Numerical Analysis of Shallow Tunnels Under Static Loading: A Finite Element Approach

In the present world, the need for underground structures has grown many folds due to the increasing population, the advancement of public infrastructure, and the scarcity of land. Underground structures also provide attractive alternatives for storage of explosives and other military hardware. Being at shallow depth, their potential impact on the environment and surrounding structures can be significant. Therefore, it is crucial to understand the surrounding material behaviour for the safe and economical design of underground facilities. In the present work, an attempt is made to simulate the in situ condition using finite element, which has been validated by experimental results, to understand the tunnel deformation behaviour under static loading condition in soft rocks. 3D non-linear finite element analysis has been carried out by using Abaqus. Rockmass-tunnel model considered in this study has dimensions of 30 cm x 30 cm and 35 cm. The diameter of the tunnel has been varied from 2.5 cm, 3.5 cm to 5.0 cm. Similarly, the overburden depth is taken as 2.5 cm, 3.5 cm and 5.0 cm. Both the lined and unlined cases of the tunnel have been considered. Geo-material has been prepared in the laboratory having four different compositions of POP, sand, clay and mica. The weathering effect of the rockmass is also considered in the study. Fresh, slightly weathered, medium weathered, and highly weathered are the four different weathering stages of basalt rock taken into consideration. The elasto-plastic behaviour is considered for the natural and synthetic rock, and a Mohr-Coulomb constitutive model is incorporated. Stress and deformation behaviour is monitored for different rocks and geo-materials. Longitudinal and transverse profiles of the tunnel have been plotted to understand the response of tunnel lining and the surrounding rockmass, along and across the point of loading. The paper concludes that the diameter of the tunnel, overburden depth of the tunnel, and weathering of rock has a significant effect on the stability of tunnels under severe loading conditions.