Mechanisms of Rib Sloughing and Methods of Controlling Thick Bolted Ribs

摘要

Coal ribs can roughly be divided into three types: (1) roof and floor rocks are similar to ribs, (2) roof and floor rocks are stronger than ribs, and (3) roof and floor rocks are weaker than ribs. Different types of roof/floor and ribs will exhibit different types of ground pressure around an entry. Most coal mines belong to the 2nd type with ribs fractured or even broken. In this case the mine ground pressure depends on the occurrence and development of deformation and fracture of the ribs. If it's not properly controlled, the following deteriorating process will follow: the roof bent downward > the ribs squeezed and broken > rib sloughing and collapse > rib support to the roof reduced > the roof bent downward further > the ribs broken further. In this paper rib instability is divided into four types: compression/shear sliding rib fall, gravity sliding rib fall, cross arch rib fall, and cleavage rib fall. Since failure in thin walls is largely of tension type whereas that of thick walls by compression or shear, a concept is proposed here to utilize a thick bolted rib to stabilize entry ribs, i.e. the thickness to height ratio of the thick bolted ribs should be larger than that of thin ones or t/M △(right) 1/5. The idea is to better utilize its compressive/shear strengths, which are larger than tensile strength. Based on this principle, a rib bolting design method, in which methods for determining the bolt length, bolt and row spacings, bolt anchorage capacity, and bolt strength were specified, was developed.