Rock cutting mechanics related to the design of primary excavation systems

Abstract

Estimates of the future need for tunnelling activity in this country are of necessity tenuous, but a recent survey (1) predicts a threefold increase during the next decade. It is reasonable to assume that, where ground conditions permit, the use of mechanised systems to excavate these tunnels will be considered. The decision to utilize machines in place of labour intensive cyclic tunnelling systems is normally based on their relative costs. A tunnelling machine, while representing a considerable capital investment, eliminates the wages and uncertainties associated with a large labour force. In addition, tunnelling activity within urban areas is subject to environmental constraints. The use of explosives to fragment the rock can cause inconvenience, while the irregular profile obtained, together with the damage caused to adjacent strata, can lead to surface subsidence. During the last fifteen years machines have, on several occasions, been used to bore development tunnels at metaliferrous mines (2). At the same time the successful introduction of coal winning machinery has ensured the need for, and the acceptance of, mechanised roadway drivages in coal mines. Although the design of tunnelling machines has developed substantially during the last twenty years a complete understanding of the action on which these designs are based has yet to be achieved. This thesis is concerned with a detailed investigation into fundamental aspects of excavating Bunter Sandstone and Magnesian Limestone by drag picks and rotary cutters. These rocks were chosen because they appear at shallow depth and in considerable thicknesses throughout large parts of the Midlands and North of England. They are, therefore, of considerable interest to tunnelling-engineers operating in these areas. In order to maintain the experimental work at a manageable size a partial factorial approach, proposed by Protodyakonov (3), is used for the design of the experiments. Existing theories of rock cutting are reviewed and a simple mathematical model of the action of a disc cutter is proposed. Experimental data from both pick and disc cutting is compared with theoretical predictions.