Experimental investigation on the water migration and deformation of widely graded coarse soil under drying–wetting cycles

Widely graded coarse soils are widely distributed in the mountainous regions of China and are extensively utilised in engineering construction. The volume change of these materials upon intense seasonal wetting and drying cycles presents challenges for the long-term performance of the engineering structures. In this study, the water migration and deformation behaviour of widely graded coarse soils are examined over multiple cycles of drying and wetting. Laboratory tests are integrated with nuclear magnetic resonance and scanning electron microscopy. The results demonstrate that capillary and adsorption forces drive the gradual expulsion and absorption of inter-aggregate and intra-aggregate pore water during the drying–wetting cycles. These two mechanisms differentially influence the microstructure and soil water characteristic curve, soil shrinkage curve, and their hysteretic response. The water content corresponding to the transition between these two regimes is influenced by the coarse content of the soil. In the capillary regime, capillary forces generate normal stresses on the soil skeleton, inducing significant volume changes and resulting in a denser soil structure. In the adsorption regime, on the other hand, adsorption forces control the drainage of pore water within the aggregates. This causes the clay aggregates to shrink without significant deformation of the soil skeleton, the latter being influenced by the coarse content. The hysteresis effect is notably more pronounced in the capillary regime.