Analysis of Geomechanical Factors Affecting Rock Bursts in Sedimentary Rock Formations

Abstract

Rock bursts are defined as sudden, violent failures of rock mass that are of such a magnitude that they expel large amounts of coal and rock into the face area during longwall or pillar extraction in sedimentary rocks. In an attempt to develop tools for assessing stress bump potential, the first author initiated a comprehensive study using site specific information from 25 case studies undertaken in U.S. mines. This effort builds on an initial study while expanding the data base and including additional variables and analyses. Multiple linear regression and numerical modeling analyses of geological and mining conditions were used to identify the most significant factors contributing to stress bumps in coal mines. Twenty-five factors were considered initially (mechanical properties of strata, stress fields, face and pillar factors of safety, joint spacings, mining methods, and stress gradients, among others). Allowances were made for favorable local yielding characteristics of mine roof and floor in reducing damage severity. Pillar and face factors of safety were calculated using displacement-discontinuity methods for specific geometries in case studies having experienced both violent and nonviolent failures. This work identified the most important variables contributing to coal bumps and violent failure of near seam strata. These are [1] mechanical properties of strata, including local yield characteristics of a mine roof and floor, [2] gateroad geometry and/or gate pillar factors of safety, [3] stress gradients associated with the approach of mining to areas of higher stress concentrations such as abutment stresses from multiple seam mining, and [4] roof beam thickness, joint spacing, and stiffness characteristics, which influence cave conditions and dynamic loading. The latter variables, combined together to form a new variable called “strata rigidity-cavability”, reflect some of the most important aspects of violent failure, i.e. having massive and stiff near-seam stratigraphic units capable of absorbing high strain energy, forming high stress on mine structures and poor cave conditions, and thus the potential for dynamic loads.