EAGER: Solar Thermal Soil Improvement over Different Depths

这一初步构想的探索性研究资助(AGER)项目致力于评估利用太阳能改善不同深度范围内软土沉积物的力学性质的可行性所需的岩土工程研究。具体地说，从太阳能热板收集的加热流体通过地下的闭环式地热换热器循环，用于诱导土壤目标区域的热体积收缩和剪切强度的相应增加。将对垂直和水平配置的地热换热器阵列进行调查，以改善不同深度范围和区域分布的土壤。这种方法的优点是可以利用可再生能源以有针对性的方式改善土壤，之后地热换热器可以用于长期的地下热能储存，与现有的软土改良技术相比，可以节省成本。这项研究计划旨在更好地了解控制不同深度范围内软土热体积变化的基本过程，并改进先进的计算机模拟所需的软土本构模型，以满足美国国家科学基金会“促进科学进步”的使命。如果可行，太阳能和地热热交换器将成为经济有效地改善在民用基础设施项目、近海或河流沉积物、尾矿坝和灰库中遇到的具有挑战性的软土沉积物的重要工具。这个项目将介绍来自不同背景的本科生通过加州大学圣迭戈分校现有的暑期项目，如STARS和Enlace，进行研究。本研究研究的一个影响太阳能热土改良可行性的基本问题是初始平均有效应力(或初始孔隙率)对正常固结土在给定温度增量下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，由于热硬化，所有正常固结土都应该具有相同的热体积变化，但可用于正常固结土加热的有限数据表明，初始平均有效应力较低、初始孔隙比较大的土在改进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，对于给定的温度增量，浅层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试验室中进行全面的实验测试，更好地了解正常固结粘土的热体积变化，并利用实验结果来改进现有的排水热弹塑性模型和不排水热加压模型。沿着这些思路，本项目试图研究温度和平均有效应力对关键材料特性的潜在影响，包括热硬化参数、土骨架的热膨胀系数和土壤的压缩系数。利用从这次调查中获得的知识，该项目试图将与太阳热力土壤改良相关的瞬时耦合换热和水流过程的预测与观测到的排水热量变化趋势统一起来。统一的模型将被用于模拟太阳能热土壤改良过程，以了解热交换器几何形状和太阳能热边界条件在达到不同程度的土壤改良中的作用。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.