Laboratory study on cutting tool-rock interaction at the interface of rock layers in TBM tunneling

In mechanized mining and tunneling operations, the interface of existing rock layers poses a variety of challenges including cutter failure, machine vibration, and low advance rate without prior notification. As a result, proper understanding of the cutting process and the tool-rock interaction is critical for machine design in these circumstances. In this study, a lab-scale investigation on the interaction between rock and tool to determine the cutting forces at transition zone was carried out in mechanized excavation laboratory of Tarbiat Modares University using a small-scale linear cutting machine equipped with a disc cutter of 54 mm in diameter. Three types of samples were used in the study: sandstone with a strength of 37 MPa and two types of synthetic gypsum with strength of 21 and 24 MPa, in a mixed condition. Experiments with linear cutting machine were carried out at cutting depths of 1, 2, and 3 mm. Laboratory studies revealed that as the cutter approaches the interface between two soft and hard specimens in mixed specimens, the normal and rolling forces vary dramatically, and the difference in the ratio of forces in the transition zone diminishes as the cutting depth increases. It was also revealed that the ratio of boundary cutting forces to rock cutting forces in unmixed mode reduces as cutting depth increases. Taking into consideration the highly variation of cutting forces pattern at the interface, the importance of selecting appropriate cutters for tunneling machines and mechanical excavators increases in the field conditions. Also, it was revealed that the difference in cutting forces results in shock load at the transition zone where the cutter passing the interface of two rock layers. This shows the importance of proper design of cutter in respect of toughness and hardness and its impact in reducing cutter failure in mixed face conditions.