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TitleAn earthflow in sensitive Champlain Sea sediments at Lemieux, Ontario, June 20, 1993, and its impact on the South National River
AuthorEvans, S G; Brooks, G R
SourceCanadian Geotechnical Journal vol. 31, no. 3, 1994 p. 384-394,
Alt SeriesGeological Survey of Canada, Contribution Series 29193
PublisherCanadian Science Publishing
Mediapaper; on-line; digital
File formatpdf
AreaSouth Nation River; Lemieux
Lat/Long WENS -75.5000 -75.0000 45.5000 45.2500
Subjectssurficial geology/geomorphology; landslides; mass wasting; sediment stability; water quality; clays; sensitive clays; Lemieux Landslide; Champlain Sea Sediments; Quaternary
Illustrationssketch maps; photographs
AbstractA large (est. volume 2.8-à 106 m3) landslide occurred in sensitive Leda clay on the east bank of the South Nation River at Lemieux, Ontario (45.4°N, 75.06°W), on June 20, 1993. The earthflow involved an area of about 17-ha and retrogressed a total of 680 m, 555 m into the flat plain above the river. No lives were lost but a motorist was injured when he drove into the landslide crater. The 1993 landslide occurred 4.5 km downstream of the well-known 1971 South Nation River landslide along a stretch of river that had experienced other historical landslides in 1895 and 1910. A band of earlier, undated, retrogressive sliding, between 100 130 m in width, was present at the base of the slope that failed in 1993, and the earthflow was probably triggered by a reactivation of these failures. Borehole information obtained in 1986 and 1987 in the vicinity of the landslide indicates that a zone of soft, sensitive marine clay existed beneath the flat farmland, which was overlain by a stiffer cap consisting of laminated marine-estuarine sands and deltaic silts and sands. The morphology of the debris suggests a mechanism that involves the fluidization of much of the landslide mass and subsidence, translation, and rotation of cap blocks. The stability number for the site was approximately 9.6, suggesting that the flow could have occurred as a result of extrusion of the soft sensitive clay layer due to undrained cap loading. Landslide debris temporarily blocked the South Nation River, causing flooding upstream and adversely affecting water quality downstream. Key words : landslide, earthflow, sensitive clay, debris hazards, water quality.