Influences of water on the microstructure and mechanical behavior of the Xigeda formation

As a typical hard soil and soft/weak rock, the Xigeda formation is a set of Cenozoic lacustrine semi-rock discontinuously distributed in south-western China. Engineering practice shows that water exerts a significant influence on the mechanical properties of the Xigeda formation. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and triaxial compression tests were conducted by means of in situ sampling of the Xigeda formation in Zhaizi village along the Jiasha River. The mineral composition and microstructure were determined, the deformation and failure mechanisms were investigated, and influences of the water content on both deformation and strength indices were discussed. The results show that (a) the Xigeda formation has a characteristic weakly cemented structure, which differs from that of soil and rock, and this cemented structure is easily damaged under saturated conditions; (b) with increasing water content, both average modulus and shear strength of the Xigeda formation decrease significantly, and influence of water content on peak strength is much greater than that on residual strength; and (c) in the range of tested conditions ( w = 17.79 ~ 30.83%, σ 3 = 200 ~ 800 kPa), both the peak and residual strengths meet the Mohr–Coulomb criterion. The results can provide an experimental basis and mechanism informing engineering practice in the Xigeda formation.