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|Title:||Verification of empirical method for determining riverbank stability, 1968 and 1969 data|
|Authors:||United States. Mississippi River Commission.|
Torrey, Victor H.
|Publisher:||U.S. Army Engineer Waterways Experiment Station.|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Potamology investigations report ; no. 12-21.|
Summary: This report is the sixteenth of a series in which new data obtained from borings made for revetment construction are analyzed to determine the applicability of an empirical method for predicting riverbank stability with regard to flow (liquefaction) failure. Boring data obtained in 1968 and 1969 are analyzed, and stability predictions are made for 68 new areas. Failures that occurred during 1968 and 1969 at sites previously analyzed also are discussed. Based on analyses made in 1958 of previous performance data, the classification criteria for zone A and zone B sands were modified in 1959. The failures at sites previously studied, new site predictions, and current year performance are analyzed using the modified criteria. During 1968, 31 bank failures (24 flow type and 7 shear type) occurred along the Lower Mississippi River at 16 revetment sites within 500 ft of boring locations for which stability predictions with regard to flow failure had been made. Twenty-three flow failures occurred near 16 boring locations predicted to be unstable with regard to flow failure, and one flow failure occurred near a boring location predicted to be stable with regard to flow failure. Also, seven shear failures occurred near six boring locations predicted to be stable. Two flow failures (at two revetment sites) and nine shear failures (at five revetment sites) were reported in areas where no borings were located within 500 ft . Two flow failures occurred near boring locations for which no prediction was possible because the thickness of zone A sand had not been determined. Four revetment breaches were judged to be the direct result of severe local scour. During 1969, 26 bank failures (21 flow type and 5 shear type) occurred at 12 revetment sites within 500 ft of boring locations for which stability predictions with regard to flow failure had been made. Among these were 17 flow failures near 13 boring locations predicted to be unstable, 4 flow failures near 4 boring locations predicted to be stable, 3 shear failures near 3 boring locations predicted to be stable, and 1 shear failure near a boring location predicted to be unstable with regard to flow failure. The one other shear failure was reported near a boring location for which no prediction was possible because a sufficient thickness of zone A sand had not been penetrated. Four flow failures and 12 shear failures occurred in areas that were more than 500 ft from any boring. From 1954 (when riverbank stability predictions were initiated) through 1968, 1492 boring locations at 118 revetment sites on the Mississippi River were studied. Almost all of these borings were in the Memphis and Vicksburg District areas. Data on sites in the New Orleans District were included only in the first report of this series (Report 12 -3). However, boring and failure data for 1968 and 1969 from the New Orleans District are included herein. Flow failures reported through 1969 have occurred within 500 ft of 19 boring locations in the Memphis District and 84 boring locations in the Vicksburg District; of these, 83 occurred near locations that had been predicted to be unstable according to the modified criteria, 12 occurred at boring locations predicted to be stable, and 8 occurred at boring locations for which no prediction had been made because the thickness of zone A sand had not been determined. The modified criteria have proven reliable in predicting stability with regard to flow failure. Of the total of 103 flow failures recorded since 1954 within 500 ft of analyzed borings, only 12 (12 per cent) were near boring locations predicted to be stable. However, many locations predicted to be unstable have not experienced flow failure, and it is possible that either the density of the 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.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Potamology Investigations Report|
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