Undrained cyclic shear strength and stiffness degradation of overconsolidated soft marine clay in simple shear tests

Abstract

In offshore engineering, overconsolidated marine clays are frequently encountered, which may be subjected to simple shear loading conditions (e.g. soil beneath a gravity-type foundation). The corresponding monotonic and cyclic behaviours of overconsolidated clay are of vital importance for offshore foundation design. Therefore, in this study, the undrained shear behaviour of soft marine clay is experimentally investigated through a series of monotonic and cyclic simple shear tests. The shear strain, pore water pressure, stiffness, and shear strength are presented and discussed by considering the effects of the overconsolidated ratio (OCR) and cyclic stress ratio (CSR). Consequently, an OCR-independent linkage is established between cyclic and monotonic shear strengths. In addition, a new empirical model considering the influences of both OCR and CSR is proposed. It predicts the evolutions of shear stiffness and strain with the number of cycles for soft clay specimens when the CSR is sufficiently large to induce cyclic failure. Furthermore, the datasets of shear modulus (G) and residual pore pressure (Δur) of different CSRs converge to a narrow band, which is dependent on the OCR. The peak of negative Δur is rarely affected by CSR but increases with increasing OCR.