Effect of wet-dry cycles on shear strength of residual soil

In order to investigate the relationship between the shear strength of residual soil and the number of repeated wet-dry cycles it underwent, the direct shear for different water content, different numbers of wet-dry cycles and different vertical stresses were examined. Based on the comprehensive structural potential theory, a comprehensive structural potential model considering the water content, the number of wet-dry cycles and the vertical stress was constructed. Then, the internal relationship between microstructure characteristics and structural strength attenuation of residual soil under alternate wet-dry conditions was emphatically discussed. As the number of wet-dry cycles and water content increased, the structural variability provided by the soil particle arrangement increased, while the structural stability provided by the soil particle cohesion decreased, causing the shear strength and structural potential of the soil to show a downward trend. As vertical stress increased, the variation range of structural stability was greater than that of structural variability. This asynchronous change caused decreased soil structural potential m_τ. Under the same levels of vertical stress and the same water content, the model we constructed well represented the exponentially decreasing trend between the structural potential and the increased number of wet-dry cycles. Scanning electron microscope (SEM) images of the sample after five wet-dry cycles showed that the surface layer had lifted and the overhead structure between aggregates was clear, suggesting that unit cell connections had changed from face-to-face contact to edge-to-surface contact. That is, the overall orientation of the unit cell was diminished. As the successive wet-dry progressed, the overhead structure provided a more open water migration channel. The nuclear magnetic resonance (NMR) test showed that the content of micropores in soil samples decreased, and the proportion of mesopores and macropores increased, resulting in a decrease in soil strength. With the further development of wet-dry cycles (>5), the peak and residual strength of the soil gradually decreased to be stable. This is related to the decrease in soil saturation after multiple wet-dry cycles, and the stronger the stabilizing effect provided by soil–water meniscus.