Long-term operational experience of an energy pile wall in a tunnel: Construction, repairs, energy performance and thermal behavior

Abstract

Energy geostructures offer the possibility of synergistically using structurally required elements as heat exchangers. Despite the growing interest in their applications as energy walls and energy tunnels, comprehensive field measurements from operational installations remain limited, restricting both practitioners in system design and researchers in validating thermal performance and thermo-hydro-mechanical models. This study presents investigations of the Lainzer Tunnel in Vienna, Austria, which incorporates different geothermal systems across several tunnel sections. The focus is placed on section LT24, which uses energy piles in an intermittent bored pile wall serving as a thermo-active tunnel wall. The project offers a comprehensive monitoring dataset spanning over 24 years, encompassing three distinct operational phases: construction, active operation, and idle conditions. The complete measurement program is presented to provide a thorough project characterization. Discussed data is limited to the energy performance of the entire system, including comparison with newly constructed borehole heat exchangers adjacent to the tunnel section, as well as temperature evolution of both the heat exchanger fluid and the energy wall. Construction procedures, encountered damages, and implemented repair measures are documented in detail, alongside observations on system operation and consequences of neglect in maintenance, leading to sub-zero heat exchanger fluid and pile temperatures. Based on the observed long-term behavior under real operational conditions, recommendations are formulated to provide insights to both engineering practice and scientific research.