Effect of Pre-Shearing during Cyclic Simple Shear Testing of Clean Sand for Liquefaction Analyses

The undrained cyclic direct simple shear test (CSS) is an advanced laboratory method well-suited to characterising the in-situ liquefaction resistance of clean and silty sands. This is due to it approximating the one-dimensional consolidation that occurs beneath level ground, and the shear deformations that develop in a soil mass during earthquake shaking. Currently however there is no recognised international standard for this test, which can lead to significant cyclic resistance ratio (CRR) errors being observed when factors such as the sand-apparatus interface and induced stress concentrations are not properly considered. Thus, while CSS testing may be employed as a critical component of detailed liquefaction analyses for important geotechnical projects, the testing itself must be executed with care and guidance. To further clarify CSS testing methodology, this paper systematically examines the effect drained cyclic pre-shearing has on the undrained cyclic resistance of Toyoura sand. The results from this study suggest pre-shearing clean sand CSS specimens under drained cyclic loading conditions, before carrying out undrained cyclic testing, produces CRR values that may be considered reasonably representative of in-situ cyclic response. Conversely, a lack of pre- shearing is shown to produce CRRs approximately 20 % lower in value, which would lead to unnecessary conservativism in design when undertaking detailed liquefaction assessments.