This paper focuses on the main issues discussed during a session on the impact of thermo-hydromechanical behavior of soils on thermo-active geotechnical systems, and how they could affect the performance of thermo-active geotechnical systems. Both soil behavior as well as soilstructure interaction behavior were discussed. The main observation from the session was that the thermo-hydro-mechanical behavior of saturated soils has reached a mature understanding, with several established constitutive models that can be used by engineers. However, there are still opportunities to enhance these constitutive models by considering issues such as unsaturated conditions, anisotropic stress states, cyclic heating and cooling effects, and changes in the preconsolidation stress during heating and cooling. Further there are still opportunities to improve our understanding of soil-concrete interface behavior, including the development of novel testing approaches.
Ground-source heating of bridge decks can be an alternative to the use of salts and chemicals to deice bridge decks. Energy foundations, geothermal boreholes, shallow trenches or the approach embankment can be utilized as energy storage media and heat exchange pathways to utilize the ground as a heat source. Coupling the energy foundation with a ground-source heat pump can provide higher inlet fluid temperatures and result in more effective bridge heating but at the expense of greater energy consumption and increased system complication. Circulating the fluid directly from the energy foundation to the bridge deck relies heavily on the in-situ ground temperatures. This paper outlines the operational principles and how these are related to the design parameters of bridge deck deicing systems. A series of parametric analyses was performed to investigate the bridge deck heating process. The analyses considered a variety of tube spacings, inlet fluid temperatures (i.e. ground temperatures), flow rates, wind speeds, ambient temperatures and thicknesses of concrete cover over the circulation tubes. The results serve as a benchmark to gauge the operational conditions and the energy requirements for designing ground-source bridge deck deicing systems. Ground temperatures serve as a baseline for heating of bridge decks with passive heating in the winter and this study underlines its limitations at colder environments. This can be overcome by collecting heat from the bridge deck in the summer and injecting the collected heat into the ground to raise the ground temperatures. This stored energy can be reclaimed in the winter when needed. The issues related to heat collection and ground storage are part of a broader study and fall outside the scope of this paper.
This paper presents a review of current design tools used for thermo-active geotechnical systems, along with validation efforts. The capabilities of available analytical methods used for the thermal and thermo-mechanical design of these systems are evaluated and shortcomings of the existing methods are identified. Although the analytical methods permit accurate prediction of the thermal stress and strain response of thermo-active piles from readilyavailable soil and concrete properties, current shortcomings consist of the ability of the methods to simulate cyclic heating and cooling effects, transient pore water pressure generation and dissipation, and the effects of radial stress changes. Recommendations are provided on how to properly address the current design requirements and the efforts to overcome shortcomings with the development of constitutive relationships from further full-scale and laboratory-scale experimental studies on thermo-active piles. Furthermore, the need for the development of both simplified analytical tools and advanced finite element models is emphasized. In addition, the existing analytical tools should be validated using field data from recently available case studies of thermoactive piles in varying soil deposits. An urgent need for an extensive design guide for energy geostructures was identified. The guidelines should be targeted towards practitioners and include field observations and measurements, as well as laboratory and numerical studies.
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