Thermo-mechanical behaviour of thermo-active diaphragm walls

Thermo-active geo-structures are becoming more common as a renewable energy solution that both reduces carbon emissions and reduces the whole-life cost of building and civil infrastructure projects. These structures have an added benefit, which is that the additional cost to install is relatively small since the geo-structure is already required for structural stability. Thermo-active geo-structures can significantly reduce CO2 emissions from building and civil infrastructure and can have a significant impact on the Government’s attempts to de-carbonise commercial and residential heating and cooling. The first thermo-active diaphragm wall in the UK was constructed in 2010, and was designed to provide 150 kW of heating and cooling to satisfy the building’s heating and cooling requirements, however, they are still under utilised. Most of the research carried out to date on thermo-active structures has focused on the use of thermo-active piles, which are primarily axially loaded foundation elements and the most commonly used deep foundation. This topic has been more extensively studied, with field tests, numerical models and centrifuge models used to determine their behaviour on their own and in groups over the short and long term and so their use is more widespread as their behaviour is more understood. In 2012, the Ground Source Heat Pump Association (GSHPA) published design guidance. However, diaphragm walls are becoming more common as the main foundation element carrying both vertical loads and horizontal loads. These structures are being adapted to for ground heat exchangers in a similar way to foundations piles. The diaphragm wall behaviour differs from pile behaviour since the main purpose is to retain lateral loads, as opposed to axial loads which is the primary function of a foundation pile. Also, while the pile is fully surrounded by soil, the diaphragm wall is only in contact with the soil at one side, with the other side open to the excavated soil above excavated ground level. Therefore, the understanding of pile behaviour cannot be applied to thermo-active diaphragm walls where the loading conditions and geometry are different. This research is the first of its kind where a thermo-active diaphragm wall is modelled in a geotechnical centrifuge, which is used to validate a Finite Element Model that utilises commercially available software. This study uses a newly-developed model concrete which is capable of capturing both the thermal and the thermo-mechanical properties of thermo-active diaphragm walls in a geotechnical centrifuge. The validated model is then used to conduct a detailed study into the behaviour of thermo-active diaphragm walls in sand long-term, including theoretical best and worst case climate change scenarios and under combined axial and lateral loading. The results indicate that increasing relative density of the sand increases thermally-induced wall deformation and internal stresses compared with sands of lower relative density. The results from long-term modelling show that the behaviour is sensitive to increases in temperature between heating and cooling phases. With increasing ambient atmospheric temperatures expected due to climate change, as well as more frequent spells of extreme heat, action on climate change and efficient GSHP design to limit the changes in temperatures between heating and cooling phases are crucial to stabilising internal stresses and wall deformations. These effects were also more prominent in cases where the wall is laterally-loaded only as opposed to combined axial and lateral loading which appears to limit the thermally-induced deformations and internal stresses.