SHEAR STRENGTH THEORY the shear strength of any material is the load per unit area or pressure that it can withstand before undergoing shearing failure.

1. SHEAR STRENGTH
THEORY
GEOTECHNOLOGY
SUBMITTED TO: DR. N. SHANKAR

2. INTRODUCTION
 In general, the shear strength of any material is the load per unit
area or pressure that it can withstand before undergoing shearing
failure.
 Shear strength:
o Soil’s ability to resist sliding
o Primarily depends on interaction between soil particles
Important for:
o Foundation design
o Lateral earth pressure calculations
o Slope stability

3. SHEAR STRENGTH OF SOIL
 Soil is weak in tension
 Soil can resist compression
 For excessive compression, failure occur in the form
of shearing along the internal surface within the soil
 The failure in soil occurs by relative movement of
the particles and not by breaking of particles

4. SHEAR STRENGTH IN SOILS
 Soil derives its shear strength from two sources:
• Cohesion between particles(stress independent component)
• Frictional resistance between particles(stress dependent
component)

5. General curve for shear strength of
soil
S
=ANGLE OF INTERNAL FRICTION
C



tan
tan


cs
s

6. COULOMB EQUATION
S: shear strength
C: cohesion
: Angle of internal friction
Effective intergranular normal pressure
 tan cs

:σ

7. MOHR-COULOMB THEORY
 According to Mohr, the failure is
caused by a critical combination of
normal and shear stress
 The soil fails when the shear
strength ‘s’ on the failure plane at
failure is a unique function of
normal stress ‘ ’ acting on that
plane
s=f( )
 Failure of material occurs when the
Mohr circle of stresses touches the
Mohr envelope
'tan''   cf
’
X Y
~ stable
X
Y
~ failure

8. MOHR-COULOMB FAILURE
CRITERIA
 This theory states that a material fails because of critical combination of normal
stress and shear stress and not from their either maximum normal or shear stress
alone
 Mohr-coulomb failure criteria
(In terms of total stress)


 tan cf
c

f


9. MOHR-COULOMB FAILURE
CRITERIA
(in terms of effective stress)
u=pore water pressure
 f is the maximum shear stress the
soil can take without failure, under
normal effective stress of ’.

’
'tan''   cf
c’
’
f
’
u '

10. METHODS OF INVESTIGATING
SHEAR STRENGTH
 Unconfined compression test (for cohesive soil)
 Direct shear test
 Triaxial compression test
 Vane test (for soft clay)
 Standard penetration test (for cohesionless soil)
 Penetrometer test
In which UCC,Direct shear and Triaxial compression are mainly
performed laboratory test.

11. UNCONFINED COMPRESSIVE
STRENGTH TEST
 It is performed mainly on cylindrical,moist clay specimens sampled from bore
holes
 Measures vertical stress applied to soil sample with no confining pressure
 Shear stress on failure plane is determined similarly to undrained triaxial
compression test
; =unconfined compressive strength
Soil
Specimen
2
uq
c 
uq
uq
uq

12. DIRECT SHEAR TEST
 It can be performed on all type of soil, moist or dry
 Measure shear stress at failure on failure plane for various normal stresses
 Failure plane is controlled parallel to direction of applied load
Shearing
Force
Shearing Force
Shearing
Force
Shearing
Force
Normal Load
Normal Load

13. TRIAXIAL COMPRESSION TEST
 It can be performed on all type of
soil ,moist or dry and can
consolidate sample to in situ
conditions by tracking pore water
pressure
 Measure vertical stress applied to
soil sample and confining pressure
 Shear stress on failure plane must
be calculated from principal
stresses.
Porous
stone
impervious
membrane
Piston (to apply deviatoric stress)
O-ring
pedestal
Perspex
cell
Cell pressure
Back pressure Pore pressure or
volume change
Water
Soil
sample

14. VANE SHEAR TEST
 The vane shear test can be used to determine the undrained shear
strength of soft clay in laboratory.
 It can also be conducted in the field on the soil at the bottom of a bore
hole.
 The test is simple and quick.
 It is ideally suited for the determination of the insitu undrained shear
strength of non-fissured,fully saturated clay.
 The test can be easily used to determine the sensitivity of soil.

15. SOME OTHER SHEAR STRENGTH
THEORIES
 Hvorslev’s strength theory
According to Hvorslev’s hypothesis, the shear strength of remoulded
saturated clay is given by
s=ce+ tan e
ce=true cohesion; e=true angle of internal friction
=effective stress on the failure plane at failure
The constants ce and e are also known as Hvorslev shear strength
parameters.


16. Shear strength of partially saturated
soils
 Bishop (1959) proposed shear strength equation for unsaturated soils as follows
Where,
n – ua = Net normal stress
ua – uw = Matric suction (ua=pore air pressure;uw=pore water pressure)
c = a parameter depending on the degree of saturation
(c = 1 for fully saturated soils and 0 for dry soils)
 Fredlund et al (1978) modified the above relationship as follows
Where,
tanb = Rate of increase of shear strength with matric suction
  'tan)()(' c waanf uuuc 
b
waanf uuuc  tan)('tan)(' 

17. CONCLUSION
 Hence the shear characteristics of soil can be summarized as-:
• The shear strength of cohesionless soil such as sand &non-plastic silt, is
mainly due to friction between particles.
• In dense sand, interlocking between particles also contributes significantly
to the strength.
• The shear characteristics of a cohesive soil depend upon whether a soil is
normally consolidated or over-consolidated.
• The stress-strain curve of an over-consolidated clay is similar to that of a
dense sand and that of a normally consolidated clay is identical to that of a
loose sand.