The composite rock strata is highly detrimental to the cutters of tunnel boring machines (TBMs) during the tunneling construction. To improve the tunneling efficiency and reduce the construction cost

it is necessary to study the efficiency of composite rock fragmentation by TBM cutters. In this study

the rock breakage experiments were carried out on the specimens of the sandstone and granite composite (composite ratio of 4:6). A series of rotary synchronous cutting tests was conducted on three cutters with three penetration depths at five cutter spacings. During the experiments

the total normal force and total torque of the rock fragmentation were monitored

and slags of two rock types were separately collected for screening and weighing. The relationships between normal force

torque

and specific energy under different cutter spacings and penetration depths were analyzed. The results showed that the average normal force and the average torque increased with the increase of the penetration depth under different cutter spacings. However

their increase trends were different

which meant that with the increase of the penetration depth

the average normal force increased almost linearly

while the average torque increased slowly. In addition

the optimal cutter spacing varied at different penetration depths. It was found that the efficiency of rock fragmentation was the highest when the ratio of cutter spacing to penetration depth was about 14

and the cutting length ratio of the sandstone to granite was 4:6.