Experimental study on the microscopic swelling–shrinking mechanisms of clay-sand mixtures from wetting to drying

In this paper, microcomputed tomography and low-field nuclear magnetic technologies were employed to investigate the evolution of the water occurrence and particle behavior of montmorillonite–kaolinite–quartz sand mixtures during a wetting–drying cycle. The experimental results showed that the cutoff values between the adsorbed water, capillary water and bulk water were approximately 2.5 and 50 ms, respectively. During wetting, 1D distribution of pore water was not uniform due to the differences in the characteristic time scales of unsaturated flow and swelling. At the early stage of wetting, the adsorbed water dominated, and the macroscopic deformation was mainly determined by the crystalline swelling of montmorillonite. At 25% water content, adsorbed water was nearly saturated. With continuous wetting, the capillary water content increased and caused some initially horizontally oriented clay particles to rotate or shift, greatly enlarging the size of pores and clay aggregates. During drying, 1D water distribution curves can be divided into an apparent dry region, a capillary flow region and a saturated region, which is opposite to that of wetting stage. From 40 to 10% water content, capillary water changed from T₂ = 10 ms to 5.7 ms, and the variations of its content was closely related to the trend of shrinkage deformation. If water content further dried to 5%, only adsorbed water remained. At the same water content, there was a significant difference in the pore water type and water distribution for wetting and drying states. Along a wet–dry cycle, the characteristic water content between adsorbed and capillary water was approximately 19 and 15%, corresponding to 16.4 and 20% saturation. Owing to the delayed evaporation of small-sized liquid bridges, the proportion of capillary water and adsorbed water was higher in the drying state. Accordingly, there was a hysteresis in aggregate size, particle orientation, and pore size, leading to the inconsistency of macroscopic swelling and shrinkage deformation. This work provides basic experimental data and guidance for subsequent simulations and theoretical models.