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Shear Strength
of Soils
N. Sivakugan
Duration: 17 min: 04 sec
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Shear failure
Soils generally fail in shear
strip footing
embankment
At failure, shear stress along the failure surfacereaches the shear strength.
failure surface mobilised shear
resistance
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Shear failure
The soil grains slide overeach other along thefailure surface.
No crushing ofindividual grains.
failure surface
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Shear failure
At failure, shear stress along the failure surface() reaches the shear strength (f).
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Mohr-Coulomb Failure Criterion
tan cf
c
cohesionfriction angle
f is the maximum shear stress the soil can take
without failure, under normal stress of.
f
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Mohr-Coulomb Failure Criterion
tanff c
Shear strength consists of two
components: cohesive and frictional.
f
f
c
ftan
c
frictionalcomponent
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c and are measures of shear strength.
Higher the values, higher the shear strength.
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Mohr Circles & Failure Envelope
X
YSoil elements atdifferent locations
XY
X
Y
~ failure
~ stable
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Mohr Circles & Failure Envelope
Y
Initially, Mohr circle is a point
c
c
c
c
+
The soil element does not fail if
the Mohr circle is containedwithin the envelope
GL
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Mohr Circles & Failure Envelope
Y
c
c
c
GL
As loading progresses, Mohr
circle becomes larger
.. and finally failure occurs
when Mohr circle touches the
envelope
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Orientation of Failure Plane
Y
c
c
c
GL
c+
90+
45 + /2
Failure planeoriented at 45 + /2to horizontal
45 + /2
Y
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Mohr circles in terms of &
X X X
v
h
v
h
u
u
= +
total stresseseffective stresses
vhvh
u
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Envelopes in terms of & Identical specimens
initially subjected to
different isotropic stresses(c) and then loaded
axially to failure
c
c
c
c
f
Initially Failure
uf
At failure,
3= c; 1 = c+f3= 3 uf; 1 = 1 - uf
c,
c,
in terms of
in terms of
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Triaxial Test Apparatus
porous
stone
impervious
membrane
piston (to apply deviatoric stress)
O-ring
pedestal
perspex cell
cell pressure
back pressurepore pressure or
volume change
water
soil sample at
failure
failure plane
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Types of Triaxial Tests
Under all-around
cell pressure cShearing (loading)
Is the drainage valve open? Is the drainage valve open?
deviatoric stress ()
yes no yes no
Consolidatedsample
Unconsolidatedsample
Drainedloading
Undrainedloading
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Types of Triaxial TestsDepending on whether drainage is allowed
or not during
initial isotropic cell pressure application, and
shearing,
there are three special types of triaxial teststhat have practical significances. They are:
Consolidated Drained (CD) test
Consolidated Undrained (CU) testUnconsolidated Undrained (UU) test
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Granular soils have
no cohesion.
c = 0 & c= 0
For normally consolidatedclays, c= 0 & c = 0.
For unconsolidatedundrained test, in
terms of total
stresses, u = 0
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CD, CU and UU Triaxial Tests
no excess pore pressure throughout the test
very slow shearing to avoid build-up of porepressure
Consolidated Drained (CD) Test
gives c and
Can be days! not desirable
Use c and for analysing fully drainedsituations (e.g., long term stability,very slow loading)
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CD, CU and UU Triaxial Tests
pore pressure develops during shear
faster than CD (preferred way to find c and )
Consolidated Undrained (CU) Test
gives c and
Measure
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CD, CU and UU Triaxial Tests
pore pressure develops during shear
very quick test
Unconsolidated Undrained (UU) Test
analyse in terms of gives cu and u
Not measured
unknown
= 0; i.e., failure envelope is
horizontal
Use cu
and u
for analysing undrainedsituations (e.g., short term stability,quick loading)
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1- 3 Relation at Failure
X
soil element at failure
3 1
X 3
1
)2/45tan(2)2/45(tan231 c
)2/45tan(2)2/45(tan213 c
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Stress Point
t
s
h v
(v-h)/2
(v+h)/2
stress pointstress point
2
hvs
2
hvt
X
v
h
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Stress Path
t
s
Stress pathis the locusof stresspoints
Stress path
Stress path is a convenient way to keep track of the
progress in loading with respect to failure envelope.
During loading
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Failure Envelopes
t
s
c
c cos
tan-1 (sin )
failur
e
During loading (shearing).
stress path
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Pore Pressure Parameters
Y
1
3
u = ?
A simple way to estimate the porepressure change in undrained
loading, in terms of total stress
changes ~ after Skempton (1954)
)( 313 ABu
Skemptons pore pressure
parameters A and B
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Pore Pressure Parameters
For saturated soils, B 1.
A-parameter at failure (Af)
For normally consolidated clays Af 1.
B-parameter
B = f (saturation,..)
Af= f(OCR)
For heavily overconsolidated clays Af is negative.