Laboratory investigation of the cyclic loading behaviour of intact and de-structured chalk

Abstract

Chalk is a soft biomicrite composed of silt-sized crushable CaCO3 aggregates. Chalk’s response to cyclic loading depends critically on its sensitive micro fabric and state, which may be altered significantly by high-pressure compression, dynamic impact or prior large-strain repetitive shearing. This paper reports high-resolution undrained cyclic triaxial experiments on low- to medium-density intact chalk and chalk de-structured by dynamic compaction to model the effects of percussive pile driving. The intact chalk manifested stable and nearly linear visco-elastic response under a wide range of the one-way, stress-controlled cyclic loading conditions imposed. However, high level cycling led to sudden failures that resembled the fatigue response of metals, concretes and rocks, with little sign of cyclic damage before sharp pore pressure reductions, non-uniform displacements and finally brittle collapses. However, the de-structured chalk’s response to high-level undrained cycling resembles that of silts, developing both contractive and dilative phases that led to pore pressure build-up, leftward effective stress-path drift, permanent strain accumulation, cyclic stiffness losses and increasing damping ratios. Results from exemplar tests are presented to illustrate these key features and demonstrate how chalk’s undrained cyclic shearing characteristics depend also on effective stress level. The experimental outcomes provide significant scope for developing constitutive and empirical relationships or predictive tools to enable the interpretation and design of driven pile foundations in chalk and other chalk-structure interaction related problems under cyclic loading.