Soil Mechanics Homework-5

Due May 8, Friday, 17:00
Soil Mechanics Homework-5
State all assumptions clearly. Graphical solutions will be accepted when applicable.
1. Direct shear test was done under a vertical load of 885 N, on a dense clean sand specimen
that is initially 6cm by 6cm square. The shear stress vs. horizontal deformation data from this
test is given in the figure.
250
200
Shear stress (kPa)
a) Determine the peak friction angle
of this dense sand specimen.
b) Illustrate the Mohr circle at peak
shear stress, at the middle of the
specimen. Also draw the peak
strength envelope on the same figure.
c) Determine the magnitudes and
orientations of principal stresses at
peak shear stress, at the middle of the
specimen.
d) Estimate the friction angle for a
loose specimen of the same sand.
150
100
50
0
0
1
2
3
4
Shear deformation (mm)
5
6
7
2. A saturated clay layer is tested by a field vane that has a diameter of 10cm and height of
15cm. A torque of 70Nm is required to shear the soil. The vane was then rotated to remould
the soil completely. The torque required to continue this rotation decreased to 28Nm.
a) Calculate the undrained shear strength of this clay.
b) Calculate the residual shear strength and sensitivity of this clay.
c) An undisturbed sample from the same clay layer is cut to the shape of 5cm diameter and
10cm tall cylindirical specimen. When this specimen is tested by unconfined compression, the
maximum axial load it can carry is measured when the axial deformation is 8mm. Determine
this peak axial load.
3. A series of CU triaxial
compression tests are done on a
clay sample, with the results
given in the table.
Cell
Deviator
Test Pressure Stress
(kPa)
(kPa)
Pore Pressure
(kPa) during
consolidation
& start of shear
200
300
250
Pore
pressure
at failure
(kPa)
145
430
700
1
300
315
a) For Test 3, draw total stress
2
700
410
Mohr circle at failure, and
3
1050
470
effective stress Mohr circle at
failure, on the same graph.
b) Plot the failure stresses of all three tests on one q vs p´ plot. Hint: q=dev /2 and
p´=(1´+3´)/2
c) Determine Mohr-Coulomb effective strength parameters of this soil (c´ and ´).
d) Determine Skempton’s A parameter for all tests. What can you guess about p´ of this soil?