Experimental study on the evolution of micropore structure softened by nonpressure water absorption strength of sandstone

With the increase of mining depth of a coal mine, the strength of the surrounding rock in deep roadways decreases due to the influence of groundwater, which seriously affects the safety of on-site personnel in production. To study the influence of water-rock interaction on the properties of surrounding rock, taking the sandstone in the deep roadway of Wanfu Coal Mine as the research object. The nonpressure water absorption experiment was carried out. According to the compression tests of key water content (0%, 0.8%, 1.6%, 2.4%, 3.3%), the mechanical evolution law of sandstone during water absorption was studied. At the same time, mercury porosimetry (MIP), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) experiments are used to study the evolution of experimental micro-scopic pores. The strength-softening mechanism of sandstone in Wanfu Coal Mine is discussed. The results show the following: (1) The NMR T2 spectrum curve of sandstone presented a single peak characteristic. With the increase of water content, the total spectrum area increases continuously, and the corresponding relaxation time of peak shifts to the right. According to the ratio of spectral area in the experimental process, the traditional process of water absorption is further divided into four stages: rapid water absorption, uniform water absorption, pore transformation, and stable water absorption, in which the pore transformation mainly occurs in stage III (31-113 h). (2) The strength of sandstone decreases nonlinearly with the increase of water content, and the degree of attenuation is affected by confining pressure. The softening degree of strength decreases with the growth of confining pressure. With the rise in water content, both cohesion and internal friction angle show a negative exponential decreasing trend. (3) Expansion of clay minerals and the restraint of a quartz skeleton during the experiment are reasons for the closure of pores; misalignment of quartz particles, the flow of pore water, and dissolution of soluble minerals will lead to pore enlargement during water absorption. (4) In addition to the lubrication of water film and the decrease of the cementation ability of clay minerals, the expansion and connectivity of the pore are also important aspects of the softening of sandstone and influence the final destructive form of sandstone.