Understanding the response of geo-materials and civil infrastructure to temperature variations is crucial in relation to the frameworks of shallow geothermal energy exploitation through energy geostructures, urban heat islands, as well as the climate change. Energy geostructures enable the use of renewable energy resources for efficient heating and cooling of buildings, by combining their conventional structural support role with the contemporary one of heat exchange  (Fig. 1). Any structure (piles, walls, tunnels) in contact with geo-materials can be equipped with geothermal loops, connected to a ground source heat pump, allowing heat exchange with the ground. With the use of energy geostructures, heat energy can be extracted from the ground during winter for space-heating and similarly, extra heat can be injected into the ground during winter for space-cooling. Undoubtedly, these heat exchange operations result in cyclic temperature variations along energy geostructures, within the surrounding geomaterials (i.e. soil and rock), as well as at their interface.
Besides energy geostructures, the global temperature increase (up to 10˚C in cities by 2080), as well as urban heat island effects caused by human activities (e.g. 5-14˚C temperature increase around London Underground) will have consequences on soils, rocks and their interface, particularly in shallow depths. Thus, considering infrastructure in mixed-face ground, the soil-rock interaction will become increasingly crucial in close future.
So far, research on energy geostructures mainly focused on in-situ tests , laboratory-scale tests  and numerical tools , aiming to understand cyclic temperature change effects on the behaviour of geomaterials, infrastructures and their interfaces. Yet, these all emphasised soils and soil-concrete interfaces, overlooking the key impact of shallow rock formations. Regarding interfaces, extensive research has been performed on their response to structural actions . Limited efforts were also devoted to temperature effects on soil-concrete interfaces , which showed that sand-concrete interface has fairly thermo-elastic behaviour whereas a clay-concrete interface shows a decrease in interface friction angle and an increase in adhesion with temperature rise.
How the aforementioned knowledge can be applied to soil-rock interfaces is still obscure due to several differences concrete and rock interfaces possess, including: (i) soils around concrete structures are usually disturbed due to construction efforts, whereas the ones around rock formations are naturally deposited over long geological periods; (ii) concrete structures usually have uniform roughness, while rock surfaces might have irregularities due to potential unconformities during its geological history; and (iii) concrete structures are usually accepted as isotropic, while rock formations can exhibit highly anisotropic behaviour due to diagenetic and/or deformational processes. Regarding these disparities, an extensive experimental investigation of soil-rock interfaces considering confining pressure, surface impurities and rock anisotropy is essential, the outcomes of which will greatly benefit geoenergy, climate change and urban heat island fields.
The aim of this project is to establish an observational framework to understand the fundamental mechanics of soils, rock formations and their interaction caused by thermo-mechanical actions through a cross-scale experimental campaign. The outcomes will help predict potential soil-rock interface deformation and failure triggered by thermal variations, potentially leading to improvement of their interactions.
The specific objectives of the project are:
O1: Investigate the role of geomaterial characteristics and environmental factors on soil-rock interfaces subjected to mechanical (M) and thermo-mechanical (TM) actions in macro-scale, using a direct shear device.
O2: Examine the geomaterial characteristics and environmental factors from O1 by evaluating the size-dependence of soil-rock interfaces subjected to M and TM actions in meso-scale, using a tribometer.
O3: Refine the outcomes of O1 and O2 by performing additional (pre- and post- M and TM actions) analysis in micro-scale to reveal the driving mechanisms behind the response of soil-rock interfaces.
O4: Evaluate the outcomes of O1, O2 and O3 to establish a complete qualitative and quantitative framework for key mechanisms leading to the response of soil-rock interfaces to TM actions.
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Fig. 1: Illustration of energy geostructures concept, providing structural and energy support to buildings