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|Title:||Verification of empirical method for determining riverbank stability, 1970 and 1971 data|
|Authors:||United States. Mississippi River Commission.|
Torrey, Victor H.
Gann, Albert R.
|Publisher:||U.S. Army Engineer Waterways Experiment Station.|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Potamology investigations report ; no. 12-22.|
Abstract: This report is the seventeenth 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 1970 and 1971 are analyzed, and stability predictions are made for 56 new areas. Failures that occurred during 1970 and 1971 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 1970, 16 bank failures (9 flow type and 7 shear type) occurred along the Lower Mississippi River at 10 revetment sites within 500 ft of boring locations for which stability predictions with regard to flow failure had been made. Six flow failures occurred near five boring locations predicted to be unstable with regard to flow failure; one flow failure occurred near a boring location with no prediction possible; and two flow failures occurred near one boring location predicted to be stable with regard to flow failure. Also, two shear failures occurred near two boring locations predicted to be stable; four shear failures occurred near four boring locations predicted to be unstable; and one shear failure occurred near a boring location for which no prediction was possible because the thickness of zone A sand had not been determined. Five failures occurred that could not be classified as to type failure. Seven flow failures (at five revetment sites) and two shear failures (at two revetment sites) were reported in areas where no borings were located within 500 ft. Three revetment breaches were judged to be the direct result of severe local scour. During 1971, 20 bank failures (13 flow type and 7 shear type) occurred at 14 revetment sites within 500 ft of boring locations for which stability predictions with regard to flow failure had been made. There were nine flow failures near eight boring locations predicted to be unstable and three flow failures near three boring locations predicted to be stable. One other flow failure was reported near a boring location for which no prediction was possible because boring depth was not sufficient and zone A sand had not been penetrated. One shear failure was reported near a boring location predicted to be unstable with regard to flow failure, and six shear failures were reported near six boring locations predicted to be stable. Seven flow failures (at six revetment sites) and five shear failures (at four revetment sites) occurred where no borings were located within 500 ft. One revetment breach was judged to be the direct result of severe local scour. From 1954 (when riverbank stability predictions were initiated) through 1971, 1935 boring locations at 181 revetment sites on the Mississippi River have been studied. The majority of the 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 beginning in 1968 from the New Orleans District are included herein. Flow failures reported through 1971 have occurred within 500 ft of 19 boring locations in the Memphis District and 106 boring locations in the Vicksburg District. Of these, 98 occurred near locations that had been predicted to be unstable according to modified criteria, 15 occurred at boring locations predicted to be stable, and 10 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 125 flow failures recorded since 1954 within 500 ft of analyzed borings, only 15 (12 percent) 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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