Time-dependent jamming mechanism for Single-Shield TBM tunneling in squeezing rock

The entrapment risk of Tunnel Boring Machine (TBM) tunneling in squeezing rock associated with high overburden is a challenging and time-dependent consideration. However, there is limited advance knowledge of the shield jamming mechanism due to the difficulties in predicting the complex interactions among the ground, the TBM, its system components and the tunnel support. In this paper, time-dependent interactions between rock and shield, i.e., contact, squeezing and friction, for single-shield TBMs in squeezing ground are investigated. An extended Vlachopoulos-Diederichs approach, instead of the Hoek equation, is used to capture the longitudinal deformation profiles. The plastic behavior of squeezing rock is characterized by the Drucker-Prager criterion with the non-associated flow rule. Viscoelastic deformation is determined using the Burgers model. The contact behaviors are captured through the closure of the radial gap. Moreover, a linear relationship exists between the squeezing loads and the shield deformation. Ultimately, the required thrust force is determined by integrating the squeezing loads over a contact interval. Taking the Saint Martin La Porte access adit (Lyon-Turin Base Tunnel) as an example, the effects of standstill time, relaxation time and overcut on the size of the loaded area and the magnitude of squeezing loads after a standstill are investigated in detail. It is found from the obtained results that a longer standstill, smaller overcut or smaller relaxation time leads to a larger frictional contact area, as well as a larger squeezing load acting upon the TBM shield and a larger deformation of the shield. Moreover, the analytical results show that the TBM jamming is mainly caused by the increase of the squeezing loads and the loaded area.