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E1-E2

Civil Technical

Overview of Soil Mechanics and

Foundation Design

WELCOME

• This is a presentation for the E1-E2 (Civil) Module for the

Topic: Overview of Soil Mechanics & Foundation Design

• Eligibility: Those who have got the Up-gradation to from

E1 to E2.

• This presentation is last updated on 15-3-2011.

• You can also visit the Digital library of BSNL to see this

topic.

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AGENDA

Soil Mechanics –Basic Concepts

Plasticity Characteristics of Soils

Unified Soil Classification and ISC

Consolidation of Soils

Bearing Capacity

Deep Foundation

Failures in Soil

Soil Investigation

Tests Required for Classification of Soils

Soil Mechanics –Basic Concepts

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Three Phase System

Soil Mechanics –Basic Concepts

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Unit Weight:

• The weight of soil per unit volume is defined as unit

weight or specific weight. In SI units is expressed as

N/m3 or kN/m3.

Bulk Unit Weight (γ ).

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

unit of its total volume.

Thus,

γ = W

V

Soil Mechanics –Basic Concepts

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Dry Unit Weight (γd) : The dry unit weight is the weight

of soil solids per units total volume of the soil mass.

γd = Ws/V

The dry unit weight is used to express the denseness of

the soil.

Water content :

w = Weight of water x 100

Weight of soil solids

The water content is generally expressed as a percentage.

Soil Mechanics –Basic Concepts

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

of soil solids to that of water:

G = γs / γw

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

Porosity (n) The porosity n of a given soil sample is the

ratio of the volume of voids to the total volume of the given

soil mass.

n = Vv /V

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Soil Mechanics –Basic Concepts

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

Permeability of soils:- The property of soil which

permits flow of water through it, is called the

permeability.

It may be noted that the shear strength of cohesive

soils decreases, on wetting.

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Soil Mechanics –Basic Concepts

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 loosest state and its natural voids ratio (e) to

the difference between the voids ratios in the loosest and

densest states:

emax - e

ID = emax – emin

where emax = voids ratio in the loosest state

emin = voids ratio in the densest state

e = natural voids ratio of the deposit.

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Plasticity Characteristics of Soils

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• Definition:- Plasticity of soil is its ability to undergodeformation without cracking or fracturing. Plasticity is animportant index property of fine grained soils, especiallyclayey soils.

Plasticity Characteristics of Soils

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Atterberg Limits:-

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.

Plastic limit (wp). Plastic limit is the water content

corresponding to an arbitrary limit between the plastic and

the semi solid states of consistency of a soil. It is defined

as the minimum water content at which a soil will just

begin to crumble when rolled into a thread approximately

3 mm in a diameter.

Plasticity Characteristics of Soils

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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.

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

Unified Soil Classification & Indian Standard

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

are broadly divided into three divisions.

• Coarse grained soil. In these soils, 50% or more of the

total material by weight is larger than 75 micron IS sieve size.

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

material by weight is smaller than 75 micron IS sieve size.

• Highly organic soils and other miscellaneous soil materials.

These soil contain large percentage of fibrous organic matter,

such as peat, and the particles of decomposed vegetation. In

addition, certain soils containing shells, cinders and other

non soil materials in sufficient quantities are also grouped in

this division.

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Unified Soil Classification & Indian Standard

Classification• Coarse grained soils. Coarse grained soils are further

divided into two sub – divisions:

• 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. Its particle size is 80mm to 4.75mm.

• 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.

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Unified Soil Classification & Indian Standard

Classification• Each of the above sub-divisions are further sub divided into

four groups depending upon grading and inclusion of other

materials.

C : Clay binder

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,

GM means Silty Gravel, SM means Silty Sand and PT means

Peat and other highly organic soils.

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Unified Soil Classification & Indian Standard

ClassificationFine grained soils. Fine grained soils are further divided into

three sub divisions.

Inorganic silts and very fine sands :M

Inorganic clays :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:

Silts and clays of low compressibility, having a liquid limit

less than 35, and represented by symbol L.

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Unified Soil Classification & Indian Standard

ClassificationSilts and clays of high medium compressibility, having a

liquid limit greater than 35 and less than 50, and represented

by symbol I .

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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Consolidation of Soils

Over Consolidation of Soil:- A soil is said to be over

consolidated if it had been subjected to a pressure in excess

of the present pressure. It is caused due to a) Erosion of

over burden b) Melting of ice sheets after glaciations

c) permanent rise of water table.

Coefficient of Compressibility:- It is defined as decrease

in void ratio per unit increase in effective stress. It is the ratio

of strain to stress.

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Bearing Capacity

Foundation:- A foundation is that part of the structure

which is in direct contact with and transmits loads to the

ground.

Bearing capacity:- The supporting power of a soil or rock

is referred to as its bearing capacity.

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.

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Bearing Capacity

Net ultimate bearing capacity (qnu):- It is the net increase in

pressure at the base of foundation that causes shear failure of

soil.

qnu = qu– γD

Net pressure intensity (qn) :- It is defined as the excess

pressure, or the difference in intensities of the gross pressure

after the construction of the structure and the original

overburden pressure.

Thus, if D is the depth of footing

qn = q – γD

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

base.

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Bearing Capacity

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

Gross Safe bearing capacity (qs) :-The maximum pressure

which the soil can carrying safely without risk of shear failure

is called the safe bearing capacity. It is equal to the net safe

bearing capacity plus original overburden pressure.

qs = qns + γ D.

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Bearing Capacity

Effect of water table

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

correction factor w’ in the equation below:

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

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Bearing Capacity

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

Dw below the G.L. such that D<Dw<(D+B), then w’ be

obtained by linear Interpolation. It may be noted that if the

water table rises above the base of footing, w’ will remain at

its minimum value of 0.5.

Hence, the bearing capacity of soil decreases by

increase in water table.

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Deep Foundation

• When the soil at or near the ground surface is not capable

of supporting a structure, deep foundations are required to

transfer the loads to deeper strata.

• The most common types of deep foundation are piles,

piers & caissons.

Pile Foundation

• Piles can be classified according to (i) The material used,

(ii) The mode of transfer of load, (iii) The method of

construction, (iv) The use, (v) The displacement of soil.

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Deep Foundation

(i) Based on material used:-

a) Steel Piles, b) Concrete Piles, c) Timber Piles,

d) Composite Piles

(ii) Based on Mode of Transfer of Load:-

a) End Bearing Piles, b) Friction Piles, c) Combined End

Bearing & Friction Piles

(iii) Based on Method of Construction:-

a) Driven Piles b) Driven & Cast-in-situ c) Bored & Cast-in-

situ d) Screw Piles e) Jacked Piles

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Deep Foundation

(iv) Based on Use:-

a) Load bearing piles b) Compaction Pile c) Tension Piles d)

Sheet Piles e) Fender Piles f) Anchor Piles

(v) Based on Displacement of Soil:-

a) displacement Piles b) Non-displacement Piles

Negative Skin Friction :- When the soil layer surrounding a

portion of the pile shaft settles more than the pile, a

downward drag occurs on the pile. The drag is known as

negative skin friction. This friction develops when a soft or

loose soil surrounding the pile settles after the pile has been

installed.

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Failure in Soil

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

deformations associated with the load and the extent of

development of failure, three types of failure of soil support

beneath the foundations, they are (a) General Shear Failure;

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

occurs on soils of high compressibility. In such a failure, there

is vertical shear around the footing, perimeter and

compression of soil immediately under the footing, with soil

on the sides of the footing remaining practically uninvolved.

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Soil Investigation

• Soil samples are obtain during sub-surface exploration to

determine the engineering properties of the soils & rocks.

• Soil samples are generally classified into two categories

i. Disturbed samples:-

These are the samples in which the natural structure of

the soil gets disturbed during sampling. It can be used

to determine the index properties of the soil, such as

grain size, plasticity, specific gravity.

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Soil Investigation

ii. Undisturbed samples:-

These are the samples in which the natural structure of

the soil & the water content are retained. It can be used

to determine the engineering properties of the soil,

such as compressibility, shear strength and

permeability.

Soil sample can be collected mainly by following methods:-

i. Open Drive Method

ii. Piston Method

iii. Rotary Drilling Method

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Tests Required for Classification of Soils

• Tests required to determine safe bearing capacity of

shallow foundations ( including raft)

• Static cone penetration test

• Direct shear (controlled strain) test

• Standard penetration test

• Unconfined compressive strength test for highly cohesive

clays except soft/sensitive clays.

• Vane shear test for impervious clayey soils except stiff or

fissured clays.

• Tri-axial shear tests for predominantly cohesive soils. If shear

strength is likely to be critical.

• Box shear test for clayey soil.

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