Influence of Soil Structure on the Evolution of Small-Strain Shear Modulus During Drying-Wetting Cycles

One. Influence of Soil Structure on the Evolution of Small-Strain Shear Modulus During Drying-Wetting Cycles

Hydroclimatic variability intensifies suction fluctuations in near-surface tropical soils, directly affecting stiffness at small strains and, consequently, serviceability performance of shallow foundations and earth structures. This study investigates the evolution of small-strain shear stiffness of a natural lateritic clayey soil from southern Brazil subjected to controlled drying and wetting paths under unconfined conditions. Thirty-three tests were performed on specimens prepared in three structural states: undisturbed, remoulded at field void ratio, and compacted at standard compaction energy. Hydraulic paths were imposed on the same specimen to ensure consistency in structural history and to capture hysteretic effects. Suction was obtained from the soil-water retention curve following ASTM D five two nine eight, and small-strain stiffness was measured after hydraulic equilibration using bender elements. Results demonstrate a systematic increase in small-strain stiffness during drying and a reduction during wetting, evidencing hydraulic hysteresis in the mechanical response. Compacted specimens exhibited consistently higher stiffness and reduced sensitivity at low water contents, followed by a pronounced degradation beyond a threshold moisture range. Remoulded specimens showed marked structural instability under wetting, with progressive disaggregation associated with the loss of natural cementation at high void ratio. The results confirm that structural condition governs both the magnitude of initial stiffness and its path-dependency under hydraulic cycling. The findings provide experimental evidence linking suction-controlled stiffness to microstructural stability in lateritic soils and highlight the necessity of incorporating hydraulic path effects into stiffness-based design frameworks for tropical unsaturated deposits.

One point one. Introduction

Climate change has been inducing alterations in the magnitude, intensity, and temporal distribution of precipitation events, thereby increasing the complexity of problems associated with soil-structure interaction. This scenario has motivated a growing body of research aimed at understanding the implications of such variations on the performance and safety of geotechnical structures, with soil type playing a significant role in the observed mechanical response. In this context, unsaturated soils assume a central role, as they are commonly encountered in nature, particularly within near-surface soil layers, engineered fills, and regions subjected to seasonal climatic variability. Such soils are also characteristic of arid and semi-arid environments, as well as areas with deep groundwater tables, where matric suction exerts a significant influence on their hydraulic and mechanical behaviour.

In southern Brazil, the region addressed in the present study, unsaturated soils are predominantly found within near-surface deposits and natural slopes. In recent years, the increasing frequency of extreme precipitation events has resulted in adverse and, in some cases, catastrophic impacts on geotechnical structures founded on these soils, as well as on the stability of natural and engineered slopes. Rainfall infiltration associated with high-intensity events promotes a progressive loss of matric suction within residual soils, leading to a reduction in shear strength and an increased susceptibility to instability. These regional manifestations highlight the need to understand how large-scale climatic drivers are translated into soil-scale hydro-mechanical responses.

At the soil scale, the translation of regional climatic processes into mechanical behaviour depends largely on soil properties such as permeability, water retention behaviour, and hydraulic conductivity, which govern infiltration dynamics, matric suction loss, and pore-water pressure fluctuations during wetting-drying. Among the mechanical parameters influenced by these processes, the shear resistance mobilised at small strain levels, G zero, plays a key role in controlling the initial stiffness response of unsaturated soils.

Accordingly, a substantial body of experimental research has focused on the relationship between suction and small-strain stiffness. A consistent outcome of previous investigations is that the small-strain shear modulus is linked to suction. Nevertheless, hysteresis effects have been observed in small-strain stiffness, indicating that the dependence of G zero on suction may be nonlinear. Experimental studies combining bender element testing and suction-controlled resonant column tests have shown that G zero of moderately unsaturated soils increases with matric suction following a power-law relationship.

2016; Hoyos et al., 2015; Liu et al., 2020; Ng and Yung, 2008). Nevertheless, hysteresis effects have been observed in small-strain stiffness, indicating that the dependence of G0 on suction may be nonlinear (e.g., Ng and Yung 2008b; Dong et al. 2016). Experimental studies combining bender element testing and suction-controlled resonant column tests have shown that G0 of moderately unsaturated soils increases with matric suction following a power-law relationship (Hoyos et al., 2015; Ng and Yung, 2008).