Rock Fracture Pattern Kerfed by HPWJ and its Influence on TBM Cutter Indentation in Granite

The advance rate of a tunnel boring machine (TBM) excavating in high-strength rock masses is limited. Creating kerfs on the working face using pure high-pressure water jets (HPWJ) is a feasible way to improve TBM performance. However, the influence of the cracks created by HPWJ on the subsequent cutter indentation has not been investigated. To get a comprehensive understanding of the rock breakage mechanism within the TBM cutter indentation assisted with HPWJ, a full factorial granite kerfing experiment was conducted, the depth and width of the kerfs produced by HPWJ with various jet pressures and nozzle traverse speeds were measured. Indentation tests including different cutter indentation positions and various shapes of kerfs created by HPWJ were performed, the indentation force and acoustic emission rate were continuously monitored. Finally, typical kerfing samples and indentation samples were sectioned to observe the internal rock fracture pattern using fluorescent dye. The kerfing test results indicated the granite kerfing mechanism by HPWJ consisted of the grain erosion and the dynamic impingement. A new index of unit loading energy ( E u ) of HPWJ was proposed, which was the energy input into the rock as the jet traversed a unit distance of the nozzle diameter. The influence of E u on the kerf shape and inner cracks pattern was analyzed. The indentation test results indicated a shallow wide kerf with less cracks produced by HPWJ using a low E u could drastically reduce the high indentation force if the cutter loaded on the kerf. However, a deep narrow kerf with long inner cracks created by HPWJ using a high E u could facilitate the crack expansion during the cutter indentation if the kerf was arranged on the side of the cutter. In this case, a critical spacing between the kerf and cutter existed, below which the rock chip could be created with the lower indentation force, and the critical spacing increased with the increasing E u . This study is conducive to a rational cutterhead design of TBM assisted by HPWJ.