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|Title:||Development of a quantitative method to predict critical shear stress and rate of erosion of natural undisturbed cohesive soils|
|Authors:||University of California, Davis. Dept. of Civil Engineering.|
|Publisher:||Geotechnical Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical report (U.S. Army Engineer Waterways Experiment Station) ; GL-80-5.|
Abstract: To determine streambank recession with time, resulting from erosion due to current action and slope failure, requires measurement of the rate of erosion and shear strength properties of undisturbed natural soils. However, it is recognized that for many stream bank erosion problems, it may not be economically feasible to obtain undisturbed soil samples and conduct laboratory tests to measure rate of erosion and shear strength. For this type of problem, it may suffice to determine the susceptibility of the streambank to erosion and slope failure, without determining the bank recession at selected intervals of time. The study was conducted in an attempt to develop a quantitative method to predict the hydraulic shear stress at which erosion is initiated (critical shear stress) and the rate of erosion of natural soil (with sufficient cohesiveness to allow undisturbed samples to be taken) along the stream bank. In order to obtain a wide range of properties with sufficient geographical distribution, soil and river water samples were requested from Corps of Engineers (CE) Districts in the United States. Preliminary tests conducted on 42 samples included: (a) free swell test on undisturbed soil; (b) dielectric dispersion on remolded soil; (c) slaking test on undisturbed soil; (d) amount of clay present in soil; (e) soil chemistry (pH and soluble cations); and (g) river water chemistry (pH and soluble cations). Based upon the results of the preliminary tests 30 soils were selected for detailed analysis which included (a) index property tests (hydrometer analyses, specific gravity, organic content, gypsum content, moisture content, unit weight, and Atterberg limits); (b) soil chemistry (exchangeable cations); (c) dielectric dispersion on undisturbed soil; (d) flume erosion tests on undisturbed soil; and (e) rotating cylinder erosion tests on saturated remolded soil. The results of the study showed that remolding the soil generally decreased both the critical shear stress and the rate of change of erosion rate. The salt concentraton of the eroding fluid significantly influenced the erosion of remolded soil samples. Usually, as the salt concentr ation of the river (eroding) water decreased, the critical shear stress of the soil decreased, and the rate of change of erosion rate increased. The results of the study did not yield a quantitative method to predict critical shear stress and/or rate of erosion for undisturbed natural soils. However, it was shown that a previously developed chart for remolded soil (Figure 3) gave a predicted value of critical shear stress that was generally less than the measured value for undisturbed soil using distilled water as eroding fluid. Since the critical shear stress usually increases as the salt concentration of the eroding fluid increases, the chart should give a reasonable estimate of the critical shear stress for a natural undisturbed soil subjected to hydraulic shear stress from river (eroding) water. Using the estimated critical shear stress, a predictive chart (Figure 10), which was developed in this study of the relationship between critical shear stress and rate of change of erosion rate for undisturbed soil, with distilled water as the eroding fluid, can be used to estimate the rate of change of erosion rate for a natural undisturbed soil subjected to hydraulic shear stress from river (eroding) water. The estimated rate of change of erosion rate will represent an upper bound because as the salt concentration increases, from distilled water to river water concentration, the rate of change of erosion rate decreases. NOTE: Some of the appendixes of this report were included with the report in Microfiche format only. The microfiche is still included with the report, but has not been digitized at this time (1/4/2012). Contact the ERDC Library (email@example.com) if you need to examine the microfiched appendixes.
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