The swelling behaviour of natural expansive stiff clay under different constraint conditions during cyclic wetting and drying

In this study, the intact Nanning stiff clay were subjected to cyclic wetting-drying under different axial constrain conditions. Experimental results showed that specimen presented vertical stress-dependent axial strain under one-dimensional condition, and gave varying axial stress responses under limited-axial swelling condition. At the first wetting stage, the swelling pressure extrapolated from the swelling strain-vertical stress relationship was found well comparable to that measured on the intact specimen hydrated under constant-volume condition. The proceeding wetting-drying cycles caused structural deterioration of the intact specimens, which differed under different constraint conditions, thus giving rise to different swelling behaviour of the soil. Under one-dimensional condition, the swelling deformation directly destroyed the diagenetic structure of the natural expansive stiff clay. Increasing the vertical stress helps prohibiting such destructive effect. While under limited-axial swelling condition, the specimen swelled to develop varying axial stress in response to hydration during each wetting-drying cycle. More interestingly, the developed axial stress presented a considerable decrease after it remained comparatively stable over three-consecutive wetting-drying cycles. By further analysing such evolving trend of the axial stress from the perspective of structural degradation, the de-structuration of expansive stiff clay under limited-axial swelling condition could evolve in forms of the desiccation- and hydration-induced cracks growth, diagenetic bonds breakdown and crack sealing during cyclic wetting-drying. Such degradation mechanism needs further verification and quantification by microstructural observations.