Compressibility of Clays under Repetitive Loading: A New Perspective on Consolidation State, Loading Frequency, and Partially Drained Conditions

Abstract

Many structures built on saturated clays are subjected to repetitive loads from sources such as waves, wind, and traffic. This loading can induce excess pore water pressure within saturated clays, leading to additional volumetric deformation and the long-term degradation of geostructures. This study investigated the compressibility of saturated clays under repetitive loading, focusing on the coupled effects of the initial degree of consolidation (U<inf>i</inf>) and loading frequency (f). A newly developed loading system was used to perform tests on sand, kaolin, and Ca-bentonite specimens across a wide range of frequencies (f = 0.011, 0.046, 0.139, 0.278, 1.67, 8.33, 25, 125, and 250 mHz) and consolidation states (U<inf>i</inf> = 0, 0.2, 0.4, 0.6, and 1.0). The results indicated that the cyclic-induced void ratio change (Δ e) was highly sensitive to both U<inf>i</inf> and f . For underconsolidated soils (U<inf>i</inf> ' 1), high-frequency loading significantly increased Δ e , whereas the response was minimal and largely independent of these factors under low-frequency conditions or for normally consolidated soils (U<inf>i</inf> = 1). This study identified three distinct drainage regimes: drained, partially drained, and undrained based on the cyclic loading ratio (T / t<inf>100</inf>), establishing a quantitative threshold of T / t<inf>100</inf> ≤ 0.001 − 0.01 for the transition to pseudo-undrained behavior. Furthermore, repetitive loading was found to increase the horizontal effective stress, leading to a higher overconsolidation ratio and a corresponding reduction in post-cyclic compressibility. The findings provide new experimental evidence on the complex, coupled behaviors of saturated soils and offer critical insights for the reliable design and performance assessment of structures on soft clay deposits.