The resuits of three Direct Shear tests conducted on a coarse-grained soil from Isle of Wight, Virginia are given below. Remember that the direct shear test is a drained test. Each sample was constructed by first adding portions of the soil to a direct shear box and then compacting it to reach a relative density, D,, of 80% that corresponded to a Relative Compaction of 95% of standard Proctor maximum achieved in the field. The relative density was the same for each sample. A normal stress was then applied to the sample that remained constant during the test. Finally the shear test was conducted by moving the upper box at a constant rate of displacement with respect to the lower box (see the class handout). The stress-displacement and volume change curves were recorded using automated data acquisition.
Part 1. Use the resulting stress-displacement information to determine the peak state effective stress strength parameters, ¢' and c'. The normal stress for each stress-displacement curve is indicated in the figure.
Part 2. The lower plot shows how the samples changed in height (therefore volume) during the test. Positive values indicate an increase in volume and negative values indicate a decrease. Explain the volume change behavior shown-how does it relate to the stress-displacement curves? In engineering practice, is this volume change behavior desirable? Why or why not?