IAP2-22-480

From energy geostructures to urban heat islands: how subsurface interfaces will respond to temperature variations?

Understanding the response of geo-materials and civil infrastructure to temperature variations is crucial regarding several motives such as shallow geothermal energy exploitation through energy geostructures, urban heat islands, as well as the undeniable consequences of 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 [1] (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 [3], laboratory-scale tests [4] and numerical tools [5], aiming to understand cyclic temperature change effects on the behaviour of geomaterials, infrastructures and their interfaces. Yet, emphasis was on soils and soil-concrete interfaces, overlooking the impact of shallow rock formations. The latter has recently attracted concerns following an in-situ test on energy piles whose bottom portions were socketed in sandstone [6]. Regarding interfaces, extensive research was performed on their response to structural actions [7]. Limited efforts were also devoted to temperature effects on soil-concrete interfaces [8], which showed that sand-concrete interface has fairly thermo-elastic behaviour whereas clay-concrete interface shows decrease in interface friction angle and 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: (i) soils around concrete structures are usually disturbed due to construction efforts, the ones around rock formations are naturally deposited over long geological periods; (ii) concrete structures usually have uniform roughness, rock surfaces might have irregularities due to potential unconformities occurred during the geological history; (iii) concrete structures are usually accepted as isotropic, 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 benefit geoenergy, climate change and urban heat island fields.

Aims
The objective of this project is to forge an observational framework in understanding the fundamental mechanics of soils, rock formations and their interaction in consequence of 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 the geomaterials in contact.

Objectives
The objectives of the project are summarised below:
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 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 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.

The project methodology is structured around four key objectives:

(1)Investigation of the soil-rock interfaces subjected to M and TM actions in macro-scale: the initial investigation of soil-rock interfaces by large-scale direct shear testing at Geomechanics and Materials Laboratory at Heriot Watt University (HW). Current apparatus will be modified for thermal loading. The tests will allow fundamental understanding of the effects of geomaterial characteristics and environmental factors on the soil-rock interface shear strength. Temperature variations of +20°C to -10°C from room temperature will be employed; remaining within the practical cases related to energy geostructures and urban heat islands.

(2) Investigation of the soil-rock interfaces subjected to M and TM actions in meso-scale:
the fundamental meso-scale investigation of soil-rock interfaces performed by temperature controlled tribometer, by employing similar geomaterial and environmental factors as macro-scale analysis. The current apparatus will be customised to study the rock-soil behaviour.

(3) Examination of the response of soil-rock interface in micro-scale: The structure of the tested materials will be investigated using X-ray tomography (XRT) and scanning electron microscopy (SEM) methods (both available at Institute of GeoEnergy Engineering at HW). Once the direct shear and tribometer tests are terminated, the interfaces will be assessed again by XRT and SEM. Thus, this step will proceed in parallel to the former two steps.

(4) Assessment of key mechanisms leading the behaviour of soil-rock interfaces: The outcomes from macro-, meso- and micro-scale tests will be converged in this step to examine in a holistic approach: (i) how the overall soil-rock interface response (O1 and O2) is influenced by the micro-structural characteristics (O3) and (ii) how the soil, rock type and the environmental factors (O1 and O2) affect the changes in microstructure (O3).

Year 1

Literature review and training:
• Literature review on soil-structure interfaces
• Training for direct shear testing
• Attending HW training sessions for first year PhD and ALERT Doctoral School (with remote training/attendance possibility)

Direct shear tests:
• Modification of conventional equipment for thermal loading (remote working will be made possible, if required, by close collaboration of HW technical staff)
• Direct shear tests on soil-rock interfaces subjected to M and TM actions
• XRT and SEM (performed by instrument scientist) investigation of post-tested interfaces

Year 2

Training:
• Training for Tribometer testing
• Attending HW training sessions for second year PhD and GETE Winter School

Tribometer tests:
• Tribometer testing apparatus customisation for geomaterial testing
• Tribometer testing
• XRT and SEM (performed by instrument scientist) investigation of post-tested interfaces

Year 3

Training:
• Attending HW training sessions for third year PhD

Incorporation of the experimental outcomes:
• Comparison and synthesis of the test results for: (i) temperature variation effects; (ii) geomaterial characteristics (rock surface roughness, anisotropy, soil grain size); (iii) environmental factors (-applied stress conditions and shear displacement)
• Upscaling the understanding of the soil-rock interface processes from pore-scale to larger-scales to perceive its consequences on real-scale engineering problems.

• Writing up the thesis

• Dissemination of results, write-up publications

Year 3.5

• Writing up the thesis
• Attending International Symposium on Energy Geotechnics

• Dissemination of results, write-up publications

Research Futures Academy at HW provides skills/career development workshops to facilitate doctorate and future research career of PhD students. The student will attend these workshops shown in chronological order.

First-year workshops focus on developing basic skills for successful research: Essential skills for researchers, Literature searching, Citing and referencing, Managing research data.

Second year aims at developing communication and dissemination skills: Advanced presentation master class, Conference talks, Data visualization.

Third year workshops target skills for research publishing: Strategy for publishing, Preparing a document for publication, Citation and impact.

Finally, the last group of workshops focus on the development of doctoral thesis: Preparing for Viva, Performing in Viva. In addition, the School of Energy, Geoscience, Infrastructure and Society provides group seminars for visiting and internal speakers.

Bespoke technical training will also be provided by the research supervisors and technical staff in both universities regarding the use of direct shear apparatus, tribometer, XRT and image analysis. SEM tests will be performed by trained full-time research fellow, but the student will be trained for the interpretation of the results. Finally, the student will be encouraged to attend two doctoral schools (GETE and ALERT) and an international conference (ICEGT).