E1-E2 Civil Technical
Transcript of E1-E2 Civil Technical
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
For internal circulation of BSNL only
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
For internal circulation of BSNL only
Three Phase System
Soil Mechanics –Basic Concepts
For internal circulation of BSNL only
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
For internal circulation of BSNL only
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
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
Plasticity Characteristics of Soils
For internal circulation of BSNL only
• 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
For internal circulation of BSNL only
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
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
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,
GM means Silty Gravel, SM means Silty Sand and PT means
Peat and other highly organic soils.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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,
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
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
For internal circulation of BSNL only
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.
For internal circulation of BSNL only
For internal circulation of BSNL only