Numerical investigation of the thermo-hydro-mechanical performance of energy tunnels in stiff ground conditions

Energy tunnels represent a promising innovation in the integration of tunnel structures with shallow geothermal energy applications. However, the thermo-hydro-mechanical behaviour of energy tunnels in stiff ground conditions, particularly in rock formations, remains largely understudied. This research addresses this gap by employing a thoroughly validated finite element modelling approach, incorporating both tunnel excavation and geothermal operation stages. The study explores the influence of key rock parameters and site conditions such as rock stiffness, permeability, in-situ stress ratio, and Biot’s coefficient on thermally induced tunnel structural and geotechnical responses. Results indicate that tensile stresses exceeding 4.0 MPa are observed in the tunnel lining during ground cooling (i.e., geothermal heating extraction) when hard, low-permeability rock is encountered. This poses a risk of concrete cracking and potentially compromises tunnel durability. Additionally, the in-situ stress ratio is found to have an insignificant effect during thermal operations, whereas neglecting Biot’s coefficient can lead to over- or underestimations of the thermally induced ground deformation and tunnel stress, due to its effect on hydro-mechanical coupling. The discussion on pore water pressure and effective stress provides insights into the thermo-hydro-mechanical behaviour of energy tunnels under different geological and hydrogeological conditions.