Transcript Shear Strength

Shear Strength of Soil
By
Kamal Tawfiq
Fall 2007
Soil Shear Strength
Soil in weak in shear
To determine the shear strength of soil Mohr-Coulomb Yield Criterion is used
t
t = c + sn tan f
Soil Cohesion
Angle of Friction
Yield Surface
f
c
sn
Determination of soil strength parameters ( c and f ) in the lab
1- Direct Shear Test
2- Unconfined Compression Test
3- Triaxial Compression Test
Direct Shear Test
Normal
Force N
Shearing
Force
T
t = T/A
D = Horizontal Displacement
s = N/A
T Failure
D
Direct Shear Test
t
t
s1
t
s2
t Failure
t Failure
t Failure
D
s3
D
D
Test #
1
2
3
Normal Stress (sn)
√
√
√
Shear at Failure (t)
√
√
√
t
f
c
t1
s1
t2
s2
t3
s3
sn
Unconfined Compression Test
Deviator Stress
Ds
Ds Failure
e
t
t= cu
Cu = qu/2
s1
qu = Unconfined compressive strength
Cu = Unconfined shear strength
qu = s1
sn
TRIAXIAL COMPRESSION TEST
Failure Surface
By Kamal Tawfiq, Ph.D., P.E.
Triaxial Compression Test
Deviator Stress
1- Consolidated Undrained Test (CU)
Step 1
Ds
s2
Step 2
Ds Failure
e
t
s1 = Ds + s2
Ds
Ds
Confining
Pressure
s2
s2
s2
s2
s2
fu
s2
s2
s2
s2
s2
s2 cu
s2
Ds
s2
s2 s1 s2
s1 - s2
s1
s1 - s2
s1
sn
Triaxial Compression Test
2- Consolidated Drained Test (CD)
Deviator Stress
Ds
Step 1
s2
Step 2
Ds Failure
e
s1 = Ds + s2
Confining
Pressure
s2
s2
s2
fd
s2
s2
t
Ds
s2
s2
Ds
cd
s2
s2 s1 s2
s1 - s2
s1
s1 - s2
s1
sn
Triaxial Compression Test
3- Unconsolidated Undrained Test (UU)
Deviator Stress
Ds
Step 1
s2
Step 2
Ds Failure
e
t
s1 = Ds + s2
Confining
Pressure
s2
s2
Ds
s2
s2
t= c
s2
s2
s2
Ds
cu
s2
s2
s1 - s2
s2 s1
s1
s1 - s2
s1
sn
Triaxial Compression Test
1- Applying
Confining
Stresses
2- Loading
F2
Each Circle
=
One Test
F2
F2
Axial
= F1 stress
)F
Deviator stress = )Ff
+
Confining stress = F2
F2
F2
3- At Failure
F2
F2
F2
J
F2
2
F2
F2
+
F1 = Major Principle Stress
F2 = Minor Prencipal Stress
+
Failure
Surface
+
)F
)F= deviator stress
)Ff
= F1
Failure Surface
Cohesion =
c
F2
2
F2
F2 F1
F1
F1
Fn
By Kamal Tawfiq, Ph.D., P.E.