In the US, for purpose of accessing remote reserve area, some coal mines have to maintain aged underground belt entries for a great distance. However, due to high humidity, warm temperature, and time effect, progressive roof deterioration and unexpected roof fall pose a great challenge to ground control engineers. With active belt structure in place and limited space, rebolting becomes very costly, less effective, and sometimes impractical and unfeasible. To gain long-term entry stability and serviceability, operators typically rehabilitate the aged belt entries installing standing steel set supports. In the last several years, Keystone Mining Services, LLC (KMS) has assisted many coal mines with their belt entry rehabilitation projects, evaluated ground condition of various aged belt entries, and designed different standing steel set support systems. This paper presents a case study of a large scale roof fall that occurred at an aged belt entry in a mine located in eastern coal field, analyzes root cause of excessive deformation of square sets that were installed in adjacent entry, evaluates adequacy of existing rehabilitation square set, and develops remedial recommendations for future rehabilitation practice. Based on the case study, the paper outlines a rehabilitation steel set design guideline that includes field evaluation, engineering considerations, design assumptions, steel structural analysis, and field installation quality control.
With gradual depletion of nearby coal reserves, some underground coal mines have to maintain long-term entries (belt, track, return airway, etc.) for a great distance in order to access remote coal reserve area. However, due to high humidity, warm temperature, and aging effect in the belt entry, roof strata weathering and pillar degradation become more and more severe. Progressive roof deterioration poses a great challenge to ground control engineers. Pillar rib sloughs at certain locations, entry width increases at some areas, immediate roof sags, immediate roof rock pops out between bolts, existing bolts lose functionality due to corrosion, cutters develop deeper in corners, and strata separation develops deeper up into the main roof. As a result, unexpected roof fall occurs in a random manner, which results in loss of production, damage of belt structure, and even injuries to underground personnel.