In some areas, coal mine roadways inevitably pass through mudstone strata. Under the action of groundwater and mechanical loads, the roadway floor mudstone quickly softens and eventually slakes, seriously affecting mining tasks and construction safety. The process and mechanism of the argillization has been a research hotspot. Relevant studies have proposed various theories for mudstone softening and disintegration from different angles. As a kind of soft rock collection, mudstone has varying physical mechanics and hydraulic properties according to its varied mineral composition and micro-structure. For a specific engineering project, special research should be carried out based on the actual geological conditions. The material composition and microstructures of the mudstone in the Shendong coal area were tested by conventional mechanical experiment, scanning electron microscopy, and X-ray diffraction. The change of the mudstone microstructure characteristics during the softening process was studied. The main reason and mechanism of mudstone softening were analyzed. The results showed that: ① The argillization process of the roadway floor went through four stages: 1) the soluble salts gradually dissolved and void regions appeared on the surface and inside of the rock; 2) the uneven stress and inhomogeneity swelling led to the development of micro-voids; 3) the initial irregular shape of the contact edge between particles became smooth, which caused the reduction of cohesion and internal friction angle; and 4) the cementation between clay minerals and coarse particles was gradually destroyed under the action of water, and the sliding between particle edges led to the complete destruction of mudstone strength. ② Water was the most important factor causing the argillization, instead of the rock component. ③ Mechanical loads accelerated the softening process. The research results provide useful guidance for research into the reinforcement of roadway mudstone. Future research should focus on reinforcement measures and water treatment.

1. Introduction

In China, most of the strata traversed by coal mine roadways are sedimentary rock strata. The bottom of the roadway is mostly formed from muddy sediments or sedimentary mudstones, which are widely distributed and cover almost all coal-bearing areas in the whole country (Cai et al., 2005). As a kind of fine-grained soft rock collection, mudstone is mainly composed of clay minerals (kaolinite, montmorillonite, hydromica, etc.), followed by detrital minerals (quartz, feldspar, etc.) and secondary minerals (epidote, chlorite, etc.) as well as a little iron manganese matter and organic matter. The mechanical properties of mudstone are different under different circumstances: it is relatively intact, hard, and has good mechanical properties in the natural and dry state. When the mudstone meets water and becomes saturated, it softens quickly and finally slakes.

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