Physical model test on the influence of the cutter head opening ratio on slurry shield tunnelling in a cobble layer

Abstract

To study the influence of the opening ratio of a cutter head on slurry shield tunnelling in a cobble stratum, a slurry shield test system that could realize the action of a cutter head and the automatic circulation of the slurry was developed. The mechanical response of cobble stratum during excavation and the instability of the excavation face in the slurry shield tunnel was studied. Moreover, the influences of the cutter head opening ratio on the shield parameters, surface settlement and failure mode of the excavation face were investigated. Finally, the opening ratios adaptability for the slurry shield in the cobble strata was discussed. Results show that the smaller the opening ratio is, the smaller the active failure chamber pressure and ultimate support ratio will be, as well as the settlement area during active failure. With an increase in the opening ratio, the active failure mode of the excavation surface changes from local slip failure to integral slip failure, and the volume of failure soil increases. The cutter head thrust and torque decrease with an increase in the opening ratio. The increase in shield thrust induced by a low opening ratio can be attributed to three factors, i.e. the increasing contact area between the cutter head and the excavation face, a large friction resistance between shield shell and stratum and the slurry cake phenomenon on the face of the cutter head. A modified theoretical calculation method suitable for cutter head torque of slurry shield in cobble stratum was proposed based on the experimental results. Finally, the opening ratios adaptability of the slurry shield in the cobble stratum was obtained.