Numerical modelling of thermo-active retaining walls

The exploitation of the energy stored in the ground through geotechnical structures poses new challenges to geotechnical engineers due to effects related to temperature changes that have to be considered in the design of these structures. Underground structures, such as piles, retaining walls or tunnel linings, can be equipped with heat exchanger pipes through which thermal energy is exchanged with the ground to provide low-carbon space heating and cooling. The exchange of energy imposes temperature changes to the structure and the ground, which can induce additional stresses and strains within the structures, as well as leading to thermo-hydro-mechanical (THM) interactions within the ground. The research presented in this thesis focuses on the analysis of these phenomena in relation to the utilisation of retaining walls as heat exchangers, also termed thermo-active retaining walls. The aim of this research is to assess the impact of temperature variations on the behaviour of thermo-active retaining walls and the surrounding soil and to provide efficient modelling approaches for their design. In recent years, the Imperial College Finite Element Program (ICEFP) has been upgraded to include a fully coupled THM formulation for saturated soils as well as special types of elements for the simulation of the heat exchanger pipes, allowing the simulation of complex boundary value problems including thermo-active structures. Firstly, the phenomena taking place within the soil when temperature changes are applied are analysed in detail to provide the basis for the assessment and interpretation of the performance of thermo-active retaining walls. Subsequently, modelling approaches for the accurate simulation of the pipe-structure-soil interaction within three-dimensional analyses of thermo-active retaining walls are established and validated against field data. The findings are employed to develop simple and computationally efficient modelling approaches to simulate thermo-active walls in two-dimensional analyses, focussing both on their energy efficiency and structural behaviour.Open Acces

Simplified methods for designing thermo-active retaining walls

Thermo-active retaining structures are geotechnical structures employed to provide thermal energy to buildings for space heating and cooling through heat exchanger pipes embedded within the concrete structure. Consequently, the design of these structures needs to consider both the long-term energy efficiency as well as the thermo-mechanical response in terms of stability and serviceability. Transient finite element analyses can be carried out to evaluate the behaviour of thermo-active walls, where the heat exchanger pipes are explicitly modelled, thus requiring three-dimensional (3D) analyses. However, performing long-term 3D finite element analyses is computationally expensive. For this reason, in this study, new approaches are presented that allow the thermal or thermo-mechanical design of thermo-active walls to be carried out by performing two-dimensional (2D) plane strain analyses. Two methods, which are based on different design criteria, are proposed and their performance in replicating the three-dimensional behaviour is assessed. Furthermore, the factors affecting the 2D approximations for the two modelling approaches are evaluated, where particular emphasis is given to the influence of the simulated boundary condition along the exposed face of the retaining wall

Assessing the impact of vertical heat exchangers on the response of a retaining wall

Shallow geothermal energy systems, e.g. borehole heat exchangers or thermo-active structures, provide sustainable space heating and cooling by exchanging heat with the ground. When installed within densely built urban environments, the thermo-hydro-mechanical (THM) interactions occurring due to changes in ground temperature, such as soil deformation and development of excess pore water pressures, may affect the mechanical behaviour of adjacent underground structures. This paper investigates the effects of vertical heat exchangers installed near a deep basement by performing fully coupled THM finite element analyses using the Imperial College Finite Element Program. Different heat exchanger configurations are considered and their influence on the response of the basement wall is assessed in two-dimensional plane strain analyses, where different methods of modelling the heat sources in this type of analysis are employed to evaluate their effect on the temperature field and the non-isothermal soil response

Evaluating the impact of different pipe arrangements on the thermal performance of thermo-active piles

Thermo-active piles are widely utilised for low carbon heating and cooling, and their uses are further encouraged in cities where there are obligations for developments larger than a certain threshold to generate a portion of their estimated energy use on site in a renewable manner. It is therefore important to model accurately the thermal performance of the designed thermo-active piles to ensure that such obligations are complied with. In this paper, the thermal performance of a thermo-active pile is quantified by the evolution with time of the power that can be harnessed from the pile, obtained from 3D thermo-hydro-mechanically coupled finite element analyses which include the simulation of a hot fluid flowing through heat exchanger pipes. Different pipe arrangements are considered in this study, in order to demonstrate the potential gains in efficiency arising from the installation of multiple U-loops within the pile. Furthermore, detailed analysis of the heat fluxes resulting from pipe-pile-soil interaction is carried out, illustrating the contribution of the different components of the system (concrete, near-field and far-field) to the overall storage of thermal energy

Snowmass 2021 CMB-S4 White Paper

This Snowmass 2021 White Paper describes the Cosmic Microwave Background Stage 4 project CMB-S4, which is designed to cross critical thresholds in our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. We provide an overview of the science case, the technical design, and project plan

Snowmass 2021 CMB-S4 White Paper

This Snowmass 2021 White Paper describes the Cosmic Microwave Background Stage 4 project CMB-S4, which is designed to cross critical thresholds in our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. We provide an overview of the science case, the technical design, and project plan

Snowmass 2021 CMB-S4 White Paper

This Snowmass 2021 White Paper describes the Cosmic Microwave Background Stage 4 project CMB-S4, which is designed to cross critical thresholds in our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. We provide an overview of the science case, the technical design, and project plan