Analysis of Twin-Bored Tunnel Using Plaxis 3D for the Proposed Metro Rail at Coimbatore District

India, a fast-growing country due to increase in population, urbanization, and industrialization, has focused towards infrastructure development in the recent years. This includes metro rail construction in Tier-1 and Tier-2 cities. In Tier-2 cities, the identified cities for the construction of metro rail are Lucknow, Patna, Jaipur, Kochi, and Coimbatore. Coimbatore metro rail construction was initiated by the Government of India in the year 2017. The feasibility studies and the geotechnical investigations are being carried out. The line of metro rail starts at Ukkadam and extends to five different places covering a total length of 144 km. During the study, it was identified that near the state highway flyover at Koundampalayam, Coimbatore, a tunnel is required to be constructed as there are space constraints above ground due to congestion of existing road traffic. The general soil profile at the location consists of layers such as Clay, Clay mixed with Kankar, Silty Sand, Soft Disintegrated Rock up to 21 m, followed by Completely Weathered Rock where the pile foundation of the existing flyover is placed. Tunnel analysis was carried out using Plaxis 3D V20, a finite element modelling software. In this analysis, 10-noded element model was used to study the stress–strain and strength characteristics of soil. The soil is modelled as Mohr–Coulomb failure model; tunnel lining, pile, and pile caps are modelled based on the linear elastic failure model. The tunnel was analyzed with two different cover depths and with three different water table conditions such as groundwater table at 0.00, −15.00, and −30.00 m. The stress and strain were found to decrease with an increase in cover depth, but deformation increases. When the water table was at 0.00 m and at −15.00 m, the adjacent tunnel lost its face stability during tunnel advancement. The maximum and minimum stresses, strains, and deformations were found from the numerical analysis. The zone of influence in longitudinal and lateral directions was also determined from the analysis.