Numerical Modelling of Thermo-Active Piles

Thermo-active piles, as one form of ground source energy systems (GSES), utilise heat energy stored in the ground to provide low-carbon space heating and cooling, in addition to their primary function of providing structural stability as building foundations. This relatively new technology for generating renewable energy creates challenges for geotechnical engineers in predicting temperature effects on both the pile and the surrounding soil, usually requiring advanced numerical analysis to illuminate the complex thermo-hydro-mechanical (THM) soil-structure interaction. This paper presents the experience gained from the authors’ decade-long research in developing and applying numerical methods in the analysis of GSESs. The paper first demonstrates the ability of the numerical model to reproduce a field test of a pile under thermo-mechanical loading, investigating the effects of the manner in which the thermal load is applied and of the uncertainties in determining the soil properties such as thermal conductivity and permeability. Further modelling implications are then discussed for a realistic inclusion of the heat-exchanger pipes within a pile. The paper concludes with a proposal for a simplified practical procedure to determine thermally-induced stresses in the pile, which is validated against advanced three-dimensional (3D) modelling of piles as heat exchangers. The emphasis throughout the paper is on the appropriate modelling of coupled THM behaviour of soils and its effect on the soil-structure interaction.