Excavation of long deep tunnels through adverse ground conditions by shielded TBMs requires thorough rock stability analyses to evaluate the possibility of entrapment of the machine. This refers to the assessment of ground pressure and displacement in tunnel walls. For this purpose, the Ground Reaction Curve (GRC) can be estimated using empirical and numerical models. Accurate estimation of the magnitude of ground convergence is essential to assure the success of the tunneling operation by TBM, especially for shielded machines. The analysis will yield contact forces between shield and ground for calculating of pressures around shield and linings. The result of the preliminary study has indicated that the shape of GRC for shielded machines will be different from the conventional tunneling method. This paper offers a quick review of the issue and discussion on ground behaviour and redistribution of the in-situ stresses around opening within tunneling by shielded TBM in adverse ground conditions. The 3D numerical analysis was used to estimate ground behaviour and for assessment of GRC when a tunnel is excavated by using a shielded TBM. The paper will also offer some recommendation for evaluation of the possibility of shield entrapment based on the results of empirical and analytical calculation of GRC for mechanized tunneling for future projects.
The shielded Tunnel Boring Machines (TBMs) are amongst the technically most sophisticated construction machines in use by the tunnelling industry. However, there is an intrinsic risk associated with tunnel construction by using this type of machines because of limited access to the excavated walls for observation of ground conditions. This will be more challenging when machine passes through rock mass with squeezing behaviour. In some cases, TBM may get stuck in the complicated geological structures, which includes jamming of the shield and back-up, blocking of the cutter-head and even loss of a machine. This is a time consuming, costly, unsafe, slow, and labour intensive work that must be avoided as much as possible. Therefore it is important to realistically realize the ground behaviour and the redistribution of in situ stresses around tunnel when it is excavated by using a shielded TBM especially in adverse ground conditions.