Integration of small strain stiffness in the Thermo-Hydro-Mechanical analysis of energy tunnels using critical state soil mechanics

Energy tunnels represent a promising approach for sustainable urban energy systems, yet their widespread adoption is hindered by uncertainties associated with complex thermo-hydro-mechanical (THM) soil interactions. This paper presents an advanced numerical modelling approach specifically developed to simulate the coupled THM response of energy tunnels, with particular emphasis on realistically capturing soil behaviour at small strains in the framework of critical state soil mechanics. The proposed approach integrates strain-dependent stiffness degradation into conventional elastoplastic constitutive models, enabling accurate predictions of nonlinear and irreversible soil responses characteristic of tunnelling operations. Model validation is performed in three stages: firstly, the hydro-mechanical (HM) component incorporating small-strain stiffness is verified against three well-documented tunnel excavation case studies across diverse geological conditions; secondly, the thermo-mechanical (TM) aspect is benchmarked by reproducing laboratory results of cooling-induced volumetric strains in Boom clay; and thirdly, the full THM formulation is validated against field data from a full-scale experimental energy tunnel prototype. Comparisons with field observations confirm the model’s ability to reproduce both excavation-induced settlements and thermally induced lining stresses. Subsequently, the validated framework is applied to simulate an idealised energy tunnel scenario subjected to cyclic cooling and recharge, demonstrating key phenomena including cooling-induced consolidation, cyclic excess pore pressure generation and the progressive stabilisation of ground temperatures. Results highlight the significance of incorporating small-strain stiffness effects, revealing substantial improvements in predicting ground settlements compared to traditional constitutive approaches. The proposed framework yielded, on average, a 67% smaller cooling-induced ground settlement compared to the conventional Modified Cam–Clay model. This reduction highlights the significant influence of incorporating small-strain stiffness in reproducing the deformation response of energy tunnels.