Failure Mechanisms of Boreholes in Soft Coal Seams: Comparative Analysis of Protective Sieve Pipes

Abstract

A key strategy for mitigating gas-related disasters involves the effective drainage of gas through in-seam short boreholes. Maintaining the integrity of these boreholes in soft coal seams, prone to deformation, collapse, and closure, poses a significant challenge that substantially impacts gas drainage efficiency. This study investigated the use of protective sieve pipes made from PVC, polyethylene (PE), and polypropylene (PP) materials to support and prevent the collapse of the borehole. A series of similarity experiments were conducted to assess the effects of support strength, the coefficient of horizontal pressure, and borehole inclination on borehole deformation and failure. The results indicated that boreholes supported by sieve pipes exhibited significantly reduced displacement and cumulative acoustic emission (AE) counts compared to unsupported boreholes. Among the tested, the displacement was largest in boreholes supported by PVC pipes, followed by PE pipes, with PP pipes showing the smallest displacement, indicating that support strength decreases on the order of PP, PE, and PVC. Also, sieve pipes were found to mitigate AE activities by stabilizing the borehole walls, particularly under high horizontal pressures. Moreover, the PFC3D-FLAC3D coupling method was used to explore the stress and displacement distribution around boreholes with varying support strengths. The results demonstrated that enhanced support strength leads to a reduction in vertical displacement and stress concentration around the borehole, effectively maintaining the integrity of the gas drainage channels. Building on these insights, an integrated drilling and sieve pipe installation technology was developed and field-tested at the Shoushan Mine. The results showed a threefold increase in gas drainage efficiency compared to traditional, unsupported boreholes. This study not only underscores the importance of failure analysis in designing safer gas drainage systems but also presents practical solutions to enhance gas drainage efficiency and coal mine safety.