Risk Assessment Study for Mega Tunnel in Fault Zone of the Lesser Himalayas Using Finite Element Analysis

Mechanized tunneling technology using a tunnel boring machine (TBM) has a great space in the modern era as it provides a safe and workable environment for the working crew and a high advance rate in favorable geologic conditions. Despite being a good solution in the favorable ground scenario, it will create a challenging situation for the weak and fragile rock masses. Himalayan mountain ranges are the most unstable region in the world, and seeking the researcher’s attention for proposing a suitable methodology for successful tunneling operations using TBM. Complex geology, tectonically intricate zones, squeezing ground, faults, and ingress of water are some of the inevitable challenges in this region. The selection of appropriate TBM machines and other pressure parameters is a significant task. Keeping all in view, this research work proposed the study approach in the assessment of geo-risk evolved in TBM tunneling. A finite element-based program using Midas GTS Nx was used for the three-dimensional analysis of the mega tunnel. Mega tunnel refers to a large dimensional artificial passageway as an underground structure beyond 10 m in diameter. Soft computing models were prepared considering actual ground conditions and fault characteristics. Maximum vertical displacement of the surface considering plastic region up to three times of tunnel diameter (3D) was observed along with the crown settlement. Specification of single shield machine was considered as per literature at initial analysis. It was found that applied machine parameters were inadequate at the actual fault zone and led to excessive displacement of the ground, which may lead to blocking and jamming of the particular machine. The application of a combination of drilling thrust and cutter head torque provides a workable solution to avoid the above-mentioned risk. This research work anticipates a viable solution in the application of machine pressure parameters and provides insight for decision-making in the practical condition of the Lesser Himalayan region.