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?