EAGER: Solar Thermal Soil Improvement over Different Depths

这一探索性研究（EAGER）项目的早期概念资助旨在开展岩土工程研究，以评估利用太阳能热能改善不同深度软土沉积物力学特性的可行性。具体来说，通过地下闭环地热热交换器循环的太阳能热板收集的加热流体用于诱导目标区域土壤的热体积收缩和相应的抗剪强度增加。研究了垂直和水平配置的地热交换器阵列，以改善不同深度范围和面积分布的土壤。这种方法的优点是可以利用可再生能源有针对性地进行土壤改良，之后地热热交换器可以用于长期地下热能储存，与现有的软土改良技术相比，可以节省成本。该研究计划旨在更好地理解控制软土在不同深度范围内热体积变化的基本过程，并改进高级计算机模拟所需的软土本构模型，解决国家科学基金会“促进科学进步”的任务。如果可行，太阳能热能和地热热交换器将是有效改善民用基础设施项目、近海或河流沉积物、矿山尾矿坝和煤灰蓄水池中遇到的具有挑战性的软土沉积物的重要工具。该项目将通过加州大学圣地亚哥分校的STARS和ENLACE等暑期项目，引入来自不同背景的本科生进行研究。本研究探讨了影响太阳热改良土壤可行性的一个基本问题，即在给定温度增量下，初始平均有效应力（或初始孔隙比）对正常固结土排水热体积变化幅度的可能影响。虽然现有的热弹塑性模型表明，所有正常固结土由于热硬化应具有相同的热体积变化，但有限的正常固结土加热数据表明，初始平均有效应力较低、初始孔隙比较高的土，在改善后可能会经历更大的热体积变化和更大的不排水抗剪强度增加。如果这些有限的数据是有效的，这将表明，在给定的温度增量下，较浅的土壤可能会经历更大的改善。本项目的目的是通过在热三轴箱中进行综合实验测试，更好地了解正常固结粘土的热体积变化，并利用实验结果来完善现有的排水热弹塑性模型和不排水热压模型。沿着这些思路，本项目旨在研究温度和平均有效应力对关键材料性能的潜在影响，包括热硬化参数、土壤骨架的热膨胀系数和土壤的可压缩性系数。利用从这项调查中获得的知识，该项目试图将与太阳热土壤改良相关的瞬态耦合传热和水流过程的预测与观测到的排热体积变化趋势统一起来。采用统一模型模拟太阳热改良土壤过程，了解换热器几何形状和太阳热边界条件在达到不同程度土壤改良中的作用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.