In order to guarantee perpendicularity of rotary cast-in-place pile in Karst areas, filling, like sand, etc., is commonly used between the inner steel casing and the outer steel sleeve; while the outer steel sleeve is pulled out or a vertical load is applied on the top of the pile, the sand will move, contact, and push against each other, which leads to recrushing; therefore, shear characteristics and secondary breakage are a meaningful problem to study with regard to the influence of different sand densities. Through experimental direct shear tests, with different dry densities, shear stress versus shear displacement curves are obtained and internal friction angles are analyzed. The results show that internal friction angles increase along with increment of dry densities; more interesting is that the secondary breakage ratio also increases. A comparative numerical 2D model is set up using PFC2D, with lower porosity used to simulate a large dry density, and similar results are obtained, that is to say, with increase of dry densities, internal friction angles also increase.
The load-unload response ratio (LURR) reflects damage evolution of rocks and thus is a good way for describing rock stability. With the background of stopping in a metal mine, this paper studies the relationship between LURR and damage of brittle rocks. Based on the elasto-brittle damage theory, the analytical expressions of LURR were first derived for brittle rocks under both uniaxial and conventional triaxial loading/unloading conditions. Then uniaxial and conventional triaxial loading/unloading tests were conducted on brittle rocks. The experimental results indicate that the LURR varies greatly at the initial stage possibly due to the compaction and heterogeneity of rock. As the load increases, the LURR keeps almost constant with occasional fluctuation possibly due to propagation of local cracks. When the rock is close to failure, the LURR presents abrupt increase or great fluctuation. The experimental results are in reasonable agreement with the derived analytical expressions of LURR.
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