Jointed rock mass surrounding underground excavations, whose tensile strength is almost negligible, can only be competent with the help of several arching actions. One of these actions is that of a linear arch formed in the jointed rock beams in the immediate roof of a cavern.
The rock mass outside the ground arch is mechanically stable. But the rock mass between the ground arch and the immediate roof where the stress, normal to the excavation wall surface, becomes relatively low is mechanically unstable. In case of a self supporting excavation, the rock inside this unstable area is supported by the linear arch formed in the rock beams in the immediate roof. The span that gives an equilibrium between the dead weight of the unstable area and the bearing strength of the rock beam, is the critical span of the self supporting excavation.
To investigate the supporting mechanism of such linear arch, model experiments have been performed using rock beams of plaster-cement mixture. According to the results of experiments, simple but effective idea to explain the supporting mechanism of laterally stressed jointed rock beam is proposed. Also the effect of lateral stress to the bearing strength of the rock beam is clarified.
The following procedure to estimate the critical span of an underground cavern based on the idea of a linear arch formation was also considered. 1) Evaluate the shape of the ground arch to obtain a value for the possible load operating on the rock beam under several stress conditions using FEM analysis. 2) Calculate the relationship between the span of the rock beam of particular thickness and its bearing strength. 3) Estimate the critical span of the cavern, comparing 1) and 2), for various conditions.
