Influence of the assisted kerf depth on cracks pattern and cutting performance of TBM cutter

Abstract

Kerfing on the working face of tunnel boring machine (TBM) can reduce the integrity of intact rock mass, it's one of feasible methods to improve the TBM performance. When the TBM cutter cut along the midline of two kerfs, the chipping mechanism changed to the interaction between the cutter and adjacent kerfs, the lateral cracks generated from the cutter tip could propagate to the bottom of adjacent kerfs. To study the effects of kerf depth on the cracks pattern and the cutting performance of the TBM cutter, a series of full-scale linear rock cutting tests were conducted. In addition, conventional cutting tests without kerfs were also performed to make a comparison. The cutter force and rock muck were collected, and the rock samples were sectioned after the tests. The cracks patterns beneath the cutter tip were observed by fluorescent dye. The results show that the dip angle of the lateral cracks increase with the increasing kerf depth, the vertical cracks become shorter and less. Besides, the area of the crushed zone decreases. The required normal force and rolling force are much reduced even for the large cutter penetration compared with the conventional cutting. The cutter forces decrease, while the characteristic grain diameter and coarseness index of the rock muck increase with the increasing kerf depth. The specific energy of kerf-assisted cutting is lower than that of the conventional cutting, and it declines as the kerf depth increases. A critical kerf spacing to depth ratio is found to efficiently reduce the specific energy. The results are useful for the rational parameter design of the kerfing system installed on the new generation TBM. The kerf spacing could be enlarged to save energy if the kerfs are deeper than 12 mm. However, it's still a challenging task to create deep kerfs on site.