Parallel Session C-1 - Mini-Symposium: Energy geotechnics for underground climate change

A city-scale subsurface thermal characterisation methodology is showcased, in which subsurface archetypes - sets of features that represent a common thermal behaviour - are identified, using the city of Cambridge as a case study. The geothermal potential for the area is estimated in terms of: (a) the amount of power a typical 100 m geothermal borehole can provide - resulting in between 2.3 and 2.6 kW of heating, (b) the percentage of the residential demand that can be fulfilled by deploying boreholes in all spaces available - resulting in an average of 80%.

British Geological Survey are focused on three topics: - Data sharing of ground investigation information and how providing a centralised repository to improve the largely unknown subsurface setting - Delivery of key underground scientific research using ground observatories in three locations around the UK, looking at geothermal and mine water heating and the impact of underground management - Engagement of key stakeholders to encourage the use and re-use of data is vital to a global sustainable and greener future

- The impact of subsurface urban heat islands on the performance of civil infrastructure is investigated with reference to the Chicago Loop district. - Thermally induced strains are found to be significant and potentially detrimental to civil infrastructure. - Subsurface urban heat islands represent a silent hazard for the operational performance of civil infrastructure in Chicago and other cities worldwide.

The impact of the presence of heated basements near a field of 88 borehole ground heat exchangers is explored, for borehole depths of 50 m and 100 m, in terms of the GHEs performance over a 50 years operating period. The basements act to increase the ground temperature and average fluid temperature (in heating mode), raising the COP, albeit by only a small amount (by 0.05 for 100 m, and 0.08 for 50 m GHEs). While heated basement presence does not significantly improve GHE performance, the results suggest that capital cost savings could be achieved by having fewer boreholes.

The subsurface of many urban areas worldwide is warming up causing a phenomenon widely known as subsurface urban heat islands (SUHIs). Currently, limited knowledge is available on the intensity and features of the sources of SUHIs. To understand the inherent characteristics of SUHIs, this study presents a unique sensing network composed of >150 wireless temperature sensors deployed in the surface and subsurface environments across the Chicago Loop district. Highly heterogeneous temperatures are observed to characterize the underground-built environments with evidence of a severe subsurface urban heat island in downtown Chicago- the Loop.

This study investigates the coupled effects of groundwater and tunnel air flows on the energy tunnel system via 3D thermo-hydraulic numerical modelling. It is found that the combination of groundwater flow parallel to the tunnel and limited airflow velocity results in reduced operational efficiency of ground source heat pump (GSHP) due to the strong thermal interference along the tunnel. The study also highlights the importance of real scale modelling to evaluate the thermal yield of long energy tunnels, particularly when dealing with parallel groundwater flow in geothermal tunnel design.

This study presents an experiment based on a small-scale pile model installed in saturated kaolin clay to study the long-term behavior of energy piles under thermal cycles and inclined mechanical loads. Three levels of horizontal loading of 35, 71, and 109 N were applied to the pile head while keeping the constant vertical mechanical loading of 100N. Fifteen thermal cycles with an amplitude of ±4.5℃ were applied to the pile under each inclined mechanical level. The results show an increase in irreversible pile head settlement and horizontal displacement with thermal cycles but with a gradually decreased accumulation rate.

Numerical analyses conducted in the past to study energy tunnels use simple soil constitutive models. Using simple constitutive models for tunnel design may not accurately depict the complex behaviour of soil materials under different conditions, and the complex interplay among the deformation, flow and thermal models governing the soil behaviour. In this study, the coupled thermo-mechanical behaviour of energy tunnels is investigated employing a thermo-elasto-plastic constitutive model developed in the framework of the critical state soil mechanics and considering thermal softening effects.