Effects of Imperfect Simple Shear Test Boundary Conditions on Monotonic and Cyclic Measurements in Sand

The direct simple shear (DSS) test is commonly used to assess the shear strength of soil, estimate liquefaction resistance, or calibrate constitutive models under the assumption of idealized simple shear conditions even though this may not be necessarily achieved in the test. Near-frictionless vertical boundaries cannot develop the complementary shear stresses necessary for equilibrium. Difficulties in maintaining constant height during equivalent-undrained tests may violate the constant volume assumption. The cap holding the top of the specimen may tilt during shear, violating the assumed perfect simple shear strain conditions. These boundary effects on constant-volume monotonic and cyclic DSS test results were parametrically studied by simulating a DSS device using an advanced plasticity model. For practical purposes, near-frictionless vertical boundaries can be used to approximate simple shear conditions, and the effect of top-cap tilting is negligible in well-designed devices. However, vertical compliance can greatly affect the stress-strain response.