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|Title:||Verification of empirical method for determining riverbank stability, 1958 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. 12-9.|
Summary: This report is the sixth 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 liquefaction failures. Data obtained principally in fiscal year 1958 are analyzed, and stability predictions are made for 17 new areas. Failures at sites previously analyzed also are discussed. Based on analyses made during 1958 of previous performance data, the classification criteria for overburden soils, zone A sands, and zone B sands were modified. Consequently, this report discusses (a) the failures at sites previously studied, and (b) new site predictions and 1958 performance with regard to both the original and modified classification criteria. During 1958, 45 bank failures were reported at 41 boring locations on 18 sites for which stability predictions had been made. Nine of the failures were believed to be of the flow type; the others were considered to be of the shear type, resulting from current scour or other causes. Six of the nine flow failures occurred at boring locations predicted to be unstable and three occurred at boring locations predicted to be stable according to the original classification criteria. According to the modified classification criteria, all nine boring locations at which flow failures occurred were predicted to be unstable. Since 1954 when stability predictions were initiated, data have been studied from 535 boring locations at 65 revetment sites on the Mississippi River in the Memphis and Vicksburg Districts. Flow failures have occurred at 5 boring locations in the Memphis District and at 18 boring locations in the Vicksburg District. Of the 23 locations where flow failures have occurred, all except three are considered unstable according to the modified classification criteria. At these three locations the borings did not penetrate the full depth of zone A sand, and the predictions are not considered valid. Analysis of performance data indicates that the modified classification criteria for predicting susceptibility to liquefaction-type failures are superior to the original criteria and that their use should be continued in future verification studies. From the analysis of boring data at locations where flow failures have occurred, it is concluded that an unstable condition is indicated by an R value (ratio of overburden thickness to zone A sand thickness) of 0.85 or less and a thickness of zone A sand greater than 20 ft. An R value higher than 0.85 or a thickness of zone A sand less than 20 ft indicates a stable condition. The empirical method for determining riverbank stability does not provide means of determining factors that initiate flow failures; the influence of such factors as river stage, migration of the river, turbulence, current attack, and deepening and movement of the thalweg of the river should be investigated to permit further refinement of the criteria for predicting instability. It is believed that at certain unstable locations, turbulence in the river may not become sufficiently severe to initiate flow failure until a river stage of as much as 10 ft above bankfull is experienced. Therefore, it is recommended that annual verification studies be continued until all revetted locations for which predictions have been made are subjected to river stages at least 10 ft above bankfull, and that additional studies be undertaken to determine the river conditions that initiate flow failures.
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