Geothermal Geotechnics for Urban Undergrounds

Underground urbanization as a prerequisite for sustainable development involves the design and construction of underground spaces in difficult ground and under very restricted conditions. Additionally, it offers the possibility of using ground structures also as thermo-active heat exchangers for heating and cooling of all kinds of buildings. This keynote paper focuses on near-surface geothermal geotechnics, i.e. energy piles and other energy foundations, energy walls, energy tunnels etc. This represents an innovative technology that contributes to environmental protection and provides substantial long-term cost savings and minimized maintenance. Absorber pipes filled with a heat carrier fluid are installed within conventional structural elements (piles, barrettes, diaphragm walls, basement slabs or walls, tunnel linings), forming the primary circuit of a geothermal energy system. The natural ground temperature is used as a heat source in winter and for cooling in summer. Hence not additional elements have to be installed below surface. The primary circuit is then connected via a heat pump to a secondary circuit within the building. The paper deals with earth-contact concrete elements that are already required for structural reasons, but which simultaneously work as heat exchangers. For instance, several new metro stations in Vienna are equipped with this sustainable technology. Theoretical details, pilot research projects and case histories bridge the gap between theory and practice, and special applications reveal the wide field of geothermal geotechnics. Design recommendations are given. Finally, benefits of thermo-active ground structures and recommendations for promoting geothermal energy utilization are listed.