Experimental investigation on compaction characteristics and permeability evolution of broken coal

Abstract

During coal mining, the compaction of goaf and the evolution of the permeability of coal masses in goaf are of significance to the safe, high-efficiency mining operation of coal mines, and directly influence surface subsidence, air leakage, spontaneous combustion of coal, etc. Although the stress and permeability of coal masses in goaf are important, it is hard to measure the two parameters directly. Therefore, seepage testing of 7 samples of broken coal masses after being compacted under isotropic loading was carried out by using a system for the seepage test of damaged coal and rock masses to evaluate indirectly the compaction and seepage characteristics of broken coal masses in goaf. Moreover, the energy evolution of broken coal masses after compaction was analysed. The research results are summarised as follows. Under the same stress, the strain in coal samples with a single particle size was lower than that in coal samples with mixed particle sizes. For coal samples with a single particle size, the smaller the particle size was, the lower the crushing amount of particles after testing. By contrast, for coal samples with mixed particle sizes, the smaller the particle size was, the greater the crushing of particles after the test. In compressive deformation stage, the stress sensitivity of the permeability of broken coal mass decreased with the increase of the elastic modulus, decreased with the increase of effective stress, decreased with the increase of Poisson's ratio, decreased with the decrease of particle radius. During isotropic compression testing, the proportions of stored elastic energy in the total energy of coal samples with single particle sizes were larger than those of coal samples with mixed particle sizes; however, the proportion of dissipated energy in the total energy of the former was lower than that of the latter.