Work

Study on the broken law of overlying rock in the steeply dipping medium-thick coal seam with thick and hard roofs

Year: 2025

Type: article

Abstract: In order to investigate the roof fracture mechanisms and working face structural characteristics during medium-thick coal seam mining under steeply dipping, thick, and hard roof conditions—and to enable the successful implementation of along-gob retained entry under these geological conditions—physical model tests, similarity simulations, and theoretical analyses were conducted. The study analyzed the fracture patterns of the overlying strata and the structural features of the goaf, and proposed principles and schemes for along-gob retained entry. The main conclusions are as follows: • During the mining of medium-thick coal seams with a steeply dipping, thick, and hard roof, the roof exhibits caving characteristics of a “small goaf area (1.5 m) and a large caving zone (12.5 m)” upon fracturing. The caved gangue effectively fills the goaf, resulting in the working face roof being in an unloading state, with mining-induced stress transferred to the solid coal ribs on both sides, and a stress concentration factor of approximately 2.5. • The fracture of the thick and hard roof is influenced by shear stress concentration at both ends. After fracturing, the lower rock beam undergoes rotational subsidence, while the upper rock beam arches upward, forming a “low-level hinged reverse arch–high-level simply supported beam” structure in the goaf. This structure can effectively control the sliding of overlying gangue and provides some supporting force for the deeper rock beams. • Through mechanics analysis, four potential failure points (0, 5l/8, 25l/64, and l) were identified for the inclined long beam, leading to the establishment of the roof fracture mode for medium-thick coal seams under steeply dipping, hard roof conditions. The major challenge for along-gob retained entry lies in controlling the low-level hinged reverse arch (block B). Based on this, criteria for along-gob retained entry were summarized and engineering recommendations were provided. To investigate the fracture behavior of thick and hard roofs in steeply dipping, medium-thick coal seams, similarity simulation and FLAC3D numerical modeling were conducted using the 12112 steeply dipping working face at Tieshan South Mine as the engineering background. Combined with multi-dimensional stress–displacement monitoring, the study found that the ratio of the caving zone to the gob during mining of steeply dipping, medium-thick coal seams is 8.3. After roof failure, a "low-level hinged beam–high-level simply supported beam" bearing structure forms within the gob, which can effectively inhibit deep roof deformation and failure. Based on material mechanics analysis, four potential fracture points on the inclined long beam were identified, and a fracture and gob-side entry retaining mechanical model under steeply dipping, medium-thick coal seam conditions was established. The study reveals that controlling the low-level hinged block B is the key to successful retained entry implementation. Finally, a scheme integrating roof cutting and pressure relief, roadside filling, and strong rib stabilization is proposed for retained entry construction.

Source: Results in Engineering

DOI: 10.1016/j.rineng.2025.107539