Soil Engineering BEng 2
Week 1
SHEAR STRENGTH OF SOILS
1.0
Introduction
All failures in soil are shear failures, there is no tension, and compression
leads to shear. The shear strength of a soil is its maximum resistance to
shear usually expressed as a stress. The shear stress, strain relationship is
similar to most engineering materials:Peak shear stress

Ultimate shear stress
Residual
shear stress
Strain 
%
At small strains the stress strain relationship is linear and ELASTIC. As the
soil yields the strain increases and a PEAK shear stress is reached at which
point FAILURE occurs. The stress falls away until it reaches its Ultimate
shear stress at its critical volume. The shear stress falls away with increasing
strain to approach a RESIDUAL value.
Shear stresses are used in assessing the stability of existing and designed
slopes, foundation design and forces on retaining walls.
2.0
Principal Stresses
A plane acted on by a normal stress is referred to as a principal plane and the
stress is called a principal stress. Any stress may then be equated to a
principal stress and a shear stress.
Introduction to Shear
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Soil Engineering BEng 2
Week 1

This can be converted to:
n

+
A tiny element in the soil mass will have a number of principal stresses acting
on it:-
1
2
3

Introduction to Shear
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Soil Engineering BEng 2
Week 1
The major principal stress is 1 and the minor principal stress is 3, the
intermediate stress is 2 is ignored in the commonest form of analysis.
These stresses are represented graphically in engineering by the MOHR
diagram:-

n
3
1
3.0 Soil Strength Parameters
There are two components of shear strength.
a cohesive element and
a frictional element
The cohesive element is derived from the interparticulate forces, Van de
Waals, which draw particles together. Cohesion is sensitive to water and
porewater chemistry. Cohesion is developed quickly to a maximum under
small strains and falls away with strain increase. This apparent cohesion is
usually is usually notated by cu or su.
Introduction to Shear
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Soil Engineering BEng 2
Week 1
The frictional element is derived from intergranular contact and is not
developed to its maximum value until significant amounts of strain have
occurred. The "internal friction" angle is notated as .
In diagrammatic terms
Friction
Cohesion
Strain
This characteristic of soil strength must influence our selection of design
parameters:
For a pile which we wish to restrict the movement of;
we use primarily cohesion as design parameter.
For an embankment where strains to failure may be large:
we tend to rely on the frictional component of a soil for design.
Soils can be broadly classified by their shear strength characteristics and the
way in which the failure envelope is orientated on the shear stress normal
stress axes.
Introduction to Shear
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