EAGER: Solar Thermal Soil Improvement over Different Depths

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This EArly-concept Grant for Exploratory Research (EAGER) project addresses the geotechnical engineering research needed to assess the feasibility of using solar thermal energy to improve the mechanical properties of soft soil deposits over different depth ranges. Specifically, heated fluid collected from solar thermal panels circulated through closed-loop geothermal heat exchangers in the subsurface is used to induce thermal volumetric contraction and a corresponding increase in shear strength of a targeted zone of soil. Arrays of geothermal heat exchangers in vertical and horizontal configurations will be investigated to improve soil over different depth ranges and areal distributions. Advantages of this approach are that soil improvement can be gained in a targeted manner using renewable energy, after which the geothermal heat exchangers can be used for long-term underground thermal energy storage, yielding cost savings when compared to available soft soil improvement technologies. The research plan seeks to better understand fundamental processes governing the thermal volume change of soft soils over different depth ranges and to improve constitutive models for soft soils needed in advanced computer simulations, addressing the NSF mission "to promote the progress of science." If feasible, solar thermal energy and geothermal heat exchangers will be important tools for the cost-effective improvement of challenging soft soil deposits encountered in civil infrastructure projects, offshore or river sediments, mine tailings dams, and coal ash impoundments. This project will introduce undergraduate students from diverse backgrounds to research through established summer programs at UCSD like STARS and ENLACE. A fundamental issue investigated in this study that will impact the feasibility of solar thermal soil improvement is the possible impact of the initial mean effective stress (or initial void ratio) on the magnitude of drained thermal volume change of normally consolidated soils for a given temperature increment. Although existing thermo-elasto-plastic models indicate that all normally consolidated soils should have the same thermal volume change due to thermal hardening, limited data available for heating of normally consolidated soils indicate that soils lower in initial mean effective stress and higherin initial void ratio may experience greater thermal volume changes and greater increases in undrained shear strength after improvement. If these limited data are valid, this would indicate that shallower soils may experience greater improvement for a given temperature increment. The objective of this project is to better understand the thermal volume change of normally consolidated clay through a comprehensive experimental testing program in a thermal triaxial cell and to use the experimental results to enhance existing drained thermo-elasto-plastic models and undrained thermal pressurization models. Along these lines, this project seeks to investigate potential impacts of temperature and mean effective stress on key material properties including the thermal hardening parameter, the coefficient of thermal expansion of the soil skeleton and the coefficient of compressibility of the soil. Using the knowledge gained from this investigation, this project seeks to unify predictions from the transient coupled heat transfer and water flow process associated with solar thermal soil improvement with the observed trends in drained thermal volume change. The unified model will be used to simulate solar thermal soil improvement process to understand the roles of heat exchanger geometry and solar thermal boundary conditions in reaching different degrees of soil improvement.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.