<p>Tunnelling in rockmass with elevated in-situ stresses, such as those encountered in regions of tectonic plate collision poses significant challenges such as support instability, large deformation of surrounding rock and possible collapse of the structure. This study deals with the interpretation of extensive deformation observed in primary lining of a deep tunnel in the state of Sikkim, India. It was hypothesised that high lateral stress prevalent in the rockmass was the reason for the deformation. A careful investigation of the deformation pattern from site monitoring data prompted a back analysis of observed phenomenon using numerical modelling. The realistic rockmass behaviour of the complex geology around tunnel opening is interpreted and adopted for numerical modelling. Multiple scenarios with lateral stress coefficient as a back stepping parameter is simulated in the study. The results obtained from back analysis were in good agreement with the hypothesis and contributed to the evidence that, inability to anticipate high lateral stresses in the rockmass was the major cause of deformation in primary lining following the excavation of tunnel. Further a comparative study on various tunnel shapes better equipped to counter such stresses are covered in this paper. The results demonstrate superior performance of elliptical shaped tunnel as compared to circular and modified horseshoe shaped tunnels while tunnelling through rockmass subjected to high lateral stresses. The approach used in this study will provide a useful reference to indicate the vulnerability of tunnel excavation process and support system in high lateral stresses. It also emphasizes the need for reliable estimation of lateral stress coefficient at initial stages of an underground project.</p>