# Rock Breaking Performance of TBM Disc Cutter Assisted by High-Pressure Water Jet
## 1 Introduction  
With the advantages of safe, reliable, and environmental preservation [1; 2], the tunnel boring machine (TBM) has been increasingly used in long tunnels operations [3; 4; 5] and hydraulic pipelines construction [6; 7]. However, it encounters a lot of problems in face of a full-face hard rock tunnel, such as low digging efficiency, high disc cutter wear rate, and the machine getting stuck [8; 9]. Replacing the disc cutter implies more time and cost [10], which is also a problem for microtunneling boring machines (MTBM) [11; 12]. Statistically, replacing and repairing TBM cutters in a high-abrasive stratum costs one-third of the total time of tunneling, while the cost of cutting tools is about one-third of the digging construction cost [13; 14].  
To reduce the TBM cutter wear and improve the tunneling efficiency, scholars have put forward many kinds of solutions. With a prediction model of disc cutter wear having been established, the working parameters of tunneling (penetration, diameter of the disc cutter, rotating speed of the cutter head, etc.) with the TBM have been optimized [15; 16]. The arrangement form of the disc cutters on the cutter head has been improved based on the interaction mechanism between the rock and the disc cutter being revealed further [17; 18]. According to analyzing the failure mechanism of the disccutter, [PERSON] [19] suggested to use new materials strengthened on the disc cutter to resist the rock wear. The wear rate of cutters has been monitored online to carry out the repair and maintenance of cutters and the cutter head in real time [20]. Although the above methods have achieved certain beneficial effects, the problems of the low digging efficiency and high disc cutter wear rate are still not solved.  
Being of great potential in the field of rock crushing, the high-pressure water jet is gaining attention by scholars gradually. In tunneling excavation, efficiency and capacity analyses of an abrasive water jet (AWJ) were carried out, obtaining perfect working parameters (pressure, traverse speed, abrasive feed rate, and standoff distance) and an optimum operational cycle for AWJ cutting strategies [21; 22; 23]. The water jet has been used in oil and shale oil exploitation, increasing the perforation depth and enhancing the well production [24; 25]. The water jet has been used to hydraulically slot coal rock for enhancing underground coal bed methane recovery and relieving the crustal stress of coal rock mass [26]. In geothermal exploitation, a new drilling technology, a mechanical driller assisted by a high-pressure water jet, has been developed for the challenging downhole environment [27]. The high-pressure water jet has been also used in underground mining, in which rock is broken by mechanical tools assisted with the water jet, reducing the cutting pick force and increasing the rock crushing efficiency [28; 29; 30; 31].  
According to the above literature review, the high-pressure water jet can break rock efficiently, which is of potential to overcome the problems of TBM in full-face hard rock tunnel digging. Therefore, this paper presented a new tunneling method that is a TBM coupled with a high-pressure water jet. The rock failure mechanism under the coupled forces of the disc cutter and water jet will be analyzed at first. Then, the finite element method (FEM) and smoothed particle hydrodynamics (SPH) method will be used to establish a numerical model of rock broken by the disc cutter and water jet to study the effect of parameters on the rock breaking performance. Finally, an experiment of the water jet cutting marble was carried out to verify the reliability of the numerical simulation. The results can provide theoretical guidance and data support for designing the most efficient system of a TBM with a water jet for digging a full-face hard rock tunnel.