3D Dynamic Evolution of Pore Water-Air Interaction Within Saturated Sheared Sand

许多地基系统是由含水饱和的砂土支撑的。在微观尺度上，砂粒之间的空隙（孔隙空间）充满了水。沙子通过摩擦和颗粒之间的联锁来抵抗施加的基础荷载，这被称为沙子的抗剪强度。除了水之外，孔隙空间中空气的存在引入了新的力，并从根本上改变了砂的抗剪强度。PI的初步实验表明，尽管几十年的使用和岩土工程界的广泛接受，饱和实验室砂样的标准程序可能存在重大缺陷。要消除砂土孔隙空间中的所有气泡是很困难的，而且广泛使用的假定砂土在饱和程度不变的情况下达到100%饱和状态是不正确的。如果在加载过程中，砂样的饱和程度从完全饱和变为部分饱和，那么目前的测试程序产生的抗剪强度测量结果不准确，这对解释实验测量结果有重大影响。该奖项将使用动态四维（三维空间和时间）成像来研究空气对砂土破坏行为的作用。本研究的发现促进了砂土抗剪强度的测量和解释，对岩土工程的研究、设计和实践具有广泛的影响。本研究将为土木工程本科学生提供丰富的研究经验，以少数民族/女性学生为重点，并吸引高中生参与研究，以激发他们对土木工程领域的兴趣。本研究的目的是利用6微米分辨率的快速动态同步加速器微计算机断层成像技术监测剪切载荷条件下沙-水-空气相互作用的演变。本研究将：(1)研究颗粒形态和级配对剪切砂饱和度的影响；（ii）排水条件下剪切砂孔隙空间局部水气相互作用监测；(3)评价试样密度对剪砂饱和度变化的影响。将回答以下基本问题：(1)在剪切砂中保持相同饱和度的最小孔隙水压力值是多少；(2)试件剪切时是否保持100%的饱和状态；(3)出现气泡；它们将如何发展，哪些因素会影响它们的发生和发展。动态颗粒-水-空气相互作用的三维探测有望提供独特的测量方法，以支持新的颗粒尺度理论的发展，该理论可以更好地描述饱和剪切颗粒材料的行为。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。

Many foundation systems are supported by water-saturated sand deposits. At the micro-scale, the voids between sand grains (pore space) are filled with water. The sand resists applied foundation loads through friction and interlocking between particles which is known as the shear strength of sand. The presence of air within the pore space in addition to water introduces new forces and fundamentally changes the shear strength of sand. The PI’s preliminary experiments show that standard procedures to saturate laboratory sand specimens may suffer from a major shortcoming despite decades of use and wide acceptance by the geotechnical community. It is difficult to eliminate all air bubbles from sand pore space and the widely used assumption of a 100 percent saturated state for a sand specimen with no change in degree of saturation is not correct. If the degree of saturation of a sand specimen changes from fully saturated to partially saturated during loading, then current testing procedures yield inaccurate measurements of shear strength which has a major impact on interpreting experimental measurements. This award will use dynamic four-dimensional (three space dimensions and time) imaging to investigate the role of air on the failure behavior of sand. The findings of this research advances knowledge in measuring and interpreting shear strength of sand, which have broad impacts on geotechnical engineering research, design, and practice. This research will provide an enriching research experience for undergraduate civil engineering students with emphasis on minority/female students and engage high school students in research in order to spark their interest in the field of civil engineering.The goal of this research is to monitor the evolution of sand-water-air interaction using fast dynamic synchrotron microcomputed tomography imaging at 6-micron resolution under shear loading conditions. This research will (i) investigate the influence of particle morphology and gradation on the degree of saturation of sheared sand; (ii) monitor local water-air interaction within pore space of sheared sand under drained condition; and (iii) evaluate the effects of specimen density on the change of degree of saturation of sheared sand. The following fundamental questions will be answered: (1) what is the minimum value of pore water pressure to maintain the same degree of saturation in sheared sand; (2) does the sand specimen remain 100 percent saturated when it is sheared; (3) if air bubbles develop; how they will evolve and what are the factors that affect their onset and growth. 3D probing of dynamic particle-water-air interaction is expected to offer unique measurements to support the development of a new particle-scale theory that better describes the behavior of saturated sheared granular materials.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.