Experimental and Theoretical Study of the Time-Effect-Based Instability Characteristics of the Gas Drainage Borehole Sealing Section in a Soft Coal Seam.

The instability and sealing difficulty confronted by gas drainage boreholes in a soft coal seam weakens the drainage efficiency. To address this problem, the instability characteristics of the sealing section of boreholes are investigated. On the basis of the findings of the mechanical properties of the borehole around the sealing section and in view of the physical and mechanical properties of the soft coal seam, a theoretical analysis was performed. A system consisting of the YYL200 electronic persistent creep test system and the DNS-200 electronic universal tester was used to observe the borehole under different filling conditions. The samples were subjected to a graded loading test, and the Kelvin-Voigt model was selected for parameter inversion and law analysis of the test results. The results show that the collapse of the broken area and the stress concentration and instability of the sealing section after the sealing of the soft coal seam are directly attributable to the instability of the soft coal seam and the sealing difficulty. The mechanical properties of the grouting sealing material are directly related to the loading of the hole-containing sample and dead load displacement. The maximal overall displacement of the sample in group B is close to 1.4 mm. The instantaneous deformation capacity and deformation space reflected by the generalized Kelvin model parameters , , and η are closely related to the mechanical properties of the filling material. The highly stress- and deformation-resistant sealing material can ensure the relative time-effect-based stability within the stress concentration area of the sealing section.