Descriptions of the mechanical behaviour of the Xigeda formation in Zhaizi village, Yunnan Province, China

Abstract The Xigeda formation is a set of Cenozoic lacustrine semi-rock discontinuously distributed in Southwest China. As a typical hard soil or soft/weak rock, the Xigeda formation causes problems when encountered in engineering practice due to its previously unknown mechanical behaviour. Typical samples taken from Zhaizi village along the Jinsha River have been studied. Influences of both the water content and the confining pressure on strength indices of the Xigeda formation were investigated by performing triaxial tests, and the statistical correlations between the shear strength index and the water content of the Xigeda formation, and its soft rock and soil are analysed. By introducing the tenets of the theory of damage mechanics, a damage constitutive model for the deformation of the Xigeda formation and the influence of the water content thereon was established. The results show that: (a) the peak strength, the cohesion and the friction angle decrease linearly with increasing water content; (b) the sensitivity of cohesion to water content is ranked (in ascending order) as: soft rock, the Xigeda formation, then soil, and the sensitivity of friction angle to water content is ranked (in ascending order) as: soil, the Xigeda formation, then soft rock; (c) the damage constitutive model requires few input variables, has a simple form, and can reflect the deformation and strength characteristics of the Xigeda formation under different confining pressures and water contents. The results provide a mechanism with which to understand and model (for both theoretical study and engineering application) the Xigeda formation.