Date of Award

8-19-2024

Document Type

Open Access Thesis

Department

Civil and Environmental Engineering

First Advisor

Enrica Viparelli

Abstract

The high frequency of earthen embankment failure recorded during floods has led to the general rule that embankments should not be overtopped. This notwithstanding, overtopping cannot be entirely avoided particularly during extreme events, such as winter storms and hurricanes. Reducing soil erodibility is thus fundamental to prevent and control disasters caused by embankment failure. Recent studies have explored the use of environmentally friendly additives, such as biopolymers, to improve soil properties. Geotechnical tests have shown that biopolymers can effectively increase soil strength, but little is known about biopolymer-treated soil resistance to erosion by flowing water. Results of flume experiments to characterize erosion of biopolymer-treated sand-silt mixtures are presented here. Experiments were conducted in the Hydraulics Laboratory at the University of South Carolina with xanthan gum as biopolymer additive. Proctor tests were first performed to identify the optimum water content of different sand-silt-xanthan gum mixtures differing in silt and xanthan gum content. Findings show a positive correlation between xanthan gum concentration and optimum water content of sand. In sand-silt mixtures, on the contrary, optimum water content does not appreciably vary with xanthan gum content. The erodibility of mixtures of sand, silt and xanthan gum was measured in a laboratory flume for increasing values of the boundary shear stress. To meaningfully compare results, the water content of the samples was always equal to the optimum value. Erodibility tests indicated that erodibility of xanthan gum-treated sands decreased with increasing xanthan gum concentration. Further, the mode of sediment entrainment in transport changed from grain-grain detachment at low xanthan gum concentrations, to plucking at comparatively high biopolymer contents. The same transition from grain-grain erosion to plucking was observed at relatively low xanthan gum contents for increasing bed shear stress. Characterizing the erodibility of sand-silt-xanthan gum mixtures was challenging due to the high mobility of silt size particles. In general, sand-silt-xanthan gum mixtures were harder to erode than sand-xanthan gum mixtures with the same biopolymer content. The difference between erodibility of xanthan gum-treated sand and treated sand-silt mixtures increased with biopolymer and silt content. In addition, at low xanthan gum concentration, silt was entrained in suspension and erosion of the remaining sand-xanthan gum mixture occurred as observed in absence of silt, that is grain-grain erosion at low boundary stress and plucking at high boundary stress. For increasing xanthan gum content, silt became hard to entrain in suspension, and the erosion mode transitioned from entrainment of sand grains to plucking.

Rights

© 2024, Edwin Kotey

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