In recent years, geotechnical engineers have been encountering an increasing number of problems involving the effects of temperature. The research presented in this thesis focuses on the utilisation of geothermal energy, i.e. the thermal energy stored in the ground, through ground source energy systems (GSES). This renewable energy technology has been recognised as a reliable means of providing low-carbon space heating and cooling, and is becoming increasingly popular.
Particularly attractive are systems where the heat exchanger pipes, which facilitate the transfer of energy between the building and the ground, are incorporated into geotechnical structures, such as piles, retaining walls or tunnel linings. The aim of the research presented in this thesis is to explore the effects of temperature variations on the behaviour of such geothermal piles, and provide recommendations on their modelling and design.
However, while the Imperial College Finite Element Program (ICFEP) is capable of simulating the coupled thermo-hydro-mechanical (THM) behaviour of porous materials, further developments, which allow for accurate modelling of GSES, are necessary. Firstly, a new type of finite elements – one-dimensional elements for three-dimensional analysis – is implemented into ICFEP with full THM coupling. Hence, these elements can be used for a variety of applications, including modelling structural components or heat exchanger pipes. Secondly, the problem of simulating advection-dominated heat flow using the finite element method (FEM) is addressed by replacing the Galerkin FEM with the Petrov-Galerkin FEM for the solution of the heat transfer equations. Finally, a new constitutive model capable of simulating the non-isothermal behaviour of soils observed in laboratory experiments is developed.
These developments are subsequently applied in a comprehensive computational study which explains the thermo-mechanical behaviour of geothermal piles. The study quantifies the transient behaviour and investigates the influence of various modelling approaches, application of the thermal load, soil properties, as well as thermo-plasticity.