Lateritic soils, also known as red soils, are prone to actions of external environment such as wetting and drying processes and their cycling. However, in the practice of pavement design in red soil regions, there lacks experimental studies and analytical approaches to reveal and predict the evolution of the hydromechanical characteristics of red soils under complex environmental actions. This paper investigates the variation in the hydrostructural and dynamic characteristics of a compacted subgrade red clay collected from Nanning, Guangxi, China, before and after wetting-drying cycles. The pore structure and soil-water characteristics of the red clay before and after ten wetting-drying cycles were determined to reveal the influences of moisture fluctuation history. Besides, cyclic triaxial tests were performed to determine the resilient modulus (MR) and permanent strain (εp) of the red clay and reveal the influences of external stress, moisture content w, suction s, and WD cycles. It is found that (i) compacted red clay presents typical dual porosity with distinct inter-aggregate and intra-aggregate pores. Such pore structure results in bimodal soil water retention curves (SWRCs) of the red clay; (ii) upon wetting-drying cycles, the intra-aggregate pores shrink while the inter-aggregate pores swell. Besides, the global pore space (i.e., the overall void ratio) increases after WD cycles. This results in the elimination of the SWRC’s bimodal characteristics and the reduction in the clay’s water retention capacity in the low suction range and scale of shrinkage upon drying; (iii) the εp and MR vary non-linearly with σd, w, and s. Their relationships to the external stress and soil moisture change remarkably after WD cycles. A simple model was adopted to describe the variations of the εp and MR with w and s, which has achieved close agreements with the experimental measurements; (iv) the εp and MR of the tested red clay show a unique non-linear relationship regardless of the influences of σd, w, s, and WD cycles, which highlights possible intrinsic relationships between the elastic and plastic behaviors of compacted subgrade soils.
Deng et al. (Sun,) studied this question.