Piping is known to cause catastrophic failures of levees and earthen dams, and has been studied for nearly a century. In the typical experimental studies on soil erosion, tap water or de-ionized water is used as permeating fluid; therefore, the understanding of soils' erosion behavior is mostly limited to situation where relatively pure water is the permeating fluid. In the field, permeating fluids through earthen dams or levees may contain slurry (in the case of slurry wall installation) or fines that are carried in the permeating fluid from the upstream soil erosion. The fines content may have various sizes, concentrations, and other physicochemical characteristics. Our repeatable laboratory experiments in hole-erosion tests under constant head have revealed that the piping progression of a sand differed significantly with different permeating fluids. Bentonite slurry and water that contains 1% cohesionless fines did not induce piping progression during a prolonged period, while water and water with 0.1% fines caused the pre-formed piping hole to enlarge to the side of the soil column in less than 40 minutes. This observation is counter-intuitive in the case of fluid with cohesionless fines, based on the current knowledge that permeating fluid with higher density and viscosity tends to exert higher hydraulic shear stress and cause more erosion. The gap between the current knowledge and the recent laboratory observation is due to the lack of fundamental understanding of the suspension-fluid-soil interaction. The hypothesis of this research is that the physicochemical characteristics (viscosity, fluid density, ionic strength, pH, and possibly suspension's surface electric potential) of the permeating fluids and the flow conditions accumulatively contribute to the erosion of sand. The main objective of this research is to identify the physicochemical mechanisms of suspension-fluid-soil interactions and provide fundamental explanations for the different erosion behaviors of sand under various permeating fluids and hydraulic conditions. The research project will answer the following three questions: (1) Which forces dominate the particle dislodging: inter-particle London-van der Waal forces and electrical double layer forces, or the hydrodynamic forces? (2) How do the various permeating fluids with different physicochemical characteristics (particulate concentration, size, viscosity, fluid density, electrostatic surface potential, ionic strength, pH) contribute to the dislodging forces? Which is a dominating factor? (3) How does the flow rate (i.e., laminar and turbulent flows) change the role of each of the aforementioned factors in particle dislodging? The physicochemical characteristics of eleven permeating fluids will be experimentally quantified. Microscopic flow experiments and laboratory hole erosion experiments using the eleven fluids will be conducted to reveal the individual roles of the aforementioned characteristics in the particle dislodging process at different hydraulic conditions.
This research will yield transformative knowledge because it will provide realistic understanding of the piping progression in earthen dams and levees. It will also guide us to realistically evaluate and remediate many natural and built infrastructures, such as in natural riparian habitat protection, in riverbed or bridge foundation scouring where flows can carry various particles with different concentrations at different flow rate, and in pipeline engineering where storm water, wastewater, or oil can have different viscosity or carry various particle loadings. This funding will also provide educational opportunities for the PI to mentor graduate students to become future independent and confident researchers, to timely integrate multidisciplinary research methodologies and outcomes into graduate and undergraduate courses, and to outreach to and involve K-12 students and underrepresented undergraduate students in research in order to attract future talents to the STEM fields. The knowledge developed in this project will be disseminated through websites, presentations in university and high schools, and publications written by the PI and students.
