Damping of a soft marine clay subjected to the combination of monotonic and cyclic shear stresses

Abstract

In relation to the dynamic behavior of soils, the damping (D) and the dynamic shear modulus (G) has always acquired special relevance, since its evaluation has allowed the application of simplified models of dynamic soil behavior. They depend on many factors, being necessary to include the combination of monotonic (τo) and cyclical shear stress (τc) that are inevitably present under any structure. As part of an extensive testing programme, the objective of the present paper, besides the identification and classification tests, was to perform 139 simple cyclic shear tests with different levels of static stresses (τo) before the cyclic phase. This paper refers to the influence that the combination of monotonic (τo) and cyclic shear (τc) stresses has on the damping of a soft cohesive soil susceptible to sudden failure. Laboratory tests were carried out on undisturbed samples from the Port of Barcelona, in Spain. The samples were obtained at depths between 29 m and 52 m, that correspond to an effective vertical stress of (σ′ov) between 277 kPa and 413 kPa, respectively. In general, the results indicate that: a) the combination of τo and τc controls the damping ratio (D), b) in the combined monotonic and cyclic case, it is possible to establish empirical functions of the type D = D(N, γc), D = D(N, G/σ′ov) and D = D(N, u/σ′ov), for different (τo/σ′ov) ratios and c) it is possible propose a chart to evaluate the damping ratio as a function of N, G/σ′ov and u/σ′ov, for different (τo/σ′ov) ratios.