Preliminary investigation on the deformation characteristics of lignite during dehydration process

In this paper, the water loss test was carried out to investigate the influence of lignite dehydration on the stability of coal-rock combination slope in open-pit mine.Firstly, the variation laws of water content, deformation, porosity and crack evolution of rock samples were obtained; secondly, the cracking fractal characteristics of sample's end face were analyzed; thirdly, the progressive damage characteristics of dehydrated lignite were studied; finally, based on the principle of capillarity, the law and mechanism of crack propagation for lignite were analyzed.The results show that under the condition of dehydration process, the water loss rate of rock samples presents an increasing trend of "first steep and then slow", and finally approaches or exceeds 10 %.The axial and circumferential deformation increases gradually for lignite samples, and the final strain increases to 0.012 3 and 0.020 3, respectively; meanwhile, the volumetric strain increases to 0.052 8 and has the similar growth trend to the water loss rate.The water loss volume of rock sample is much larger than that of shrinkage volume, and the porosity shows an increasing trend of "steep first and then slow".The lateral cracks of the rock sample develop gradually, and the propagation positions gradually change from upper end to bottom end, and finally to the middle of the rock; meanwhile, the axial cracks initiate and expand.The cracking characteristics of sample's end face are significant, and the fractal dimension of crack distribution shows a linear growth trend, which reveals that there is a transformation process from homogeneity to heterogeneity.The damage variable increases nonlinearly with the water loss rate, and finally increases to 0.066 5;meanwhile, the whole growth process can be divided into linear growth stage, deceleration growth stage, and accelerated growth stage.It is worth noting that the increase of matrix suction controlled by capillary action in the process of pore water loss can be account for the axial and circumferential shrinkage deformation of rock samples, and the initiation and propagation of vertical cracks.