Effect of Different Modes of Lateral Boundary Constraints of the Direct Simple Shear (DSS) Device under Monotonic and Cyclic Shear Loading

Abstract

The direct simple shear (DSS) device is commonly employed to characterize the shear behavior of soil while representing a plane strain condition. In DSS devices that use cylindrical specimens, the plane strain condition is mimicked by constraining the lateral boundaries of the specimen against radial deformations during consolidation and shear loading. The current test standard recommends using either wire-reinforced rubber membrane or unreinforced rubber membrane enclosed with stacked rigid rings as suitable modes of confinement to constrain the lateral boundaries of DSS test specimens. Only limited studies have been performed to assess the effects arising because of these different ways of lateral boundary constraints on the test results, particularly when DSS tests are conducted at relatively higher vertical effective confining stress (sigma(v0)') levels. An experimental program was undertaken to study the effect of lateral boundary constraint mode during constant-volume monotonic and cyclic DSS tests. The constant-volume tests were conducted on reconstituted cylindrical DSS specimens prepared from a natural silty soil under the following three modes of lateral boundary constraints: (a) steel wire-reinforced rubber membrane only (Mode 1); (b) steel wire-reinforced rubber membrane enclosed with a set of thin, low-friction stacked rigid rings (Mode 2); and (c) unreinforced rubber membrane enclosed with a set of thin, low-friction stacked rigid rings (Mode 3). The findings from these tests, conducted on specimens initially consolidated to sigma(v0)' level up to 900 kPa, indicate that there was no significant difference in the shear response derived from the three cases of lateral boundary constraints. This suggests that the commonly used approach of simply confining the DSS test specimen using steel wire-reinforced rubber membrane alone (i.e., Mode 1) would be effective and adequate for use in monotonic and cyclic direct simple shear testing, for sigma(v0)' <= 900 kPa.