HW10-CE357-Shear Strength of Sands and Clays

1 The University of Texas at Austin CE 357 Department of Civil, Architectural Geotechnical Engineering And Environmental Engineering Prof. El Mohtar Homework No. 10 – Shear Strength of Sand and Clay 1. [25 points] Two consolidated drained (CD) triaxial tests were conducted on a dense, angular sand at the same void ratio. Test A had a confining pressure of 100 kPa, while the confining pressure for test B was 400 kPa. At failure, the maximum deviator stress,  1 -  3 , for tests A and B were 400 and 1700 kPa, respectively. a) Plot the Mohr’s circles for both tests /specimens at failure. b) Assuming c ’ = 0, determine  ’ for each test/specimen. c) Determine the orientation of the failure plane for each test/specimen; sketch the sample with principal stresses at failure and the failure plane indicated. d) Calculate the shear and normal stress on the failure plane for both tests. Is it the same as the maximum shear stress on the sample? Why or why not? e) Use the failure stresses for each test/specimen and perform a linear regression of the two failure stresses in order to obtain the overall/average effective friction angle for the dense sand. The effective cohesion intercept should be close to zero, but you could force it to be zero by using a third data point in your regression at (0,0). What is the effective friction angle for the dense sand? 2. [25 points] Consider two CD triaxial tests at the same confining pressure. One test is performed on a dense sand and the other on a loose sand. a) Sketch (on the same graph) the shapes of the deviator stress (  1 -  3 ) vs. axial strain curves for each test. Label the curves dense and loose. b) Sketch (on the same graph) the shapes of the volume change vs. axial strain curves for each test. Label the curves dense and loose. c) If the specimens were not allowed to drain during shear, how would the pore water pressure change in each test (i.e., would u increase or decrease during shear)? Sketch the pore pressure vs. axial strain curves for each test assuming no drainage. 3. [25 Points] Four UU triaxial tests were performed on specimens trimmed from an undisturbed sample of clay. The sample came from a depth of 20 m below the mudline of San Francisco Bay (note; the mudline is the ground surface beneath the water/ocean surface) and the normally consolidated clay had a saturated unit weight of 18 kN/m 3 . The following results were obtained: Test No. Cell Pressure (kPa) Deviator Stress at Failure (kPa) 1 50 50.2 2 100 49.3 3 150 51.8 4 200 50.7 a) Plot the Mohr’s circles at failure for these tests and determine the undrained shear strength of this clay.