In coal mining, the surrounding strata is weakened and fractured around mine openings in underground mines due to the in situ stress redistribution. The main purpose of rockbolt reinforcement is to improve the internal load bearing strength of the rock mass to support itself (Bieniawski, 1984). Rockbolt performance is influenced by the rockbolt type, anchorage system, strata lithology and other geological conditions. Accordingly, various theories were proposed regarding the rockbolting mechanism in ground support. The functions of the bolt system are undertaken by one or a combination of the three fundamental mechanisms consisting of suspension, beam building and keying (Luo, 1998) as described below.
Theories of Rock Bolting
When roadways are excavated in underground coal mines, the laminated immediate roof tends to detach from the main strata. Suspension theory assumes that the immediate roof of the excavation is weak whereas the upper layer is relatively stronger. In such situations, rockbolts hold the immediate roof to the self-supporting main roof via transmitting the dead weight load of the strata between face plates and anchors to the main roof. Figure 9 shows the weak roof strata suspended to the competent strata. It is required that the bolt should be long enough to anchor into the stable rock and that bolts should hold sufficient tensile strength to maintain the rock dead weight.