Effect of Volume Change in Undrained Direct Simple Shear Tests

Abstract

In order to perform constant-volume (equivalent to undrained) Direct Simple Shear (DSS) tests using either a wire-reinforced membrane or stacked rings as the zero lateral strain confinement system, the height (and thereby volume) of the test specimen must be maintained at a constant during shear. The change in vertical stress while maintaining constant specimen height during shear is equivalent to the excess pore pressure that would have developed in a truly undrained DSS test. Depending on the control capabilities of active height-control DSS devices and the stiffness of locked (passive) height-control DSS devices, the specimen height may not be perfectly constant throughout shear in some cases. This technical note presents the results of monotonic constant-volume DSS tests where the specimen height was intentionally varied during shear to demonstrate the effect on measured shear stress and vertical effective stress. Changing the test specimen height by 0.05 % (the maximum specified by ASTM D6528, Standard Test Method for Consolidated Undrained Direct Simple Shear Testing of Fine Grain Soils) affected the test results. The percentage deviations in the measured test results are greater for stiffer soil specimens, about 2 % for soft normally consolidated clay and more than 20 % for dense sand at failure.