Macroscopic load-bearing characteristics, energy evolution, and pre-splitting control of coal-rock mass under immediate hard roof

Abstract

The load-bearing and damage characteristics of coal seams vary under different geological conditions. To investigate the failure characteristics of coal bodies under hard roof conditions, this study is set against the backdrop of the Zhangji Coal Mine's 11,129 work face. By employing a combination of physical experiments, numerical analysis, theoretical calculations, and field validation, we explore the load-bearing capacity of coal bodies and their damage characteristics. The research findings indicate the following: (1) During the coal seam extraction process, the thick hard sandstone roof has a large span and exhibits minimal deformation, thus providing support for the overlying weak rock strata within a certain range. Following the implementation of pre-splitting blasting, the initial fracture step distance of the roof was reduced to 45 m, while the periodic fracture distance decreased to between 10 and 30 m. (2) The coal rock exhibits energy accumulation and dissipation at the moment of bearing stress or fracture. When the roof initially collapses, the peak stress within the coal body reaches 33.1 MPa, with a stress concentration coefficient of 1.7. The compressive displacement of the coal body within the stress concentration zone ranges from 2.8 to 12.1 cm. Energy accumulates within the coal body, reaching a maximum value of 124 kJ/m 3 , while the energy of the load-bearing layer above the goaf accumulates between 62.3 and 103 kJ/m 3 . (3) Based on the mechanical equations governing the load-bearing state of coal rock, a quantitative analysis was conducted on how four indicators—fracture step distance, distance from the rock layer to the work face, elastic modulus of the rock layer, and thickness of the rock layer—affect the coal body under dynamic and static load conditions, revealing the primary influencing factors. (4) Considering the geological conditions of the 11,129 work face, a pre-splitting blasting depressurization scheme was proposed using a fan-shaped hole grouping arrangement to reduce the fracture step distance of the hard rock layer. Upon the initial fracture of the roof, the support pressure in the lower section of the work face was consistently between 18.8 and 22.7 MPa. When the work face advanced through the pre-splitting blasting area (exceeding 700 m), the pressure in the middle support increased from 14.7 to 18.3 MPa to between 30.0 and 32.3 MPa, with a maximum recorded support pressure of 35.3 MPa. Field data indicate that the pre-splitting blasting scheme for the work face achieved the desired results. The study outcomes provide insight into the overall failure modes of coal rock bodies under the geological conditions of directly overlying hard roofs, thereby offering references for further investigation into the load-bearing characteristics of coal bodies and the calculation of crack propagation.