Please cite this article as: N. Makasis and G.A. Narsilio, Investigating the thermal performance of energy soldier pile walls, Geomechanics for Energy and the Environment (2021) 100242, https://doi.org/10.1016/j.gete.2021.100242. Geomechanics for Energy and the Environment xxx (xxxx) xxx Contents lists available at ScienceDirect Geomechanics for Energy and the Environment journal homepage: www.elsevier.com/locate/gete Investigating the thermal performance of energy soldier pile walls Nikolas Makasis a,b , Guillermo A. Narsilio a,∗ a Department of Infrastructure Engineering, The University of Melbourne, Parkville, Australia b Department of Engineering, University of Cambridge, Trumpington Street, CB2 1PZ, UK article info Article history: Received 4 April 2020 Received in revised form 10 November 2020 Accepted 18 February 2021 Available online xxxx Editors-in-Chief: Professor Lyesse Laloui and Professor Tomasz Hueckel Keywords: Energy walls Energy geo-structures Soldier pile walls Numerical modelling Shallow geothermal energy Ground-source heat pump systems abstract Energy geo-structures are a promising application of shallow geothermal energy technologies utilising underground structures primarily build for stability to also convert them to ground heat exchangers and make thermal energy provision their secondary function. One type of energy geo-structures that has received little attention is energy soldier pile walls. This work adopts advanced numerical modelling approaches to investigate the thermal performance of these energy walls and important parameters affecting this performance including the soldier pile depth, spacing, pipe length and thermal load. The results indicate that both the soldier pile depth and spacing can impact the thermal performance with higher values being desirable. Non-linear/logarithmic performance trendlines have been identified. The scenario of activating less piles overall to increase their (thermal) spacing is also investigated, showing a decrease in the thermal performance but noting that in certain cases this decrease could be acceptable compared to the capital cost savings of activating less piles. The pipe configuration is found to result in relatively insignificant returns after utilising more than about 3 U-loops connected in series, suggesting the potential suitability of an easy to adopt rule-of-thumb for these structures. © 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Energy geo-structures and soldier pile walls Energy geo-structures are a relatively recent application of shallow geothermal energy, which can provide clean and re- newable thermal energy for heating and cooling purposes and thus contribute to the world’s ever-increasing needs of ensur- ing energy supply and reducing greenhouse gas (GHG) emis- sions. Unlike traditional shallow geothermal or ground source heat pump (GSHP) applications (such as purpose-built vertical boreholes or horizontal trenches 1–3 ) energy geo-structures utilise underground structures primarily designed for stability (mainly foundations) to incorporate energy provision as an additional/ secondary function. 4–8 The implementation of the technology in foundation structures that are already planned to be constructed can significantly reduce capital costs and open new opportunities to further utilise this technology and increase the renewable portion of the world’s energy supply. Structures that have been utilised as energy geo-structures so far consist of piles, retaining walls and tunnels, with piles having attracted most of the attention by the scientific com- munity, likely due to their geometrical similarities to the well- studied traditional vertical boreholes. There have been a number ∗ Correspondence to: Department of Infrastructure Engineering, The Univer- sity of Melbourne, Engineering Block B 208, Parkville, VIC 3010, Australia. E-mail address: narsilio@unimelb.edu.au (G.A. Narsilio). of studies on energy piles with overall positive and encourag- ing results for the industry of shallow geothermal energy. 9–14 Similarly positive results have been shown by some studies on energy tunnels 15–18 and energy retaining walls, mainly diaphragm walls. 19–23 Less attention, however, has been given to other possi- ble geo-structures, particularly soldier pile retaining walls, which this work investigates. Energy retaining wall design can be more challenging to de- sign compared to the more thoroughly investigated energy piles. A main difference between them is the fact that a retaining wall is only surrounded by ground on one of its sides, where the other one (or part of it) can form the underground space of a building. This not only results in less thermal storage being available but, perhaps more importantly, means that heat transfer must be controlled to avoid impacting on the thermal comfort of the underground building (i.e., the inner built environment), for example, by the use of thermal insulation. 24 While a recent study focuses on the impact of the wall boundary conditions on effective heat transfer yields and the potential repercussions of ill-defined boundary conditions, 24 the impact on the thermal performance of the key design physical parameters, such as the pipe length, the soldier pile spacing and the pile depth has not been studied for this type of retaining walls. Energy retaining wall design needs to carefully consider all these aspects to ensure that the system can provide as much thermal energy as possible, without impacting on the otherwise construction and operation of the building. https://doi.org/10.1016/j.gete.2021.100242 2352-3808/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/).