ON MESO-STRUCTURAL CHANGES OF BIMSOIL UNDER TRIAXIAL CYCLIC LOADS USING IN-SITU X-RAY COMPUTED TOMOGRAPHY(CT)

This paper aims to investigate the structural deterioration characteristics and mesoscopic damage evolution process of blocks-in-matrix-soils(bimsoils) under triaxial cyclic loading. An industrial CT machine equipped with a self-designed loading device is used to carry out low-confining pressure in-situ CT scanning tests on bimsoil samples with block content of 30%, 40% and 50%,respectively. The effects of rock content on macroscopic stress-strain curve, hysteretic loop pattern, crack geometry parameters and dilatancy effects are revealed. Testing results show that the cyclic loading and unloading results in the increase of plastic strain, and the stress hysteresis cycle presents a sparse-dense-sparsity pattern. Under the same stress amplitude, the existence of rock blocks in soil matrix has a certain influence on the shape of hysteresis loop. The area of hysteresis loop decreases with the increase of block content. When the block content is higher, the skeleton effect formed by the interactions between the soil matrix and rock is enhanced, and the energy dissipation in the process of structural weakening of bimsoil is weakened. A series of image processing algorithm are used to extract the rocks and cracks of samples under different loading stages. Under the same loading weeks, the damage degrees of bimsoil samples with different block contents are different, and the statistical area of cracks decreases with the increase of the content of rocks. Due to the high elastic mismatch between soil and rock mass, strain localization of bimsoil results in the difference of dilatancy behavior. The propagation of cracks and the formation of macroscopic failure surface are greatly affected by the rock block percentage. The volumetric dilatancy of bimsoil decreases with the increase of the rock block content. The testing results would be helpful for the development of the damage and degradation theory of bimsoil under stress disturbance, and provide theoretical support for the engineering construction and disaster prevention of geological mass composed of bimsoil.