Deformation and structure strength anisotropy of undisturbed loess in tunnels on high-speed railway, China

During tunnel excavation in loess, the surrounding loess at vault and waist exhibits deformation and structural strength anisotropy. Structural strength plays an important role in the mechanical and deformation characteristics of loess, which are clearly discussed in this paper. Undisturbed loess samples were taken from the Wangjiagou and Gongjiawan Tunnels in northwestern China. A series of tests including triaxial shear, unconfined compression, tensile strength, loading-unloading cycles, and confined compression were conducted on undisturbed loess in the vertical and horizontal sampling directions. Considering the influence of loess structure on its anisotropy, a scanning electron microscopy (SEM) test was also performed to analyze the main reasons for the anisotropy of undisturbed loess. The results indicated that the ratio of strain in the horizontal direction to that in the vertical direction decreased with increasing confinement pressure, but first increased and then decreased with increasing normalized deviated stress. The deformation parameters of undisturbed loess were anisotropic. The ratio of vertical to horizontal elastic modulus, the lateral compression modulus, the pre-consolidation pressure, and the compression coefficient of undisturbed loess were respectively 1.20, 1.15, 1.35, and 0.88. A geometric model of the structural strength anisotropy of undisturbed loess was also constructed, and related structural strength parameters were also proposed.