Characteristics of borehole deformation and hole collapse in typical soft and hard composite coal seam
In order to explore the deformation laws of gas extraction boreholes in soft and hard composite coal seams,a self-developed dynamic monitoring device for the stability of full-angle gas extraction boreholes was used for on-site monitoring.The deformation of gas extraction boreholes in hard coal,soft coal and soft and hard composite coal seam was systematically analyzed,and the test results were verified by COMSOL numerical simulation.The results show that,the vertical strain sensitivity of the shallow part in the borehole lags behind that of the horizontal,and the vertical and horizontal strain sensitivity increases alternatively.Before the strain decay stage of the borehole,there is a short-term pseudo-equilibrium state,during which the internal stress accumulates in the coal body,and then it releases suddenly causing the borehole collapse.Under the same external loading conditions,the soft and hard composite coal seam is divided into two relatively independent stress environments at the weak interface,and the sudden change of medium and stress occurs at the soft interface.The deformation of each part of the hard coal seam borehole is smaller than that of the soft coal seam borehole,both of which are characterized by large shallow deformation and small deep deformation,and finally achieve stable hoop strain,and the shallow deformation of the cross-bed hole of the soft and hard composite coal seam is similar to that of the two,and the deep part of the cross-bed hole and the weak interface are cross-cut to form a stress concentration area,and the range of the stress concentration area extends along the radial direction of the weak interface and the borehole at the same time and gradually attenuates,and his attenuation presents an insignificant nonlinear characteristic.
soft hard composite coal seamgas extraction boreholeweak interfaceborehole collapsenumerical simulation
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