Sandy cobble soil is a composite made of soil matrix and cobbles, and the estimation of its shear strength always requires expensive large-scale experiments. The strength of the sandy cobble soil exhibits macroscopic anisotropy with respect to the direction of the major stress due to the observed dominant distribution of the cobble dip angle. In the present paper, a numerical homogenization procedure for anisotropic strength identification of the sandy cobble soils is established, which can take into account the influencing factors of the size, shape, and inclination of the cobbles and the mesoscopic strength of the soil-rock interface. To consider the condition of plain strain, the particle size distribution of the cross of the stratum is derived based on the fractal theory and the transformation method of Walraven. The mesostructure of the sandy cobble soils is randomly produced using ellipses to model the cross of the cobbles. An iterative procedure is utilized to represent the major stress orientation-dependent macroscopic strengths. The results are validated against the data from indoor experiments and global mesoscopic computations. It is shown that the macroscopic strength of the sandy cobble mixtures can be accurately determined and the iterative multiscale limit analysis method is reliable and efficient. Parameter analysis is finally conducted to discuss the effect of the mesoscopic properties on the macroscopic strength.

The structure of sandy cobble soil is discrete, the particle size distribution is asymmetrical, and it has typical particle dispersion peculiarities. The macroscopic continuum mechanics theory method cannot accurately access the instability process and failure mode of sandy cobble surrounding rock of tunnel, which often induce instability collapse of surrounding rock of tunnel face, resulting in ground subsidence, deformation and collapse, which poses a serious threat to engineering safety. In this research, the particle discrete element theory and numerical simulation technology are utilized to conduct a fine analysis of the stability of the tunnel sandy cobble surrounding rock from the meson level. The paper focuses on the longitudinal instability failure mode of the tunnel face and failure characteristics of circumferential surrounding rock during tunnel excavation. The consequence display that after the excavation of the tunnel in sandy cobble stratum by the bench method, in the longitudinal space of the tunnel, the soil in front of the tunnel face is first destroyed, and the closer the distance is to the tunnel vault, the greater the soil deformation is, which is easy to cause surface subsidence. The stress relaxation area gradually spreads from the front of the working face to the upper right, which eventually provokes the instability of the excavation face. Correspondingly, the intrusion degree of surrounding rock and the height of vault collapse arch increase with the increment of water content in sandy cobble stratum. Meanwhile, intrusion size, collapse arch height, width and initial support effect of surrounding rock with different water content are calculated. The research results provide important reference and guidance for the design, construction and maintenance of sandy cobble stratum tunnel engineering.