Study on progressive damage and deformation law of coal body around borehole under different moisture states

Water immersion in the gas extraction borehole will reduce the stability of the borehole, lead to borehole deformation and collapse, and reduce the efficiency of gas extraction. In order to study the failure characteristics and deformation law of coal and rock bodies around boreholes with different water content, the digital image observation system of coal and rock deformation and failure was used to carry out the surface deformation observation experiment of coal samples with boreholes in dry, natural and saturated states under uniaxial compression. The time series speckle images of the surface deformation of the samples under different stress states were obtained, and the surface deformation of the samples was qualitatively and quantitatively analyzed. The results show that: (1) As the water content increases, the peak strength and modulus of elasticity of the porous specimens gradually deteriorate and decrease, with a maximum deterioration of 39.53% and 17.39%, respectively, and the peak strain gradually increases, with a maximum increase of 40%. (2) The deformation localization phenomenon of the water-containing samples started earlier than that of the dry sample. The deformation localization zones of the dry samples had a faster displacement opening speed and a smaller displacement dislocation amplitude. (3) From dry to water-saturated conditions, the borehole contracted inward by 64.1% overall in the vertical direction and expanded outward by 87.8% overall in the horizontal direction. The higher the water content, the greater the deformation and flattening of the borehole, and the greater the amount of radial displacement and circumferential displacement. (4) Under the action of water-force coupling, the bonding force between particles is reduced, the internal transformation of the specimen to loose and porous, the tensile stress in the pore-fracture stress concentration area is enhanced, and the pores and microcracks develop and expand rapidly, which weakens the bearing capacity of the coal body.