ENERGY MICROPILES AND THERMO-ACTIVE FOUNDATIONS: EXPERIMENTAL INVESTIGATION, NUMERICAL MODELLING, AND EMERGING APPLICATIONS

This doctoral thesis focuses on advancing the understanding and knowledge of the thermo-mechanical and thermo-hydraulic behaviour of innovative Energy Geostructures (EGs), specifically focusing on energy micropiles (EMPs) and large-diameter energy piles (EPs) under extreme, large-scale, and non-conventional operational conditions. EGs serve a dual purpose: providing structural stability to buildings or infrastructure while harnessing shallow geothermal energy for the heating and cooling demands of the built environment. While the behaviour of conventional EPs has been extensively researched over the past decades, the investigation into EMPs remains largely unexplored. EMPs offer crucial advantages for the structural consolidation and energy retrofitting of existing buildings. Simultaneously, the integration of EGs into high-density metropolitan areas introduces new operational challenges. Consequently, this research addresses four primary gaps (described in detail in Section 1.8): the lack of experimental parametric understanding for EMPs, the unquantified structural implications of Subsurface Urban Heat Islands (SUHI), the uncertain operational safety limits during intensive winter heat extraction at sub-zero temperatures, and the insufficient optimization frameworks for foundation-based seasonal Underground Thermal Energy Storage (UTES). To address these challenges, this study employs a unified approach combining full-scale field testing and advanced numerical modelling. The experimental component of this work builds upon a dedicated full-scale test site established in Parma, Italy. Here, pressed-in steel EMPs were installed and extensively instrumented to evaluate their thermal performance under realistic operating conditions. The field campaign systematically assessed the influence of various geometric, hydraulic, and material parameters, including varying pipe diameters, single versus double-loop configurations, and the use of special conductive grout mixtures containing aluminum chips. Extensive monitoring under varied flow regimes and hydraulic connections (series and parallel) provided robust, high-resolution data for both single-pile and group performance evaluations. Building upon the experimental evidence, calibrated three-dimensional finite element models were developed to extend the analyses beyond empirical constraints and investigate complex boundary conditions. A coupled thermo-mechanical modelling approach was utilized to investigate the long-term impact of progressive subsurface warming, driven by SUHI phenomena, on the foundations of a case-study building in Rome. This analysis aimed to quantify the evolution of axial stresses and differential settlements over decades of urban warming. Furthermore, a thermo-hydraulic numerical model was implemented to simulate freeze-thaw processes and assess the frost susceptibility of different pile geometries and soil lithologies subjected to circulating fluids at sub-zero temperatures, typical of intensive winter heating demands. Finally, multi-year transient simulations were conducted on EP groups to evaluate the hierarchical influence of design layouts, site-dependent thermophysical properties, and asymmetrical active operational strategies on the overall energy balance and storage efficiency of seasonal UTES systems. The results of this comprehensive investigation significantly advance the practical implementation of thermo-active foundations. The experimental analyses revealed that the thermal yield of EMPs is predominantly governed by the number of heat exchange loops and the imposed fluid-soil temperature differential. Parallel hydraulic configurations consistently outperformed series arrangements by facilitating uniform flow distribution and sustaining transitional or turbulent flow regimes, thereby maximizing convective heat transfer efficiency. Conversely, the inclusion of highly conductive metallic additives, such as aluminum chips, in the grout mixture paradoxically reduced the overall thermal efficiency. At the urban scale, the thermo-mechanical assessments demonstrated that long-term SUHI phenomena can induce non-negligible structural stresses and settlements; however, the active geothermal exploitation of the subsurface via EMPs serves as a highly effective mitigation strategy, absorbing excess waste heat while simultaneously improving the efficiency of winter heating systems. Regarding extreme winter operations, the coupled thermo-hydraulic simulations illustrated that frost susceptibility at the pile-soil interface is highly dependent on the thermal mass of the pile and the physical properties of the surrounding soil, rather than just the fluid temperature. These findings advocate for the adoption of physics-based, site-specific operational limits, overcoming the universally conservative and often restrictive thresholds currently dictated by international guidelines. Finally, the long-term UTES simulations confirmed that asymmetrical operational schemes can significantly elevate cumulative seasonal storage efficiencies. Overall, the conducted research facilitates significant advancements for both EMP and EP installations. The defined optimization frameworks and identified operational limits provide crucial engineering insights, ensuring the reliable, safe, and efficient integration of EGs into the next generation of sustainable urban infrastructure.