The influence of suction and density on the resilient modulus of undercompacted soils
The resilient modulus is a key parameter in road and railway subgrade design, reflecting soil stiffness under repeated loading. While subgrades are typically compacted to maximum dry density, many existing infrastructures are supported by soils with densities lower than the maximum due to inadequate compaction or long-term density loss. Understanding the behaviour of these undercompacted and often unsaturated soils is therefore essential for both infrastructure assessment and design. This paper reviews the influence of cyclic stress, suction, density, and drying–wetting cycles on the resilient modulus of four unsaturated soils with high fines content using experimental data from the literature. The analysis indicates that resilient modulus increases with suction but decreases with increasing cyclic stress for these soils. Resilient modulus is also found to decrease significantly with reductions in density at all suction levels. Moreover, drying–wetting cycles were observed to cause soil deterioration, leading to stiffness degradation and a reduction in resilient modulus of the soil examined. A review of several predictive models showed that neglecting the influence of density and drying-wetting cycles results in weaker agreement between model predictions and experimental observations. To address these limitations, a new model is proposed that incorporates the effects of density variations and soil deterioration due to drying–wetting cycles, leading to improved predictions of the resilient modulus for undercompacted soils.