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Peat flow in Mercer Slough, Lake Washington

Thomas C. Badger
Peat flow in Mercer Slough, Lake Washington (in Landslides and engineering geology of the Seattle, Washington area, Rex L. Baum (editor), Jonathan W. Godt (editor) and Lynn M. Highland (editor))
Reviews in Engineering Geology (2008) 20: 153-166


Over the past four decades, ongoing deformation of an 18-m-thick peat deposit within the flat-lying Mercer Slough has resulted in damaging deflections, and near-collapse in three cases, of pile-supported Interstate 90 bridges and a major water line on the east side of the slough. The peat is partially underlain by a dense sand unit, which includes a highly pressurized aquifer that produces artesian flow 1-2.5 m above the ground surface. Inclinometers on the east side of the slough show the peat flowing toward the structures and then apparently directed west along the interstate centerline. Large displacements recorded in several inclinometers near the center of the slough suggest a length of deforming peat that approaches 600 m, which is likely initiating retrogressively. Potential causal mechanisms include poor engineering characteristics of the peat, presence of high hydrostatic pressure transmitted within and beneath the peat, seasonal water-level variations of Lake Washington and induced hydraulic gradients within the peat, dredging of the Mercer Slough channel, puncturing of the underlying aquifer by numerous pile foundations, and fill placement along the eastern margin of the slough. The peat is flowing around the pile/shaft foundations; however, excessive lateral loads are still being applied to the foundations in a poorly understood and unpredictable manner. The most severe deflections have occurred in the outermost structures where the peat is primarily flowing transverse to the structures.

ISSN: 0080-2018
EISSN: 2169-799X
Coden: GAEGA4
Serial Title: Reviews in Engineering Geology
Serial Volume: 20
Title: Peat flow in Mercer Slough, Lake Washington
Title: Landslides and engineering geology of the Seattle, Washington area
Author(s): Badger, Thomas C.
Author(s): Baum, Rex L.editor
Author(s): Godt, Jonathan W.editor
Author(s): Highland, Lynn M.editor
Affiliation: Washington State Department of Transportation, Olympia, WA, United States
Affiliation: U. S. Geological Survey, Denver, CO, United States
Pages: 153-166
Published: 2008
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 18
Accession Number: 2009-038124
Categories: Engineering geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch maps
N47°06'00" - N47°46'00", W122°32'60" - W121°04'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200921
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