# Rock Breaking Performance of TBM Disc Cutter Assisted by High-Pressure Water Jet
## 4 Results and Discussion
### Effect of Parameters on Rock Breaking Performance  
As the first evaluation criterion to judge the performance of the water jet, the kinetic energy can reflect the forces and wear rate of the disc cutter. The initial kinetic energy of the disc cutter is set as a constant, which is related to its initial linear velocity and angular velocity set in the numerical simulation. Along with the calculation, the kinetic energy of the disc cutter will decrease for more and more energy will be used to break the rock mass. Therefore, a small decrease in the disc cutter kinetic energy represents the smaller forces and wear rate of the disc cutter at the same time. To study the effect of the jet velocity on the disc cutter kinetic energy, the jet velocity is set as the single variable, while the jet location and diameter are constant (S = 0 mm, D = 1 mm). As shown in Figure 9, the water jet can slow down the kinetic energy consumption of the disc cutter no matter what the velocity value of the water jet is in the simulation, because the water jet with the smallest velocity (100 m/s) can break the rock to create a groove on the rock mass, which can increase the freedom degree of rock breaking and then decrease the force and wear rate of the disc cutter. Moreover, the disc cutter kinetic energy used for breaking the rock mass will decrease with the increase in the jet velocity, except where the velocity of the water jet is 245 m/s. The reason is that the depth of the groove will increase with the growth of the jet velocity, leading to more freedom degrees of rock breaking. When the velocity of the water jet is more than 224 m/s, the groove depth is larger than the penetration of the disc cutter (8 mm), and in the simulations, the width of the groove created by the water jet with 224 m/s velocity is a little larger than the one by the water jet with 245 m/s velocity. Therefore, the kinetic energy of the disc cutter with the 224 m/s water jet decreases a little more slowly than the one with the 245 m/s water jet. Figure 8: Maximum principle stress of the three rock elements. Figure 9: Effect of velocity on the disc cutter kinetic energy. To study the effect of the jet location on the disc cutter kinetic energy, the jet location is set as the single variable, while the jet velocity and diameter are constant (V = 200 m/s, D = 1 mm). As shown in Figure 10, the water jet can slow down the kinetic energy consumption of the disc cutter when it is located at the traverse route of the disc cutter, and the water jet has an effect on the disc cutter kinetic energy when it is located beside the traverse route of the disc cutter. Figure 10 also shows that the center section of the disc cutter is the optimum location for the water jet, in that the lowest kinetic energy of the disc cutter will be used for breaking the rock mass. To study the effect of the jet diameter on the disc cutter kinetic energy, the jet diameter and jet velocity are set as the variables, while the jet location is constant (S = 0 mm), and values of the two variables are selected according to the rule that the energy of the water jet for rock breaking per unit time must be constant. As shown in Figure 11, the water jet can slow down the kinetic energy consumption of the disc cutter no matter what diameter is set for the water jet in the simulation, because the velocity of the water jet with the largest diameter is 96 m/s, which also can break the rock to create a groove on the rock mass. Moreover, the disc cutter kinetic energy used for breaking the rock mass will decrease at first and then increase with the growth of the jet diameter. With the kinetic energy of the disc cutter as the evaluation criterion, 1.5 mm is the optimum value of the nozzle diameter in this simulation. The second evaluation criterion is the rock crushing volume that can reflect the rock breaking efficiency. More rock crushing volume represents higher rock breaking efficiency. As shown in  
Figure 11: Effect of diameter on the disc cutter kinetic energy.