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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.

In a FE analysis of transient heat transfer, a lower limit of the time-step size exists below which numerical oscillations of temperatures may occur. Although time-step constraints for simulating 1D heat diffusion have been well established in the literature, the conclusions cannot be directly applied to 2D cases. In this paper, both analytical and computational studies are carried out to obtain the time-step constraints for 2D linear and quadratic elements. It is noted that in the simulation of 2D heat diffusion employing quadratic elements is not always beneficial. Recommendations are provided on selecting the numerical scheme to minimise numerical oscillations.

A new Petrov-Galerkin finite element method for two-dimensional (2D) highly advective flows in porous media, which removes numerical oscillations and retains its precision compared to the conventional Galerkin finite element method, is presented. A new continuous weighting function for quadratic elements is proposed. Moreover, a numerical scheme is developed to ensure the weighting factors are accurately determined for 2D non-uniform flows and 2D distorted elements. Finally, a series of numerical examples are performed to demonstrate the capability of the approach. Comparison against existing methods in the simulation of a benchmark problem further verifies the robustness of the proposed method.

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.

A temperature rise in soils is usually accompanied by an increase in excess pore fluid pressure due to the differential thermal expansion coefficients of the pore fluid and the soil particles. To model the transient behaviour of this thermally induced excess pore fluid pressure in geotechnical problems, a coupled THM formulation was employed in this study, which accounts for the non-linear temperature-dependent behaviour of both the soil permeability and the thermal expansion coefficient of the pore fluid. Numerical analyses of validation exercises (where there is an analytical solution), as well as of existing triaxial and centrifuge heating tests on Kaolin clay, were carried out in the current paper. The obtained numerical results exhibited good agreement with the analytical solution and experimental measurements respectively, demonstrating good capabilities of the applied numerical facilities and providing insight into the mechanism behind the observed evolution of the thermally induced pore fluid pressure. The numerical results further highlighted the importance of accounting for the temperature-dependent nature of the soil permeability and the thermal expansion coefficient of the pore fluid, commonly ignored in geotechnical numerical analysis.

Interface elements are frequently employed in finite element (FE) analyses to represent soil-structure interfaces or rock joints. The modelling of coupled thermo-hydro-mechanical (THM) problems in geotechnical engineering requires equally a coupled and robust THM formulation for interface elements. This paper presents such a formulation which is capable of reproducing the coupled THM behaviour of discontinuities and soil-structure boundaries, and is compatible with other types of finite elements used to discretise the soil and structural domains (e.g. solid and shell elements). The coupled THM three-dimensional (3D) zero-thickness interface element is implemented into the bespoke FE code employed in this research and its features are verified using a number of numerical exercises. To demonstrate their performance, the proposed interface elements are employed in the simulation of the coupled THM behaviour of a fissured triaxial sample subjected to a thermal load and the influence of the presence of fissures on soil behaviour is presented.

© 2016 Elsevier Ltd.Ground source energy systems, such as open-loop systems, have been widely employed in recent years due to their economic and environmental benefits compared to conventional heating and cooling systems. Numerical modelling of such geothermal system requires solving a coupled thermo-hydraulic problem characterised by a convection-dominated heat transfer which can be challenging for the Galerkin finite element method (GFEM). This paper first presents the coupled thermo-hydraulic governing formulation as well as the coupled thermo-hydraulic boundary condition, which can be implemented into a finite element software. Subsequently, the stability condition of the adopted time marching scheme for coupled thermo-hydraulic analysis is established analytically. The behaviour of highly convective problems is then investigated via a series of analyses where convective heat transfer along a soil bar is simulated, with recommendations on the choice of an adequate discretisation with different boundary conditions being provided to avoid oscillatory solutions. Finally, the conclusions from the analytical and numerical studies are applied to the simulation of a boundary value problem involving an open-loop system, with the results showing good agreement with an approximate solution. The main objective of this paper is to demonstrate that the GFEM is capable of dealing with highly convective geotechnical problems.