A Combined Discrete Element-Finite Difference Model for Simulation of Double Shield TBM excavation in Jointed Rocks

The risk of TBM shield jamming in jointed rocks is one of the main concerns in the deep tunnel construction. To realistically evaluate the possibility of shield jamming in such grounds, the property of joint sets, the behavior of jointed rocks and the interaction between the rocks and shields, segmental lining and backfilling need to be understood. Referring to these finite element models and finite difference models, a combined discrete element-finite difference model for simulating double-shield TBM excavation in jointed rocks is developed in 3DEC. To describe the behavior of jointed rocks realistically, a fish code is written in 3DEC to generate random joints considering their orientation, spacing, position and persistence. The orientation and spacing are assumed to obey Fisher distribution and uniform distribution, respectively, and then the position is determined by the corresponding orientation and spacing. Moreover, the zone of jointed rocks around the tunnel is modeled by discrete element model, while the other zones, such as the jointed rocks away from the tunnel and the hard rocks, are modeled by finite difference model, which can effectively reduce the computer runtime and hardware resource. The severest shield jamming of the DXL tunnel excavated by a double-shield TBM is conducted to demonstrate the applicability of the proposed model. The results are more consistent with the failure phenomenon on site than the finite element model or finite difference model. The proposed model can be used to quantitatively evaluate the potential risk of TBM jamming in jointed rocks.