Thesis (M. S.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Sturm, Terry; Committee Member: Burns, Susan; Committee Member: Webster, Donald. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Samples collected from 5 bridge sites from around the state of Georgia are analyzed to determine their erosion and rheologic behavior. Most sites were subject to large amounts of local scour due to flood events resulting from Tropical Storm Alberto in 1994. According to the Federal Highway Administration's Hydraulic Engineering Circular No. 18 by Richardson and Davis (2001), scouring of bridge foundations is the most common cause of bridge failure resulting from floods. The erosion rates of the soils are measured in a rectangular tilting flume capable of applying up to 21 Pa of shear stress at the bed. Samples from Shelby tubes are extruded into the flow from below the bed using a hydraulic piston. The displacement is measured as a function of time using a cable-pull potentiometer. The soils are also subject to extensive geotechnical analysis. Sieve and hydrometer analyses are performed to obtain the particle size distribution for each sample. Atterberg Limits and other standard geotechnical measures are also found. Additionally, insight into the shear strength and cohesive nature of the fine (<0.75 micrometers) particles is gained using a stress controlled rheometer to measure the rheological characteristics of the slurry. These results are used to improve and extend a relationship for the critical shear stress of soils developed in previous research that can be used in bridge scour prediction formulae as affected by soil parameters. In addition, the rheologic properties of the soil in terms of a dimensionless yield stress are related to the critical value of the Shields parameter for estimating critical shear stress for erosion.

Shelby tube sediment samples collected from the foundations of ten (10) bridges located in the state of Georgia were tested in the laboratory to find their erosional behavior and the correlation of erosion parameters with sediment properties in order to improve the prediction of scour around bridge foundations. These sites were spatially distributed in order to fall into different major river basins and in different physiographic regions. A description of the Valley and Ridge, Blue Ridge, Piedmont, and Coastal Plain physiographic regions of Georgia is included, and the erosion parameters found from flume measurements are associated with their respective regions. Flume measurements were performed using a rectangular, tilting, recirculating flume located in the hydraulics lab in the School of Civil and Environmental Engineering at Georgia Tech. Velocities up to 1.7 m/s and bed shear stresses up to 21 Pa can be achieved in the flume. Regression analysis was performed on erosion rates as a function of applied shear stress to determine the parameters of the erosion function. The resulting parameters, the critical shear stress and the erosion rate constant, were correlated with soil properties and physiographic regions. Experimental methodology was chosen to approach this problem because the involvement of interparticle forces for fine-grained materials makes it difficult to deal with the erosion phenomenon through other means. Nevertheless, analytical description of the erosion phenomenon was included in order to provide a better understanding of it. Linear, exponential and power regression mathematical models for erosion rate were compared, and the two best-fit regression models of erosion rate as a function of shear stress are proposed to formulate a methodology intended to characterize the behavior of a soil exposed to erosive flow conditions. One of them is a linear model to calculate critical shear stresses and low erosion rates. The second model, which is exponential, has the advantage of describing the erosion rate response for a wider range of shear stress values. It is shown that one of the most relevant predictors for the critical shear stress and erosion rate constant in the regression models is the fine material content present in the sample, which is an indirect indicator of the contribution of interparticle forces to the erosion process. Applying the described methodology, a more case-specific calculation of the erosion at bridge foundations can be performed taking into account the actual material in situ.