3D finite difference model for simulation of double shield TBM tunneling in squeezing grounds

Highlights

• A 3D modeling of mechanized tunneling with a DS-TBM in squeezing ground is presented.

• The effects of advance rate on the possibility of machine jamming are observed.

• Longitudinal displacement and contact force profiles on shields have been examined.

• Thrust force required to overcome friction and drive TBM forward is calculated.

Abstract

One of the main concerns in using a shielded machine in deep rock tunnels is the possibility of the shield seizure in squeezing ground. To realistically evaluate the possibility of machine seizure in such grounds, the interaction between the rock mass and shield, lining and backfilling need to be understood. This paper will explain the background theories and the application of numerical analysis for 3D modeling of mechanized tunneling by using a double shield TBM in squeezing ground. The discussions will include the effects of advance rate during excavation cycle of a shielded TBM to observe the impact of tunneling rate on the possibility of machine jamming in the squeezing grounds. Simulation results at five reference points on the tunnel circumference along the tunnel or longitudinal displacement profile (LDP) as well as longitudinal contact force profiles (LFP) on both front and rear shields have been examined. Also, maximum thrust force required to overcome friction and drive TBM forward is calculated. This exercise allows for evaluation of the effects of various parameters on convergence and contact forces between the rock and shield, which in turn determines the required propel forces and can define the possibility of machine entrapment.

Introduction

Today, almost all rock mass conditions can be bored by modern TBMs with tunnel diameter varying from less than 3 m to more than 15 m (Zhao et al., 2012). Shields TBMs are amongst the most technically sophisticated excavation machines in use by tunneling industry. Single and double shield tunnel boring machines are being increasingly considered for use in rock tunneling projects under high ground water pressure and in deep tunnels. However, using the shielded machine limits access to the walls for observation of ground conditions and presence of shield makes the machine susceptible to entrapment or seizure in weak rocks under high stresses which results in high convergence. Therefore TBM may get stuck (including shield jamming and cutter-head blocking) in the complicated geological structures, which requires manual excavation to release the machine. This is a time consuming, costly, unsafe, slow, and labor intensive work that should be avoided as much as possible. Thus, the main question in selection of shielded TBMs for many tunneling projects remains the possibility of machine seizure in the ground (Farrokh and Rostami, 2009). Also use of TBMs in very severe ground conditions is yet under discussion due to some negative experiences which resulted in very low rates of advancement and even in standstill (Zhao et al., 2012).

For the design of mechanized tunneling in such conditions, the complex interaction between the rock mass, the tunnel machine, its system components, and the tunnel support has to be analyzed in detail and three dimensional models including all these components are better suited to correctly simulate this interplay and avoid the errors introduced by assumption of plane strain conditions (Cantieni and Anagnostou, 2009). This is even more true in the case of the double shield universal TBM (DS TBM), which is indeed a more complex machine than the gripper or the single shield TBM (Zhao et al., 2012). Also double shield machines are longer than their single shield peers and thus more likely to get trapped as the ground gradually deforms behind the tunnel face.

In this study, 3D finite difference numerical simulation program FLAC^3D has been used for evaluation of the feasibility of utilizing shield TBMs in long deep tunnels in potentially squeezing ground. This paper will describe comprehensive 3D modeling used for simulation of the single shield, double shield and universal double shield TBMs for excavation of deep tunnels through various rock masses that exhibit squeezing behavior.

The tunnel excavation by a shield TBM is a continuous process, relative to the gradual movement of the ground, unless a major delay in the operation is experienced. This includes weekends, stoppages for machine repair or maintenance, or other machine downtimes. Therefore the time factor must be considered in numerical modeling. Some of the case histories indicate that interruptions of the shield TBM drive may be unfavorable in weak and deep ground, that the “time” factor may play an important role, most often detrimental to the operation. There are several cases where shielded TBM was entrapped when there was a slowdown in operation or standstill/delay in the TBM drive. This suggests that maintaining a high daily advance rate and reducing downtimes may have a positive effect in avoiding entrapment (Ramoni and Anagnostou, 2010).

Although the time-dependent stress–strain behavior of the rock mass was not included in the step-by-step analysis conducted, the time effect during advancement of the face was implemented inclusively by relaxing the unbalance forces due to excavation. This is done gradually over steps in this simulation and the procedure will be explained in this paper. The shield skin, segmental lining and backfilling were installed after some relaxation of the loads for this hypothetical shield TBM numerical simulation. This means that after relaxation of the unbalance forces, the tunnel walls are allowed to converge and the ground to advance into tunnel envelope.

Given the focus of this paper being the study of the interaction between the rock mass, the shielded TBM skin, and the support system near the face and during excavation, the use of short-term parameters can be justified. The gradual increase of ground pressure and of ground deformations in the longitudinal direction are therefore considered to be only due to the spatial stress redistribution that is associated with the advance of the working face (Ramoni and Anagnostou, 2011).