Parametric study of the impacts of various geological and machine parameters on thrust force requirements for operating a single shield TBM in squeezing ground

Excavation of long tunnels by shielded TBMs is a safe, fast, and efficient method of tunneling that mitigates many risks related to ground conditions. However, TBM applications in difficult conditions such as squeezing ground, which often happens in a combination of weaker rock and/or high depth, may lead to problems such as shield jamming. Insufficient thrust force, which cannot overcome the shield skin friction between the machine and the ground, causes shield entrapment. For successful tunneling in such conditions, it is essential to study the pertinent ground and operational factors and their impact on ground behavior, to assess the possibility of shield entrapment and to develop possible safeguards to mitigate the related risks. In this study, the controlling parameters influencing the risks of tunneling by single shield TBMs in squeezing ground are examined. For this purpose, a parametric study was performed by using a full 3D numerical simulation to quantify the influence of various factors on machine entrapment. Sensitivity analysis of most important parameters and their impacts on entrapment risks has been conducted and the results are discussed in this paper. Furthermore, nonlinear regression analysis was performed to develop a relationship between ground and machine parameters, and the required thrust force needed to overcome contact/frictional forces for propelling machine forward. The result of the modeling was compiled in a database to observe the trends and develop relationship between different parameters and risk of machine entrapment. This allows for assessment of the applicability of shield TBM in weak formations along the tunnel. Furthermore, the results of the analysis can be used to evaluate the operational parameters such as the installation of a higher thrust force to overcome contact friction loads and to make a choice between increasing the overcut or using lubricants between shield and ground. The results of this study show the role of ground conditions and in-situ stresses, as well as machine operational parameters on convergence and shield pressures.