Study on the correlation of macro-fine parameters of surrounding rock in sand and cobble tunnel based on discrete element

As a mixture of soil and rock, the deformation of surrounding rock caused by tunnel excavation in the sandy cobble stratum is affected by the meso-structure of the stratum. In this paper, the mechanical and engineering characteristics of sandy cobble soil are studied in depth using a laboratory large-scale triaxial test and discrete element numerical simulation test, and the effects of main microscopic parameters on the characteristics of sandy cobble soil are analyzed, including the same specimen size, moisture content, compactness, confining pressure as well as loading method. The microscopic parameters such as contact modulus E c , friction coefficient f r , and stiffness bond ratio n of sandy pebble soil can be obtained. Meanwhile, the accuracy and uniqueness of microscopic parameters are determined using the response surface method. Moreover, the influence of different contact modulus and friction coefficient on the macroscopic mechanical properties of sandy pebble soil is quantitatively analyzed. The obtained conclusions are drawn as follows: the elastic modulus of sandy pebble soil decreases with the increase of water content. The peak value point of the stress-strain curve of sandy pebble soil increases with the increase of contact modulus and friction coefficient, respectively. The relationship between peak stress and the two types of fine-scale parameters can both be described by logarithmic functions. Compared to fine-grained soils, sandy pebble soils start to break down at about 3.5% of the axial strain, and the sandy cobble soil can still bear a certain load after reaching the peak strength. The results can provide a reference for the calculation and discrete element simulation of tunnels in sand and pebble strata.