Effects of drying and wetting cycles on the differential evolution of pore-fracture system and mechanical properties of lignite and its potential impact on lignite-related engineering

The cracking behavior of lignite during drying-wetting cycles impacts not only the efficiency of coal mining but also the formation's stability and engineering operations. The evolution of pores and fractures in several lignite lithotypes during drying-wetting cycles was studied, thereby helping to understand the effect of material composition on the syneresis behavior of lignite. When the water in the coal matrix is removed, the lignite shrinks sharply, and a large number of fractures are formed simultaneously. There are seeming differences in the contraction characteristics and fracture propagation law of different lithotypes of lignite. The xylite lignite contraction was the greatest powerfully after dehydration, and the reduction ratio of macropores was the highest, followed by the matrix lignite, and the fusain-rich lignite has the weakest syneresis capacity. The homogeneous xylite lignite shrinks as a whole after dehydration, and its fractures are long and straight, with good orientation. Fractures formed in the detrital humic groundmass of matrix lignite are short, convoluted, and poorly oriented. Fusain-rich lignite fractures are thin, and straight, with the best orientation. The expansion of fractures can cause geological disasters such as land subsidence, collapse, and landslides. Fusian hinders fractures’ expansion and acts as a “skeleton” to support the lignite structure. The presence of fusian is advantageous to maintaining the steadiness of the lignite. The differential evolution of the physical structure of lignite during the drying-wetting cycles, which aids to avert and mitigate disasters linked to dewatering cracking in different lignite-related engineering geological fields, was discussed in this research.