It is unavoidable to have cutter bit wear during shield tunnel excavation. In recent years, because of excavation in a variety of ground conditions, problems caused by excessive cutter bit wear sometimes occur during projects. Many studies on cutter bit wear have been done in past but a quantitative evaluation method for cutter bit wear has not been established. In particular, there is little research on the characteristics of bit wear in gravel ground. This paper discusses the effects of characteristics of gravel, gravel contents and binding material on bit wear based on the results of a series of laboratory tests in order to understand the mechanism of bit wear in gravel ground and develop a prediction method of bit wear under those conditions.
The shield method is now widely applied to the construction of tunnels for infrastructure. Because this method can be applied in various geological conditions and has small impacts on road traffic and the surrounding environment. This method is that the shield is pushed into the ground with cutting and maintaining the stability of the cutting face (Clark, 1987). The shield machine has a cutter head with bits and the ground is drivaged by rotating the cutter head and pushing it by thrust. Bit wear during cutting operation is inevitable, as shown in Fig. 1.
Nowadays, as a closed type shield machine has mainly been adopted and the conditions of tunnel construction have become various, the operating issues due to the bit wear often occurs and becomes serious. As the bit wear has an obvious impact on the construction cost and constraints, such as lowering of drivage efficiency, increasing the frequency of bit replacement, etc. (Shimada et al, 1989). Therefore, the prediction of cutter bit wear is very important in order to make adequate construction plans and calculate estimated cost. However, there is little research on the characteristics of bit wear in gravel ground and the prediction method of the bit wear both theoretically and quantitatively. The mechanism of bit wear in gravel ground seems to be very complicated compared with rock mass ground, as shown in Fig. 2.
From the results of previous research (Yamamoto et al, 2016), the bit wear when a gravel ground is excavated can be evaluated and predicted based on the abrasiveness of gravel itself. So, it can be expected that the characteristics of gravel itself and the gravel contents have an obvious impact on the characteristics of bit wear when the gravel ground is excavated by shield machine.