Stability controls in subsurface interfaces subjected to thermal and mechanical actions

Overview

Understanding the response of geo-materials and civil infrastructure to thermo-mechanical actions is crucial for the shallow geothermal energy exploitation. Energy geostructures enable the use of renewable energy sources for efficient heating and cooling of buildings, by combining their conventional structural support role with the contemporary one of heat exchange [1]. Therefore, any structure (energy 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 (Fig. 1). Energy geostructures research has so far focused on in-situ tests [3], laboratory-scale tests [4] and numerical tools [5], aiming to understand cyclic temperature change effects (triggered by geothermal operations) 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]. Results showed that the pile portion within the sandstone experienced tensile stresses during heat injection into the ground; the inverse of what would be expected had the pile been embedded entirely in soils. Although this observation was attributed to larger thermal expansion of sandstone (vs that of soil or concrete), a thorough understanding of this phenomenon has never been investigated to date.
The thermo-mechanical response of soil-rock interfaces can also be linked to the global temperature increase (up to 10ËšC in cities by 2080), which will affect soils, rocks and their interface particularly in shallow depths. Finally, undisturbed ground temperature is highly affected by human activities, such as the operation of underground systems, which increases the ground temperature in urban environments (5-14ËšC temperature increase around London Underground). Considering infrastructure in mixed-face ground, the soil-rock interaction will become increasingly crucial due to temperature variations.
Extensive research was performed on mechanical behaviour of soil-structure interfaces [7], limited efforts were also devoted to temperature effects on soil-concrete interfaces [8]. According to these studies, depending on the concrete surface roughness and soils’ mean grain size, three failure mechanisms can occur: shear failure within the soil for rough surface, sliding at the interface for smooth surface, simultaneous shear and sliding at roughness close to the critical one. Limited research on thermal effects 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. Yet, 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; (iii) concrete structures are usually accepted as isotropic, rock formations can exhibit highly anisotropic behaviour. 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 engineering strengthening of the geomaterials in contact.
Objectives:
The objectives are summarised below:
O1: Investigation of the soil-rock interfaces subjected to mechanical (M) and thermo-mechanical (TM) actions in macro-scale.
O2: Investigation of the soil-rock interfaces subjected to M and TM actions in meso-scale.
O3: Examination of geomaterial and environmental effects on the response of soil-rock interface in micro-scale.
O4: Assessment of key mechanisms leading the behaviour of soil-rock interfaces

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Image Captions

Fig. 1: A full-scale energy pile constructed in Houston, TX: a) length of the pile, b) head of the pile [2]

Methodology

The project methodology is structured around four key objectives:
Investigation of the soil-rock interfaces subjected to thermal and mechanical actions in macro-scale: This step compromises 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 (HW possesses the thermal bath, the modification includes circulation tubes placement to the shear box base (below the rock sample) for water circulation at various temperatures). 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 (20°C) will be employed; remaining within the practical cases related to energy geostructures and urban heat islands.
Investigation of the soil-rock interfaces subjected to M and TM actions in meso-scale: This step compromises the fundamental meso-scale investigation of soil-rock interfaces performed by temperature controlled tribometer. The current apparatus will be customised to study the rock-soil behaviour.
Examination of geomaterial and environmental effects on the response of soil-rock interface in micro-scale: This step will proceed in parallel with the two former ones. 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.
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 outcomes) is influenced by the micro-structural characteristics (O3 outcomes) and (ii) how the soil, rock type and the environmental factors (O1 and O2) affect the changes in microstructure (O3).

Project Timeline

Year 1

Literature review, training for direct shear testing. Direct shear equipment modification for thermal loading. Direct shear tests on soil-rock interfaces subjected to M and TM actions. Pre- and post-tested soil-rock interface investigation by XRT and SEM for M and TM actions. Attending HW training sessions for first year PhD and ALERT Doctoral School.

Year 2

Training for Tribometer testing. Tribomoter testing apparatus customization for geomaterial testing. Tribometer testing. Pre- and post-tested soil-rock interface investigation by XRT and SEM for M and TM actions. Attending HW training sessions for second year PhD and GETE Winter School.

Year 3

Comparison of the test results for: a) temperature variation effects; b) geomaterial characteristics (rock surface roughness, anisotropy, soil grain size); c) environmental factors (confining pressure, applied shear displacement). Upscaling the soil-rock interface phenomenon from pore-scale to larger-scales to perceive its consequences on real-scale engineering problems. Attending HW training sessions for third year PhD.

Year 3.5

Writing up the thesis. Attending SEG.

Training
& Skills

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 symposium (SEG).

References & further reading

[1] Sutman, M., Speranza, G., Ferrari, A., Larrey-Lassalle, P., Laloui, L., 2020. Long-term performance and life cycle assessment of energy piles in three different climatic conditions. Renewable Energy, 146, pp.1177-1191.[2] Sutman, M., 2016. Thermo-Mechanical Behavior of Energy Piles: Full-Scale Field Testing and Numerical Modeling, Doctoral dissertation, Virginia Tech.[3] Sutman, M., Brettmann, T., Olgun, C.G., 2019. Full-scale in-situ tests on energy piles: Head and base-restraining effects on the structural behaviour of three energy piles. Geomechanics for Energy and the Environment, 18,pp.56-68.[4] Laloui, L., Olgun, C.G., Sutman, M., et al., 2014. Issues involved with thermoactive geotechnical systems: Characterization of thermomechanical soil behavior and soil-structure interface behavior. DFI Journal, 8(2),pp.108-120.[5] Sutman, M., Olgun, C.G., Laloui, L., 2018. Cyclic Load-Transfer Approach for the Analysis of Energy Piles. Journal of Geotechnical and Geoenvironmental Engineering, 145(1), p.04018101.[6] RottaLoria, A.F., Laloui, L., 2016. Thermally induced group effects among energy piles. Géotechnique, 67(5), pp.374-393.[7] Dejong, J.T., White, D.J., Randolph, M.F., 2006. Microscale observation and modeling of soil-structure interface behavior using particle image velocimetry. Soils and foundations, 46(1), pp.15-28.[8] Di Donna, A., Ferrari, A., Laloui, L., 2015. Experimental investigations of the soil-concrete interface: physical mechanisms, cyclic mobilization, and behaviour at different temperatures. Canadian Geotechnical Journal, 53(4), pp.659-672.

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