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

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

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.