Engineering. Of this, foundation engineering, at least as an art , is as

ancient as civil engineering whereas the roots of Soil Mechanics, which

forms its scientific base, can be traced only from 1773 with Coulombs

law for shear strength of soil given in that year. Subsequent

contributions were few upto the year 1925, which was the birth of

modern Soil Mechanics with the publication of Terzaghis celebrated

book Erdbaumechanik. Professor Karl von Terzaghi, who is rightly,

regarded as the father of modern Soil Mechanics.

Before designing a foundation for a structure it is essential to know

the behavior of soils under loads. For study of behavior of soils in

depth knowledge of soil mechanics is required. It is essential toassociate the structural engineer in drawing up the soil investigation

programme and interpretation of the report. He must visit the site to

facilitate proper scrutiny of the soil investigation report by comparing

the results and the recommendation with the information available fromsimilar sites and constructed projects.

2.0 Field Identification Of Soils

Soil grains consist of inert rock minerals (cobble, gravel, sand, silt),

often combined with significant amounts of clay (say, more than 5

percent). While inert silt grains may be angular or rounded (thus

contributing to greater or less angle of internal friction, ), particles

of clay are small platelets with negative charges on both faces which

attract the positively charged ends of water molecules. This bond isresponsible 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 soil

since the permeability of clay is of the order of 10 -7 centimeters/second

compared to 10cms/ second for sand. This capacity of the clay to hold

the water molecules for long even when pressure is applied on the soil,

greatly influences its behavior i .e. shears strength, compressibility and

permeability.


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2.1 Simple and Quick Methods of Field Identification of Soils:

(i) Fine sand is differentiated from silt by placing a spoonful of soil in a glass jar

or test tube, mixing with water and shaking it to a suspension. Sand settles

first, followed by silt which may take about five minutes. This test may also be

used for clay which takes more than 10 minutes to start settling. Thepercentages of clay, silt and sand are assessed by observing the depths of the

sediments.

(ii) Silt is differentiated from clay as follows:-

(a)

Clay lumps are more difficult to crush with fingers than

silt. When moistened, the soil lump surface texture is felt

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

silt .

(b)

A ball of the soil is formed and shaken horizontally on the

palm of the hand. If the material becomes shiny from water

coming to the surface, it is silt .

(c) If soil containing appreciable percent clay is cut with a

knife, the cut surface appears lustrous. In case of silt, the

surface appears dull.

(ii i) Field: indication for the consistency of cohesive soils are as

follows:-

Stiff : Cannot be moulded with in the figure

Medium: Can be moulded by the fingers on strong pressure.Readily indented with thumb nail.

Soft : Easily moulded with the fingers.

(iv) Color of the soil indicates its origin and the condition under

which it was deposited.

Sand and gravel deposits may contain lenses of silt, clay or even

organic deposits. If so, the presumptive bearing capacity is

reduced.

Based on the field identification of the soil, the presumptive

bearing capacity of the soi l can be guessed by referring to table

2 of IS 1904 1986. The objectives of preliminary soil

investigation are to drawn up an appropriate program for detailed

soil investigation and to examine the sketch plans and

preliminary drawings prepared by the Arch itect from the point of

suitability of the proposed structure.


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TABLE 1 : SAFE BEARING CAPACITY

.No. TYPE OF ROCKS/ SOILS

SAFEBEARING

CAPACITY

REMARKS

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

a) Rocks kN/m

1. Rocks (hard) without lamination defects,

for example, granite , t rap and diori te

3 240 -

2 . Laminated rocks, for example, stone and

limestone in sound condit ion

1 620 -

3 . Residual deposits of shattered and broken

bed rock and hard sha le, cemented

mater ia l

880 -

4 . Soft rock 440 -

b) Non-cohesive soils

5. Gravel, sand and gravel , compact and

offering high resistance to penetration

when excavated by tools

440 (See Note 2)

6 . Coarse sand, compact and dry 440 Dry means that the

ground water level is a ta depth not less than the

width of foundation

below the base of the

foundat ion

7. Medium sand, compact and dry 245 -

8 . Fine sand, si te(dry lumps easily pulverizedby the fingers)

150 -

9 . Loose gravel or sand gravel mixtures,

loose coarse to medium sand, dry

245 (See Note 2)

10. Fine sand, loose and dry 100 -

c) Cohesive soils

11. So ft sh al e, or st if f cl ay in de ep be d, dr y 44 0 This group is susceptible to

long term consolidation

settlement


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12. Medium clay, readily indented with a

thumb nail

245 -

13. Moist clay and sand-clay mixture which

can be indented with strong thumb

pressure

150 -

14.

Soft clay indented with moderate thumb

pressure

100

-

15.

Very soft c lay which can be penetratedseveral centimeters with the thumb

50

-

NOTE : Values are very much rough for the following reasons:

a)

Effect of characteristics of foundations (that is, effect of depth,width, shape, roughness, etc) has not been considered.

b) Effect of range of soil properties (that is, angle of frictional

resistance, cohesion, water table, density, etc) has not been

considered.

c) Effect of eccentricity and indication of loads has not been

considered .

3.0 Soil Mechanics Basic Concepts

3.1 Soil Mass Represented By 3-Phase System: -Soil solids, water and air are constituents of soil mass are represented

diagrammatically as three phase system shown below.

V s =Volume of soil solids. Ws =Weight of soil solids.

Va =Volume of air. Wa =Weight of air considered as

negligible.

Vw =Volume of water. Ww =Weight of water.

V=Total volume of soil mass =V s+ Va + Vw

W=Total Weight of soil mass = W s+ Ww


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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 solids

The water content is generally expressed as a percentage.

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

unit weight or specific weight . In SI units is expressed as N/m 3

or kN/m3 . In soil Engineering five different five unit weights are

used in various computations.

i) Bulk Unit Weight ().

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

Thus, = W

V

ii) Dry Unit Weight (d). : The dry unit weight is the weight of

soil solids per units total volume of the soil mass.

d = Ws

V

The dry unit weight is used to express the denseness of the soil.

i i i) Unit Weight of Soil Solids (s) : The unit weight of soil solids

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

s = W s

Vs

iv) Saturated Unit Weight ( sa t) : When the soil mass is saturated,

its bulk unit weight () is called saturated unit weight. The

saturated unit weight is the ratio of the total soil mass of

saturated sample to its total volume.

sa t= W s (saturated)

V

v) Submerged Unit Weight (): When the soil exits below water it

is in submerged condition. The submerged unit weight () of

soil is defined as the submerged weight per unit total volume.


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= W su b= sa t - w

V

3. Specific gravity G : is defined as the ratio of the unit weight of

soil solids to that of water:

G = s / w

4. Voids ratio . (e) Voids ratio e of a given soil sample is the

ratio of the volume of voids to the volume of soil solids in the

given soil mass.

Thus, e = V v/V s = n / 1-n

5.

Porosity . (n) The porosity n of a given soil sample is the ratioof the volume of voids to the total volume of the given soil mass.

Vv e

n = =

V e +1

The voids ratio e is generally expressed as a fraction, while the

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

to as percentage voids.

6 Degree of Saturation . The degree of saturation Sr is defined as

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

the total volume of voids in it .

Sr = Vw

Vv

6. Various Inter-Relations

i)

e. S r= w.Gii) e = w.G (for Sr = 1 or fully saturated soil degree of

saturation 100% )

G . w

i i i) d = 1 + e

iv) ( G + e.S r ) w

= 1 + e

v) For S r = 0 ,

G . w = d = 1 + e


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vi) For S r = 1 ,

= sa t = ( G + e)w

1 + e

vi) d = 1 + w

vii) = (G - 1)w

1 + e

7. Density Index : The term density index ID or relative density or

degree of density is used to express the relative compactness of a

natural soil deposit. The density index is defined as the ratio of

the difference between the voids ratio of the soil in its looseststate and its natural voids ratio (e) to the difference between the

voids ratios in the loosest and densest states:

emax - e

ID emaxemin

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= emaxand hence ID = 0. When the natural deposit is in

its densest state e = eminand hence ID = 1.

4.0 Plasticity Characteristics of SoilsPlasticity of soil is its ability to undergo deformation without cracking

or fracturing. Plasticity is an important index property of fine grained

soils, especially clayey soils.

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 Atterbergdivided

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.


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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 anarbitrary 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 : 14981970: Classification and

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

Soils are broadly divided into three divisions.


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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 subdivisions:

(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:


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(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.


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Table 2.0 Basic Soil Components (IS Classification)

SoilSoil

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 sieveFine : 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 sieveidentified by behavior , that it is slightly

plastic or non plastic regardless of moisture

and 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 OOrganic matter in various sizes and stages of

decomposition.


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Table 3.0 Classification of Coarse-grained Soils (ISC System)

Di vi si on Su bd iv is io n Grou p symbol Ty pi ca lNames

La bo rat or y Cr ite ri a Remark

(1) Coarse-

grained soils(More than

half of

material islarger than

75-micro

Gravel (G)

(more thanhalf of coarse

fraction is

larger than4.75 mm IS

sieve)

Clean

gravels(Fines less

than 5%)

(1) GW

(2) GP

Well gradedgravels

Poorly

graded

gravels

Cu greater than 4Cc between than 1 and 3

No t mee ti ng al l gra dat io n

requirements for GW

When fines are

be twe en 5% to12% border

line cases

requiring dualsymbols such

as GP-GM,

SW-SC, etc.

Gravels

with

appreciable

amount of

fines (Fines

more than

12%)

(3) GM Silty gravels

Clayey

gravels

Atterberg

limits

bel ow A-

line or Ip

less than

4

Atterberglimits

bel ow A-

line or Ip less than

7

Atterberg

Limits plotting

above A-line

with Ip

bet we en 4 an d

7 are border-

line cases

requiring useof dual symbol

GM-GC

(4) GC


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Table (Continued)

Di vi si on Su bd iv is io n Gr ou p sym bo l Ty pi ca l

Na me s

La bo rat ory Cri ter ia Re ma rk

Sand (S) (more

than half of

coarse fraction

is Smaller than

4.75 mm IS

sieve)

Clean Sand

(Fines less

than 5%)

(5) GW

(6) SP

Well - graded

gravels

Poorly -graded

gravels

Cu greater than 6

Cc between than 1 and 3

No t mee ti ng al l gra dat io nrequirements for SW

Sands with

appreciable

amount of

fines

(Fines more

than 12%)

(7) SM

(8) SC

Silty Sands

Clayey

gravels

Atterberg

limits

bel ow A-

line or Ip

less than

4

Atterberg

limits

bel ow A-

line or Ip

less than

7

Atterbergs

Limits plotting

above A-line

with Ip

bet we en 4 an d

7 are border-

line cases

requiring use

of double

symbol SM-SC


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(Continued)

Di vi sion

Subdivision GroupSymbols

Typical names Laboratory Criteria (see Fig 5.6) Remarks

(2) Fine

grained

soils

(morethan

50%

pas s 75

IS

Sieve)

Low-

compressibili

ty (L)

(Liquid Limit

less than35%)

Intermediate

compressibili

ty

(I)

(Liquid limit

greater than

35 but less

than 50%

(1) GW

(2) CL

(3) OL

(4) MI

(5) CI

(6) OI

Inorganic silts

with none to low

pl as ti ci ty

Inorganic clays of

low plasticity

Organic silts of

low plasticity

Inorganic silts of

medium plasticity

Inorganic clays of

medium plasticity

Organic silts ofmedium plasticity

Atterberg limits

plo t be lo w A-

line or Ip less

than 7

Atterberg limits

plo t be lo w A-line or Ip less

than 7

Atterberg limits

plo t be lo w A-

line

Atterberg limits

plo t be lo w A-

line

Atterberg limits

plo t ab ov e A-

line

Atterberg limitsplo t be low A-

line

Atterberg limits

pl ott in g ab ov e A-

line with Ip

be tw ee n 4 to 7

(hatched zone) ML-

CL

(1) Organic and inorganic

soils plotted in the same

zone in plasticity chart

are distinguished by odour

and colour or liquid limit

test after oven-drying. Areduction in liquid limit

after oven-drying to a

value less than three-

fourth of the liquid limit

bef ore ove n-d ry ing is

pos it ive id en ti fi ca tio n of

organic soils.

(2) Black cotton soils of

India lie along a band

par tl y ab ov e th e A- li ne

and partly below the A

line


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(Continued)

Di vi si o

n

Subdivision Group

Symbols

Typical names Laboratory Criteria (see Fig

5.6)

Re ma rks

High

compressibi

lity (H)

(Liquidlimit

greater than

50%)

(7) MH Inorganic silts of high

compressibility

Atterberg limits plot below

A-line

Seepla sti ci ty

chart (Fig.

56)

(8) CH Inorganic clays of highpl as ti ci ty

Atterberg limits plot belowA-line

(9) OH Organic clays of

medium to high

pl as ti ci ty

Atterberg limits plot below

A-line

(3)

Highlyorganic

soil

PT Peat and other highlyorganic soils

Readily identified by

colour, odour, spongy feeland fibrous texture


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6.0 Bearing Capacity

Definitions1. 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= qDwhere 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 = quD

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+ D.


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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 onsoils 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- 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+ D (Nq1) + 0.5 B N ---------------(1)

ii) For the case of local shear failure:

qnu= 2/3 cNc + D (Nq

1) + 0.5 B N------------(2)

For obtaining Nc, Nq, N bearing capacity facotorscorresponding to local shear

failure, calculate(m) = tan-1(0.67 ) and read Nc, Nq , N for general shear

failure as given in table 4.0 below.


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Table 4.0 Bearing Capacity Factors (Is : 64031981)

Degree Nc Nq Nr

0 5.14 1.0 0.0

5 6.49 1.57 0.45

10 8.35 2.47 1.2215 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+ D (Nq-1) Sq dq iq+1/2 B N S d i w -------(1)

ii) For local shear failure

qnu = 2/3 cNc

Sc dc ic+ D(Nq-1) Sq dq iq+1/2 B N

S d i w

---(2)

The depth factors are given as under ;

dc =1+ 0.2 (D/B ) N1/2 where N= tan

2 (45+ /2)

dq = d =1 for 100

Shape Shape factors

Sc Sq S 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- /90)2 and i = (1- / )

2

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


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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


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The net ultimate bearing capacity of shallow strip footing on cohesion

less soil deposit is then determined from Fig. given in the IS Code.

6. Bearing Capacity of Cohesive Soils ( = 0)

The net ultimate bearing capacity immediately after construction on

fairly saturated homogenous cohesive soils can be calculated from theexpression.

qnu = c Nc Sc dc ic

Where Nc = 5.14 (for =0)

The value of c is obtained from unconfined compressive strength

test. Alternatively, cohesion c may be determined from the static cone

point resistance.

7.0 Planning for Soil InvestigationSoil investigation must conform to the provisions in I.S. 1892 1979.

The scope of investigation is indicated in para 2.1 and 2.2 of this code.

Engineering properties of soil depend on the soil structure, i.e. nature

of soil grains and their arrangement, volume of air and water (degree

of saturation and porosity). Since these vary from one location to

another, the program of soil investigation needs to be evolved for each

project. It should provide for adequate data and ma ke appropriate

recommendation supported by proper calculations in respect of the

following:

1. The type of foundation.

2.

Allowable bearing capacity for the foundation.

3.

Total and differential settlements.

4.

Highest groundwater level ever reached.

5.

Anticipated construction problems and suggested solution

(sheep piling, dewatering, boulders/rock excavation,

differential, settlements, damage to adjacent property,

environment etc.)

A copy of the surveyed site plan and layout plan of buildingsindicating the type and sizes of the buildings are required. It is

essential that the location of bore holes together with the reduced

levels are marked on the site plan.

To determine the nature and extent of detailed soil investigation, a

preliminary investigation is necessary as stipulated in para 3.1.1 of

I.S. 1892 - 1979. Knowing the type of superstruct ure, the first step is

to inspect the site and its neighborhood and collect the information

about the soil profile, type of foundation generally adopted and toguess the presumptive allowable bearing pressure for the soil. This is


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

investigation details are not available for nearby sites, a test pit or a

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

Knowledge of regional soil deposits corresponding to the locality,prevalent practices of subsoil investigation and foundation design

greatly facilitate drawing up an appropriate program of soil

investigation. Major regional soil deposits of India are - Alluvial soils,

Black cotton soils, Laterities, Desert soils and Sub marine soils

(Reference may be made to Indian contributions to Geotechnical

Engineering published by Indian Geotechnical society for sources of

information of the Regional deposits).

1. Detailed soil investigation

Degrees o f app l i cab i l i ty o f var ious f i e ld and labora tory

tes t s are ind ica ted in Tab le s 1 and 2 . The s i tua t ions in

which each tes t i s app l i cab le and the l imi ta t ions o f s uch

tes t s are d i s cus s ed in the fo l lowing paragraphs .

In arriving at the allowable bearing pressure on foundations, both the

ultimate bearing capacity (based on shear strength and the permissible

settlement are taken into account. Normally settlement governs the

design but for narrow strip foundations on soft at shallow depths,

bearing capacity based on shear failure may govern.

1.1 Characteristi cs of soil in foundation

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

internal friction ( c - soils )

Sand and silt are cohesion less soils. Silt with even 5 to 8 percent of

clay has significant cohesion. Shear strength, s of soil is developed

due to resist ance to rolling, sliding and deformation of soil

particles/skeletal structure. Cohesion, c is due to inter particle

attraction due to presence of clay and the angle of internal friction is essentially due to resistance to inter particle slip of coarser grains

like silt and sand.

Shear strengths is given by s = c + tan

Where 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 claye y soils. The param eters c and

corresponding to maximum shear strength are determined by

considering effective pressures which are equal to total pressureminus pore water pressure. These are determined by consolidated


24/45



drained test for cohesion less soils (and for c - soils if insitu drainage

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

pressure is dissipated completely through a slow process of

consolidation and an equally slow process of shear. The time required

for gradual increment of load upto shear failure is determined as perappendix A of I.S. 2720 (part 13) 1986. soil in situ exists, generally,

in a consolidated state ( 3 ). As construction proceeds, additional

loads come on to the soil. If the permeabili ty of the soil is low, w hich

can occur if the fine grained soil contains more than 15 percent clay

and is classified as clay with intermediate or high compressibility, the

excess pore water pressures developed in the clayey soil can not

dissipate as fast as the rate of application of load. Hence for clayey

soils with appreciable clay content ( say more than 15 percent), the soil

parameters C and are determined from consolidated un -drained test

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

clay content is high ( say 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 results are represent ative of field conditions

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

tri-axia l shear). For semi pervious cohesive soils, the consolidated

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

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

shearing in Box Shear Test introduces an element of error. Shear

strength of stiff intact clays such as boulder clays, clayey silts arebetter determined by drained tests since the soils are generally over

consolidated.

Saturation reduces the shear strength and long term time dependant

consolidation of clay takes place during testing, only if the soil is

saturated. It is thus necessary to determine shear st rength of the soil in

saturated condition if the soil in situ is likely to be saturated due to

rising of the ground water table. Hence it is essential to ascertain the

highest ground water level ever reached. Due to the capacity of clay to

absorb water by capillar y action and the very large variation in shear

strength of unsaturated clayey soils with moisture content, results of

Box Shear Test cannot reliabl y represent in situ shear strength of

unsaturated clay. Even while considering the results of consolidated

un-drained Tri-axi al Test or in situ test on unsaturated soils, the

effect of variation of insitu shear strength due to possible change in

moisture content due to rain or rise in water table needs to be

considered.


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Satisfactory undisturbed samples of cohesion less soils are difficult to

obtain from bore holes. Soil obtained from the split spoon sampler

from standard penetration test may possess large shear strains due to

disturbance. Hence shear tests in the laboratory on cohesion less soils

do not represent the true site condition. The most common field testis the standard penetration test (Ref. I.S. 2131 1981). This test, if

carefully executed, in soil undisturbed by boring operations, enables to

estimate satisf actorily the bearing capacit y as per I.S. 6403 - 1981

and allowable bearing pressure on settlement consideration as per I.S.

8009 (Part 1) 1976. By using the same equipment and with the same

driller, N values in the same soil can be reproduced with a

coefficient of variation of about 10 percent. Use of defective

equipments such as a damaged anvil, worn out driving shoe,

old/oily/poorl y lubricat ed rope sheaves etc. can result in significantl y

erroneous N values. Pushing a boulder while driving the sampler,

rapid withdrawal of sugar or bit plug causing a quick condition at the

bottom of the bore hole by too much difference in the water levels

between the ground water table and in the hole are other sources of

error.

The original standard penetration Test was developed for sand.

However, at present it is commonly used for all types of soils.

Alluvial silt deposits are mixtures of medium dense fine sand and silt

with a small percent of clay. In some cases, layers of stiff soil areencountered at depth of 6 to 10 meters. Delhi silt has about 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 nonplastic (indicated by the inability to perform the test todetermine 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


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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/P ocket pentro-me ter. It is usualpractice 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 consolidate d.

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 necessar y to adopt low

bearing pressures or use a raft or piles. Alluvial clays , especially

marine clays, are sensiti ve 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 sandy/alluvial soils, if ground water table is lowered, ground

subsidence in the area surrounding the construction site may occur due

to consolidation of underlying clayey layers. In such a case, it may be

necessar y to provide a water retaining barrier around the site if

structures exists adjacent to the excava tion (since pumping to dewater

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

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

saturate it , the surface tension makes it possible to provide shallow

excavations with near vertical sides. With continued drainage and

evaporation or vibration, the sides collapse. Near vertical excavation

in a cohesive soil may collapse due to rainfall softening the clay andcreating excess pore water pressure.


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Excavation in sands below the water table may result in a slumping of

the sides and boiling of the bottom, unless a properly designed ground

water lowering system is adopted.

If excavation goes below the firm surface crust of alluvial clay,support by timbering or sheet pilling is and stiffened trenches are

prone to failure by heaving of the bottom and bulging of the side

supports.

1.3 Programme of detailed soil investigation

In planning the Programme, full advantages should be taken of

available informati on from preliminar y investigati on, geo technical

consultants data base and soil Investigation reports for the nearby

sites and their correlation with actual performance of buildings and

load tests on piles. If rock is encountered in a bore hole, bor ing must

extend at least 2 meters to differenti ate a boulder from bed rock. If

rock is encountered in different bore holes near about the proposed

foundation level, adequate number of bore holes are required to plot

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

knowledge, details of in situ tests and laborator y tests are worked out

keeping in view the limitation of each.

Current methods of subsoil exploration are outlined in Appendix A of

IS 1892 1979 and the tests generally required are indicated in Table

3 and Appendix A of this Code of Practice.

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

present in any typ e of soil , the description should include with. For

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

with sand or gravel is written for percentages between 15 and 29

and gravelly of sandy for larger percentages.

Sands or gravels may be classified by the standard penetration tests

into broad groups as follows:

No of S.P.T. blows N

L o o se L ess th an 1 0

Medium Dense 10 to 30

D e n s e ( o r c o m p a c t ) M o r e t h a 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)


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After establishi ng correlati on on the basis of other reliable tests,

standard penetration test results have been in use for many years for

relative density, angle of internal friction, un-drained compressive

strength, settlement and modules of sub grade reaction. Some of these

are of questionable value unless corroborated by adequate calibratio ndata for the localit y since many were originally proposed without

extensive study of the large number of variables affecting the N

values.

A . Tests required for classification of soils

1)

Classification as per IS 1498 1970 based on particle size

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 classified as clay of

intermediat e or high compressibili ty, It is necessar y to

determine the clay and silt percentages separately. Hence in

addition to sieving, pipette or hydrometer test is necessary to

determine the percentage of clay.

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

necessary to determine in situ water content in addition to

liquid limit and plastic limit of re-moulded soil.

B. Tests required to determine safe bearing capacity of shallow

foundations ( including raft)as per I.S. 64031981.

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

tests for classification of soil as per I.S. 14981970 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. csoils 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 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 arerequired if shear strength is the critical criterion.


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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 shear tests for predominantly cohesive soils. If shear strength is

likely to be critical.

C. Tests required to determine 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 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. 18881982 for cohesion less soils and c 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 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.


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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 determinedfrom 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 ( 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- 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 encounteringboth 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 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 from remoulded samples also. Drained shear


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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 withsupplementary 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 beencorrectly interpreted.

2.1 Bearing capacityFor shallow foundation, the current practice is to use an average N value in the zone

affecting soil behavior. For a 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.


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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. 64031981.

Recent geo technical studies indicate that prediction of consolidation

settlements are 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 3/4thof 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. 19041986, 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 directionsmay 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.


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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 soildeposit 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 andflexible. 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 containingless 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.


34/45



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 stiff fissured clays, bored cast in situ piles or low. Displacemen t

driven piles are usually recommended. Dense silts cause high

penetration resistance for driven piles but the capacity of the pile

remains low due to disturbance of the soil during driving.

Normally consolidated clays cause down drag on bored cast in

situ piles due to consolidated on account of drainage occurring as a

result of boring.

Point resistanc e and skin friction of pile in sand increas es as the

length of the pile increases upto the critica l depth equal to 10 times

the pile diameter for loose sand and 20 times for dense sand, Beyond

this length, the values remain constant.

Point resistance of 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 strat um upto a maximum value corresponding

to 8 to 12 times the diameter of the pile.

Except for bored piles in sand capacit y of a group of piles equals the

sum of the capacities of individual piles in the group. In case of bored

piles in sand, the capacity is about two thirds the sum of capacity.

Check is necessary for failure of the pile group as a single block.


35/45



Pile capacity may be calculat ed by several appropriate methods so as

to establish upper and lower bound values. Errors are very high when

results from one type of soil deposit in one locality or valid for one

year of pile are extrapolate d to derive the value for different deposits

in another locality or another type of pile involving a differentconstruction technique.

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

minimum, the recommendation should indicate the highest possible

capacit y of the pile considering the soil parameters , the bore log and

the appropriate type of pile.

5. Conclusion:

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


36/45



LIST OF VARIOUS FOUNDATION ENGINEERING CODES


37/45



SP 36 : Part 1 : 1987 Compendium of Indian standards on soil engineering: Part 1 Laboratory testing of soils for civil

engineering purposes

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

IS 1080 : 1985 Code of practice for design and construction of shallow foundations in soils (oth er than raft, ring and shell)

IS 1498 : 1970 Classification and identification of soils for general engineering purposes

IS 1725 : 1982 Specification for soil based blocks used in general building construction

IS 1888 : 1982 Method of Load Test on SoilsIS 1892 : 1979 Code of practice for subsurface investigations for foundations

IS 1904 : 1986 Code of practice for design and construction of foundations in soils: 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 of test for soils: Part 2 Determination of water content

IS 2720 : Part 3 : Sec 1 : 1980 Methods of test for soils: Part 3 Determination of specific gravity Section 1 fine grained so ils

IS 2720 : Part 1 : 1983 Methods of Test for Soils - Part 1 : Preparation of Dry Soil Samples for Various Tests

IS 2720 : Part III : Sec 2 : 1980 Test for Soils - Part III : Determination of Specific Gravity - Section 2 : Fine, Medium and

Coarse Grained Soils

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

IS 2720 : Part 5 : 1985 Method of Test for Soils - Part 5 : Determination of Liquid and Plastic Limit

IS 2720 : Part 6 : 1972 Methods of test for soils: Part 6 Determination of shrinkage factors

IS 2720 : Part 9 : 1992 Methods of test for soils: Part 9 Determination of dry density- moisture content relation by constant

weight of soil method

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

IS 2720 : Part 11 : 1993 Methods of test for so ils: Part 11 Determination of the Sh ear Strength Parameters of a specimen


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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 Determination of shear strength parameters of soil from

consolidated undrained triaxial compression test with measurement of pore water pressure

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 : Determination of Density Index (Relative Density) of

Cohesionless Soils

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

IS 2720 : Part VII : 1980 Methods of Test for Soils - Part VII : Determination of Water Content-Dry Density Relation Using

Light Compaction

IS 2720 : Part 8 : 1983 Methods of Test for Soils - Part 8 : Determination of Water Content-Dry Density Relation Using

Heavy Compaction

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

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

IS 2720 : Part 23 : 1976 Methods of test for soils: Part 23 Determination of calcium carbonate

IS 2720 : Part 25 : 1982 Methods of test for soils: Part 25 Determination silica sesquioxide ratio

IS 2720 : Part 16 : 1987 Methods of Test for Soil - Part 16 : Laboratory Determination of CBR

IS 2720 : Part 17 : 1986 Methods of Test for Soils - Part 17 : Laboratory Determination of Permeability

IS 2720 : Part 18 : 1992 Methods of test for Soils - Part 18 : Determination of Field Moisture Equivalent

IS 2720 : Part 19 : 1992 Methods of Test for Soils - Part 19 : Determination of Centrifuge Moisture Equivalent

IS 2720 : Part XXI : 1977 Methods of Test for Soils - Part XXI : Determination of Total Soluble Solids

IS 2720 : Part XXIV : 1976 Methods of Test for Soils - Part XXIV : Determination of Cation Exchange Capacity

IS 2720 : Part 27 : 1977 Methods of test for soils: Part 27 Determination of total soluble sulphates

IS 2720 : Part 28 : 1974 Methods of test for soils: Part 28 Determination of dry density of soils inplace, by the sand


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replacement method

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

IS 2720 : Part 33 : 1971 Methods of test for soils: Part 33 Determination of the density in place by the ring and water

replacement method

IS 2720 : Part 35 : 1974 Methods of test for soils: Part 35 Measurement of negative pore water pressure

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

IS 2720 : Part XXIX : 1975 Methods of Test for Soils - Part XXIX : Determination of Dry Density of Soils In-place by theCore-cutter Method

IS 2720 : Part 31 : 1990 Methods of Test for Soils - Part 31 : Field Determination of California Bearing Ratio

IS 2720 : Part XXXIV : 1972 Methods of Test for Soils - Part XXXIV : Determination of Density of Soil In-place by Rubber-

ba ll oo n Me th od

IS 2720 : Part 36 : 1987 Methods of test for soils: Part 36 Laboratory determination of permeability of granular soils

(constant head)

IS 2720 : Part 37 : 1976 Methods of test for soils: Part 37 Determination of sand equivalent values of 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 : Determination of Free Swell Index of Soils

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

IS 2720 : Part XXXIX : Sec 1 : 1977 Methods of Test for Soils - Part XXXIX : Direct Shear Test for Soils Containing Gravel

- Section I : Laboratory Test

IS 2720 : Part XXXIX : Sec 2 : 197 9 Methods of Test for Soils - Part XXXIX : Direct Shear Test for Soils Containing Gravel

- Section 2 : In-Situ Shear Test

IS 2809 : 1972 Glossary of Terms and Symbols Relating to Soil Engineering

IS 2810 : 1979 Glossary of terms relating to soil dynamics


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IS 2911 : Part 1 : Sec 1 : 1979 Code of practice for design and construction of pile foundations: Part 1 Concrete piles, Sect ion

1 Driven cast in-situ concrete piles

IS 2911 : Part 1 : Sec 2 : 1979 Code of practice for design and construction of pile foundations: Part 1 Concrete piles, Section

2 Bored cast-in-situ piles

IS 2911 : Part 1 : Sec 3 : 1979 Code of practice for design and construction of pile foundations: Part 1 Concrete piles, Sect ion

3 Driven precast concrete piles

IS 2911 : Part 1 : Sec 4 : 1984 Code of practice for design and construction of pile foundations: Part 1 concrete piles, 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 piles

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 for impact

type machine (hammer foundations)

IS 2974 : Part 3 : 1992 Code of practice for design and construction of machine foundations: Part 3 Foundations for 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 for 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 breakers, drop crusher and jolter)

IS 2974 : Part I : 1982 Code of Practice for Design and Construction of Machine Foundations - Part I : Foundation for

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 of test for stabilized soils: Part 1 Methods of sampling and preparation of stabilized soils for

testing


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IS 4332 : Part 3 : 1967 Methods of test for stabilized soils: Part 3 Test for determination of moisture content-dry density relation for

stablized soils mixtures

IS 4332 : Part 4 : 1968 Methods of test for stabilized soils: Part 4 Wetting and drying, freezing and thawing tests for compa cted soil-

cement mixtures

IS 4332 : Part 5 : 1970 Methods of test for stabilizd soils: Part 5 Determination of unconfined compressive strength of stablized soils

IS 4332 : Part II : 1967 Methods of Test for Stabilized Soils - Part II : Determinat ion of Moisture Content of Stabilized Soil Mixtures

IS 4332 : Part 8 : 1969 Methods of test for stablized soils: Part 8 Determination of lime content of lime stablized soils

IS 4332 : Part 10 : 1969 Methods of test for stabilized soils: Part 10 Test for soil/bituminous mixtures

IS 4332 : Part VI : 1972 Methods of Test for Stabilized Soils - Part VI : Flexural Strength of Soil-cement Using Simple Beam With

Third-point Loading

IS 4332 : Part VII : 1973 Methods of Test for Stabilized Soils - Part VII : Determinati on of Cement Content of Cement Stabilized Soil s

IS 4332 : Part IX : 1970 Methods of Test for Stabilized Soils - Part IX : Determinat ion of the Bituminous Stabilizer Content of

Bitumen and Tar Stabilized Soils

IS 4434 : 1978 Code of practice for in-sit u vane shear test for soils

IS 4968 : Part 1 : 1976 Method for subsurface sounding for soils: Pa rt 1 Dynamic method using 50 mm cone without betonite slurry

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

IS 4968 : Part II : 1976 Method for Subsurface Sounding for Soils - Part II : Dynamic Method Using Cone and Bentonite Sl urry

IS 5249 : 1992 Method of test for determination of dynamic properti es of soil

IS 6403 : 1981 Code of practice for determination of bearing capacity of shallow foundations

IS 8009 : Part II : 1980 Code of Practice for Calculation of Settlement of Foundations - Part II : Deep Foundations Subjected to

Symmetrical Static Vertical Loading

IS 8009 : Part I : 1976 Code of Practice for Calculation of Settlements of Foundations - Part I : Shallow Foundations Subjected to

Symmetrical Static Vertical Loads

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

IS 9198 : 1979 Specification for compaction rammer for soil testing


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

IS 9259 : 1979 Specification for liquid limit apparatus for soils

IS 9456 : 1980 Code of practice for design and construction of conical and hyperbolic paraboloidal types of shell foundat ions

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

IS 9640 : 1980 Specification for split spoon sampler

IS 9669 : 1980 Specification for CBR moulds and its accessories

IS 9716 : 1981 Guide for lateral dynamic load test on pilesIS 9759 : 1981 Guidelines for de-watering during construction

IS 10042 : 1981 Code of practice for site-investigations for foundation in gravel boulder deposits

IS 10074 : 1982 Specification for compaction mould assembly for light and heavy compaction test for soils

IS 10077 : 1982 Specification for equipment for determination of shrinkage factors

IS 10108 : 1982 Code of practice for sampling of soils by thin wall sampler with stationery piston

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

IS 10379 : 1982 Code of practic for field control of moisture and compaction of soils of embankment and subgrade

IS 10442 : 1983 Specification for earth augers (spiral type)

IS 10589 : 1983 Specification for equipment for determination of subsurface sounding of soils

IS 10837 : 1984 Specification for moulds and accessories for determination of density index (relative densit y) of cohesionles s

soils

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

IS 11196 : 1985 Specification for equipment for determination of liquid limit of soils cone penetration method

IS 11209 : 1985 Specification for mould assembly for determination of permeability of soils

IS 11229 : 1985 Specification for shear box for testing of soils

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


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IS 11550 : 1985 Code of practice for field instrumentation of swelling pressure in expansive soils

IS 11593 : 1986 Specification for shear box (large) for testing of soils

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

IS 11629 : 1986 Code of practice for installation and operation of single point hydraulic over-flow setting gauge

IS 12023 : 1987 Code of practice for field monitoring of movement of structures using tape extensometer

IS 12175 : 1987 Specification for rapid moisture meter for rapid determination of water content for soil

IS 12208 : 1987 Method for measurement of earth pressure by hydraulic pressure cell

IS 12287 : 1988 Specification for consolidometer for determination of consolidation properties

IS 12979 : 1990 Specification for mould for determination of linear shrinkage

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

IS 13301 : 1992 Guidelines for vibration isolation for machine foundations

IS 13468 : 1992 Specification for apparatus for determination of dry density of soils by core cutter method

IS 14893 : 2001 Non-Destructive Integrity Testing of Piles (NDT) Guidelines

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


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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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Chapter02.Oerview of Soil Mechanics & Foundation Design

Documents

Transcript of Chapter02.Oerview of Soil Mechanics & Foundation Design