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Thermo-active structures are underground facilities that enable the exchange of thermal energy between the ground and the overlying buildings, thus providing renewable means of space heating and cooling. Although this technology is becoming increasingly popular, the behaviour of geotechnical structures under additional thermal loading is still not fully understood. This paper focuses on the use of underground tunnels as thermo-active structures and explains their behaviour through a series of finite-element analyses based on an existing case study of isothermal tunnels in London Clay. The bespoke finite-element code adopted – the Imperial College Finite Element Program (ICFEP) – is capable of simulating the fully coupled thermo-hydro-mechanical behaviour of porous materials. The complex coupled interactions between the tunnel and the surrounding soil are explored by comparing results from selected types of coupled and uncoupled simulations. It is demonstrated that: (a) the thermally induced deformation of the tunnel and the ground are more critical design aspects than the thermally induced forces in the tunnel lining, and (b) the modelling approach in terms of the type of analysis, as well as the assumed permeability of the tunnel lining, has a significant effect on the computed tunnel response and, hence, must be chosen carefully.

Thermo-active foundations utilise heat energy stored in the ground to provide a reliable and effective means of space heating and cooling. Previous studies have shown that the effects of temperature changes on their response are highly dependent on their interaction with the surrounding ground. Consequently, it is necessary to consider this interaction and include both the thermal and mechanical behaviour of the ground in design. This paper addresses this issue by performing state-of-the-art finite-element analyses using the Imperial College Finite Element Program, which is capable of simulating the fully coupled thermo-hydro-mechanical behaviour of porous materials. First, the Lambeth College pile test is analysed to demonstrate the capability of the adopted modelling approach to capture the observed response under thermo-mechanical loading. Subsequently, a detailed study is carried out, demonstrating the impact of capturing the fully coupled thermo-hydro-mechanical response of the ground, the use of appropriate boundary conditions and the uncertainty surrounding thermal ground properties. It is demonstrated that the modelling approach has a large impact on the computed results, and therefore potentially on the design of thermo-active piles. Conversely, the effects of thermal conductivity and permeability of the soil are shown not to influence the pile behaviour significantly.