Face stability conditions in granular soils during the advancing and stopping of earth-pressure-balanced-shield machine

The instability of the tunnel face is one of the biggest concerns when tunneling with an earth pressure balanced shield (EPBS) machine. The mechanism by which the tunnel face fails is highly related to the advance status of the EPBS machine, which has rarely been studied. This paper aims to address this problem by examining the face stability in two situations, i.e., 1) EPBS machine advancement and 2) EPBS machine stoppage. Specifically, we present results of tests carried out using a laboratory reduced-scale model that realizes the EPBS tunneling operations in fine dry sand. A three-dimensional (3D) discrete-element method (DEM) model, which is able to simulate the main EPBS machine excavation process, is employed to gain further insight into the mechanisms of the face failure by comparing with the laboratory results of the face failure mechanism, limit face pressure and the trajectory of the soil at the face. The results show that the mechanism by which the face failure differs significantly for the studied two situations. A partial face failure mode and a larger limit face pressure were observed when the EPBS machine continued to advance. In contrast, a global face failure mode and a smaller limit face pressure were observed when the EPBS machine stopped. When the EPBS machine continued to advance, the trajectory of the soil particles at the face was spiral, with soil mainly flowing into the chamber through the upper part of the cutter head. When the EPBS machine stopped, the trajectory of the soil particles at the face was straight, and inflows of soil to the chamber occurred through the nearest cutter head opening.

Automated summary

The study found that the mechanism of face failure differs significantly between the EPBS machine advancing and stopping. When the EPBS machine continued to advance, a partial face failure mode and a larger limit face pressure were observed. In contrast, a global face failure mode and a smaller limit face pressure were observed when the EPBS machine stopped.