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|Title:||Verification of empirical method for determining riverbank stability, 1961 data|
|Authors:||United States. Mississippi River Commission.|
U.S. Army Engineer Waterways Experiment Station.
|Publisher:||U.S. Army Engineer Waterways Experiment Station.|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Potamology investigations report ; no. 12-12.|
Summary: This report is the ninth of a series in which new data obtained from borings made for revetment construction are analyzed to determine the applicability of the empirical method for predicting riverbank stability with regard to flow failure. Data obtained in fiscal year 1961 are analyzed, and stability predictions are made for 11 new areas. Failures during 1961 at sites previously analyzed also are discussed. Based on analyses made in 1958 of previous performance data, the classification criteria for overburden soils, zone A sands, and zone B sands were modified. The failures at sites previously studied, new site predictions, and current year performance are analyzed using the modified criteria. During 1961, 21 bank failures (10 flow failures and 11 shear failures) were reported along the Mississippi River near 20 boring locations at 15 sites for which stability predictions had been made. Eight flow failures occurred near eight boring locations predicted to be susceptible to flow failure. Two flow failures occurred 750 ft on either side of a boring location that was predicted to be stable; however because of the variability of point-bar deposits it is considered that this boring location is too far away to have reliably indicated the bank conditions at the failure locations. Six shear failures occurred near six boring locations predicted to be stable with regard to flow failure. Four shear failures occurred near three boring locations predicted to be unstable with regard to flow failure. One shear failure encompassed two boring locations where no predictions had been made because the full depth of zone A sand was not determined. Since 1954 when riverbank stability predictions were initiated, data have been studied from 735 boring locations at 81 revetment sites on the Mississippi River in the Memphis and Vicksburg Districts. Flow failures have occurred at 8 boring locations in the Memphis District and 26 boring locations in the Vicksburg District. Of the 34 locations where flow failures have occurred, 27 were predicted to be unstable according to the modified criteria. Five flow failures have occurred at boring locations for which no prediction can be made since the full depth of zone A sand was not determined during the field investigation. Flow failures occurred near two locations predicted to be stable, but in these cases the failure was more than 750 ft from the boring and the boring data probably were not representative of the soil conditions at the failure locations. The modified classification criteria have proved reliable in predicting flow failure. However, many locations predicted to be unstable have not experienced flow failure, and it is possible that either the density of zone A sand may be such that flow failure will not occur, or the severity of river attack has not been sufficient to initiate flow failure.
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