Roof bolting and underground roof falls

Roof bolting has been used in underground entry (roadway) support in U.S. coal mines since the Coal Mine Health and Safety Act of 1969 (US Congress, 1977) recognized roof bolting as the only means of underground entry (roadway) support. For U.S. underground coal mines, roof bolting pattern is fixed at 4 ​× ​4 ​ft (1.2 ​× ​1.2 ​m), except in the Pittsburgh Seam where longwall mining is practiced, with occasional 3.6 ​× ​4 ​ft (1.1 ​× ​1.2 ​m) pattern. However, roof falls or roof failure often occurs in roof-bolted entries in U.S. coal mines. Roof falls can roughly be divided into four types: skin falls, large falls, cutter roofs, and massive falls. Based on this situation, the roof is initially strengthened by bolt based on suspension and friction mechanism. By comparing roof bolting patterns in different coal producing countries, bolt density in all other countries is much higher (except South Africa) than that used in the U.S. In spite of its long history of successful application with hundreds of millions of units installed, roof bolting design is still the lack of a commonly accepted method.