Full-scale linear cutting tests in Chongqing Sandstone to study the influence of confining stress on rock cutting efficiency by TBM disc cutter

This study is to investigate the influence of confining stress on rock cutting efficiency by TBM disc cutter. The full-scale linear cutting test results under different equal biaxial confining stressed conditions in Chongqing Sandstone and in Beishan Granite are compared. Rock boreability index, rock cutting volume and optimum rock cutting condition are analyzed. Generally, there exist power function relationships between boreability index for normal and rolling forces and cutter penetration depth. When confining stress increases, these two indices either remain stable, increase rapidly or gradually at first, but they then decrease rapidly and significantly. The variations of boreability indices for normal and rolling forces with increasing confining stress are different for low strength and high strength rocks. Generally, under non-stressed condition, when cutter penetration depth increases, rock cutting condition changes from under-breakage to over-breakage and finally to under-breakage, and the optimum rock cutting condition corresponds to rock over-breakage. For cutter penetration depth around the optimum value obtained at the non-stressed condition, with increasing confining stress, rock cutting condition changes from over-breakage to under-breakage and finally to over-breakage. Generally, there exist quadratic function relationships, either obvious or non-obvious, between rock cutting specific energy and cutter penetration depth. With increasing confining stress, optimum cutter penetration depth and optimum rock cutting specific energy both increase firstly and then decrease. There are no common rules to describe the influence of confining stress on Chongqing Sandstone and Beishan Granite during rock cutting. Confining stress 10-10 MPa is one critical threshold for Chongqing Sandstone, but none critical confining stress can be marked for Beishan Granite. This study can serve to optimize TBM operation performance in hard brittle sedimentary and igneous rocks under highly stressed condition.