Microscale evidence for and formation mechanisms of shear-strength anisotropy of a loess-paleosol sequence since the late Early Pleistocene: The case study of the Xiushidu profile, Southern Chinese loess Plateau

The shear-strength anisotropy of loess-paleosol sequences (LPS) provides vital information for engineering and construction activities and mitigation against geological disasters. The shear-strength anisotropy of undisturbed and disturbed samples from a profile from Xiushidu village, Jingyang County, in the Chinese Loess Plateau, and the formation mechanisms of such anisotropy were studied through direct shear tests and the microstructural analysis of their characteristics. Our results suggest that the shear strength of loess layers L-1, L-2, L-3, L-4, and L-5 along the vertical direction was greater than that along the horizontal direction, under different values of axial compression. Under the same vertical pressure, the shear strength along the vertical direction was also generally greater for the layers above the seventh paleosol layer (S-6), while the opposite result appears in the layers below S-6. For the disturbed samples, their anisotropic characteristics are the same as for the undisturbed samples. The microstructure characteristics, including skeleton particles, contact relations, pore forms, cementation degree, and pore area ratio (PAR), explicated the reasons for the anisotropy of shear strength. Moreover, the influence of PAR, porosity (n), and dry density (rho d) on the shear strength in the vertical direction are greater than that in the horizontal direction, and the influence of water content is opposite to that of PAR, n, and rho d. Gravity, van der Waals forces, and hydrogeochemical reactions play important roles in the shear-strength anisotropy of the LPS. Therefore, research on the anisotropy of shear strength contributes to increasing our understanding of such layers, in preventing and mitigating against potential geological disasters and in implementing more appropriate engineering construction techniques.

Automated summary

This study investigates the shear-strength anisotropy and formation mechanisms of loess-paleosol sequences through direct shear tests and microstructural analysis. The results show that the shear strength of loess layers is greater along the vertical direction than the horizontal direction under different axial compression. The anisotropy is influenced by factors such as skeleton particles, contact relations, pore forms, and cementation degree.