Stability controls in subsurface interfaces subjected to thermal and mechanical actions

Fully funded PhD position available to UK and international students! Apply by 12 noon, 5 January 2024 (international applicants must have contacted supervisor by 11 December 2023).

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, 2]. 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 10˚C 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 (5-14˚C 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.

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 and Workshop.

Year 2

Training for Tribometer testing.
Tribomoter testing apparatus customisation 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.

Year 3

Comparison of the test results for: a) temperature variation effects; b) geomaterial characteristics (rock surface roughness, anisotropy, soil grain size and shape); c) environmental factors (confining pressure, applied shear displacement).
Upscaling the soil-rock interface phenomenon from grain-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 the 3rd International Conference on Energy Geotechnics.

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 visualisation. 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 one doctoral school and one workshop (ALERT) and an international conference (3rd International Conference on Energy Geotechnics).

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] Laloui, L. and Sutman, M., 2023. Energy geotechnology: A new era for geotechnical engineering practice. In Smart Geotechnics for Smart Societies (pp. 45-61). CRC Press.[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] Hashemi, A., Sutman, M. and Medero, G.M., 2023. A review on the thermo-hydro-mechanical response of soil–structure interface for energy geostructures applications. Geomechanics for Energy and the Environment, p.100439.[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.

Financial support

IAPETUS2’s postgraduate scholarships are tenable for up to 3.5 years and provide the following package of financial support:

• A tax-free maintenance grant set at the UK Research Council’s national rate, which in 2023/24 is £18,622;
• Payment of tuition fees at the Home rate*;
• Access to extensive research support funding; &
• Support for an external placement of up to six months.

Part-time award-holders are funded for seven years and receive a maintenance grant at 50% of the full-time rate.

*Eligibility is under UKRI Terms and Conditions, which means that UK and International candidates may apply.  For International Students, UKRI only pay the equivalent of home fees. It is expected that the differential between home and international fees will likely be self-funded (approximately £18000 per year for 3.5 years based on the fee rate in 2023/24).

Eligibility

Eligibility is under UKRI Terms and Conditions, which means that UK and International candidates may apply.  For International Students, UKRI only pay the equivalent of home fees. The differential between home and international fees will need to be self-funded. International applicants need to contact the primary supervisor of the project (Dr Melis Sutman m.sutman@hw.ac.uk) by no later than Monday 11th December 2023 in order to be considered for shortlisting.

How to Apply
All prospective students need to complete the online IAPETUS2 form (link here). Before completing this form, please read the DTP privacy policy as you will need to tick that you have read and understood this.

Both parts of the application must be made by Friday 5th January 2023 at 12pm (GMT), which is the public deadline for applications that will apply across all of the Partnership. 

If you are shortlisted you will be contacted by IAPETUS2 by 19th January 2024 and invited to submit a full application to Heriot Watt University by 9th February 2024.

Equality, Diversity and Inclusion

In order to address historical imbalances in the higher education sector, IAPETUS2 is committed to recruiting a diverse, representative community of researchers in Environmental Science. The DTP has developed an Equality, Diversity and Inclusion policy to further this. This includes the Widening Participation Scheme, which identifies Home applicants from underrepresented groups. The DTP aims to give up to 30% of interview places to those eligible for this scheme. Also we are pleased to introduce the IAPETUS2 Diversifying Talent Scholarship Scheme, a separate competition designed for those from underrepresented groups. For more, please see the IAPETUS2 website.

Further information

Please contact Dr Melis Sutman (m.sutman@hw.ac.uk) if you require further information about the project.