Hydromechanical behavior of unsaturated expansive clay under repetitive loading

Compacted layers of expansive soils are used in different engineering projects, such as subgrades, engineered clay barriers, and buffers for radioactive waste disposal. These layers are exposed to a variety of stresses and wetting conditions during field serviceability. Coupling between hydraulic and mechanical repeated loading provides insight understanding to the induced progressive deformation of expansive clay. This study was conducted to investigate the hydromechanical behavior of unsaturated compacted expansive clay under repeated loading–unloading (RLU) conditions. Two series of one-dimensional (1D) oedometer tests were conducted under controlled matric suction up to 1500 kPa using the axis translation technique (Fredlund soil-water characteristic curve device, SWC-150). The first test series was carried out at different levels of controlled matric suction for non-repeated loading–unloading (NRLU) cycles. RLU cycles were applied in the second test series at different repetitive-stress levels and under different levels of matric suction. The results indicated increasing axial wetting strain ε_a(s), axial swell pressure σ_s(s), compression index C_c(s), and swell index C_s(s) with suction reduction. The estimated load–collapse (LC) curves obtained from NRLU series (LC_N) and RLU series (LC_R) indicated increasing yield stress σ_y(s) with increasing suction. This is attributed to the developed apparent cohesion between soil particles, which in turn rigidifies the material response. Applying repetitive loading induced a notable reduction of compression index C_c(s) at the same level of suction, whereas swell index C_s(s) seems to be independent of repetitive loading. Finally, repetitive loading exceeding initial yield stresses results in plastic hardening and, hence, enlargement of yield stress locus (i.e. LC_R curve).