shear strength theory
TRANSCRIPT
SHEAR STRENGTH THEORY GEOTECHNOLOGYSUBMITTED TO: DR. N. SHANKAR
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 slidingo Primarily depends on interaction between soil particles Important for:o Foundation designo Lateral earth pressure calculationso Slope stability
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
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)
General curve for shear strength of soil S
=ANGLE OF INTERNAL FRICTION C
tan
tan
cs
s
COULOMB EQUATION
S: shear strengthC: cohesion : Angle of internal friction Effective intergranular normal pressure
tancs
: σ
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
~ stableXY
~ failure
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)
tancf
c
failure
envelope
f
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’
’
failure
envelope
f
’
u '
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.
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 strengthSoil
Specimen
2uqc
uq
uq
uq
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 LoadNormal Load
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 pressureBack pressure Pore pressure or
volume change
Water
Soil sample
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.
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+ tane
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.
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, tanb = Rate of increase of shear strength with matric suction
'tan)()(' c waanf uuuc
bwaanf uuuc tan)('tan)('
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.