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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
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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
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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.
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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.
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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.
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LIST OF VARIOUS FOUNDATION ENGINEERING CODES
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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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Recommended