Impact of Compliance on Constant-Height Simple Shear Tests

Abstract

Simple shear testing is an important laboratory tool in geotechnical engineering, providing insights into soil behavior under conditions relevant to landslides, foundation instability, and earthquake loading. Undrained conditions are often reproduced by imposing constant volume in simple shear tests, where maintaining constant height is essential. However, compliance stemming from deformations in device components can seriously affect results, often causing overestimation of strength. This study quantifies compliance in simple shear equipment and evaluates its impact on monotonic and cyclic tests. It demonstrates that measured vertical strain (nominal compliance) often meets the ASTM standard ( ± 0.05 % of the specimen’s pre-shear height), while actual compliance, which represents the specimen’s actual vertical strain, can easily exceed the ASTM threshold. The consequence for monotonic tests is a subdued response, manifested as less contraction in contractive specimens and less dilation in dilative specimens, resulting in lower peaks and higher troughs in the measured shear stress. A height correction feedback loop was implemented to correct compliance errors in cyclic tests to eliminate the difference between nominal and actual compliance. The findings indicate that compliance delays liquefaction and causes the cyclic resistance ratio to be overestimated. These observations highlight the need to improve compliance control in commercially available simple shear equipment and to improve how constant-height control is evaluated and reported. A stress-based criterion is proposed to replace the current ASTM approach.