Rock spalling

Abstract

One dynamic form of rock pressure manifestation is spalling of rock from an exposed surface [i, 2].In underground workings, intensive spalling is observed, as a rule, immediately after the formation of new exposed surfaces; the fragments shot from the surface can have considerable velocity.After a time the process usually stops. There can be cases in which rock spalling is renewed in parts of a working where there are changes in rock pressure due to the approach of the front of extraction operations or to time-dependent effects.In any case, rock spalling is observed in strongly stressed parts of the rock, and is often a precursor of larger-scale effects such as shock bumps, etc.[3]. Research on this topic is therefore becoming more urgent as deeper horizons are being opened up.Consider one possible mechanism of this phenomenon.In the undisturbed rock let us identify the periphery of the future surface of an exposure (Fig. la).Certain stresses o r act on this periphery. The exposure of the surface can be regarded as a sudden drop in o r to zero. The peripheries of real workings are never smooth. Let us consider the behavior of an individual projection.In the undisturbed rock it is in equilibrium under the action of the stresses o r . A decrease in the boundary forces causes a redistribution of the internal stresses and can lead to fracture on the periphery. As a result of the fracture, elastic potential energy is liberated, and as we will see, the separated part of the rock or ore can receive an impulse directed into the working.A similar effect is possible on account of a change in rock pressure.In solving problems on rock pressure it is usually assumed that the peripheries of the cross sections of the workings are smooth or piecewise-smooth --circles, polygons, etc.If we pose the problem of calculating the rock pressure remote from the peripheries of the workings (e.g., the mutual influence of workings), there are no fine details in the application of the boundary conditions on the periphery, i.e., it can be regarded as smooth, and also free from stresses.In this formulation the real resistance of the supports does not influence the solution [4,5].But if we investigate the processes near the exposed surface, the details of the application of the boundary conditions can be important. p