Mechanical Behaviour and Evolution of Rock Loosening Zone in in-situ Reconstruction and Expansion Construction of Highway Tunnels

Abstract This study systematically investigated the construction mechanical behaviour and surrounding rock loose zone evolution patterns during the in-situ reconstruction and expansion of highway tunnels, utilizing the GDEM discrete element program integrated with field monitoring data to analyse the excavation process of the reference Tingxin Tunnel project. The key conclusions are as follows: During the in-situ expansion of the bidirectional four-lane separated tunnel, the vertical stresses at various monitoring points primarily exhibit tensile stresses with a gradual decreasing trend; the thickness of the surrounding rock loose zone increased by 76.62% before and after the expansion construction, with the entire inter-tunnel rock wall entering the loose zone post-expansion, and the maximum extension occurring at the shoulder positions. Therefore, implementing pre-support or reinforcement measures for the surrounding rock prior to in-situ excavation in close-spaced tunnels can significantly enhance the tensile strength of the rock mass above the tunnel crown and improve the load-bearing capacity of the inter-tunnel rock wall. This holds substantial engineering significance for improving construction safety, mitigating collapse risks and associated economic losses, and accelerating project progress.