Offshore structures typically experience multiple storms during their service life. The soil around the foundations of offshore structures is subjected to cyclic loading during storm and reconsolidates between storms. Therefore, it is essential to understand the fundamental soil behaviour under episodic cyclic loading and reconsolidation to evaluate the long-term serviceability of offshore foundations. This paper presents experimental results of a comprehensive suite of cyclic DSS tests on a normally consolidated silty clay. The tests explore the soil response under different cyclic loading patterns (e.g., one-way or two-way), different cyclic amplitudes and number of cycles. A theoretical model, which combines the conventional cyclic contour diagram approach and principles of the critical state soil mechanics, is proposed and validated for predicting the cyclic soil response during undrained cyclic loading and hardening after reconsolidation. The model proposed in this paper paves a critical step for developing long-term soil-structure interaction models that are fundamentally linked to soil element level responses.

The soil response under the inherent cyclic loading conditions when dealing with offshore foundations can be considered by using contour plots. These plots are derived from several cyclic laboratory tests and characterize the general cyclic soil behaviour. In the design process with explicit numerical methods, such plots are needed in order to assess the soil behaviour under arbitrary loading conditions and hence estimate the cyclic foundation response. In the paper, excess pore pressure contour plots for a poorly graded medium sand are derived from numerous constant volume (CV) cyclic direct simple shear (DSS) tests and a new approach for parametrization of the plots is presented. Subsequently, the data are assessed regarding scaling for other sand soils, i.e., construction of contour plots with only a small number of test results by using the general trends observed.