Assessing landslide potential on coastal bluffs near Mukilteo, Washington—Geologic site characterization for hydrologic monitoring

Open-File Report 2016-1082
Prepared as part of a Technical Assistance Agreement with Sound Transit
By: , and 

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Abstract

During the summer 2015, the U.S. Geological Survey collected geologic and geotechnical data for two sites on coastal bluffs along the eastern shore of Puget Sound, Washington. The U.S. Geological Survey also installed hydrologic instrumentation at the sites and collected specimens for laboratory testing. The two sites are located on City of Mukilteo open-space land and are about 0.6 kilometers apart. The bluffs at each site are approximately 42 meters high, and rise steeply from the shoreline with 32–35° slopes. The more northerly of the two sites occupies an active landslide and is mostly unvegetated. The other site is forested, and although stable during the preparation of this report, shows evidence of historical and potential landslide activity. The slopes of the bluffs at both sites are mantled by a thin, nonuniform colluvium underlain by clay-rich glacial deposits and tills of the Whidbey Formation or Double Bluff Drift. Till consisting of sand, gravel, and cobbles caps the bluffs and rests on finer grained glacial deposits of sand, silt, and clay. These types of different glacial deposits are dense, vertically fractured, and generally have low permeability, but field observations indicate that locally the deposits are sufficiently permeable to allow lateral flow of water along fractures and subhorizontal boundaries between deposits of different texture. Laboratory tests indicate that many of the deposits are highly plastic, with low hydraulic conductivity, and moderate shear strength. Steep slopes combined with the strength and hydraulic characteristics of the deposits leave the bluffs prone to slope instability, particularly during the wet season when infiltrating rainfall changes moisture content, pore-water pressure, and effective stress within the hillslope. The instrumentation was designed to primarily observe rainfall variability and hydrologic changes in the subsurface that can affect stability of the bluffs, and also to compare the hydrologic response between areas where previous landslides have disturbed vegetation and areas where the bluff is apparently more stable and well vegetated.

Suggested Citation

Mirus, B.B., Smith, J.B., Stark, Benjamin, Lewis, York, Michel, Abigail, and Baum, R.L., 2016, Assessing landslide potential on coastal bluffs near Mukilteo, Washington—Geologic site characterization for hydrologic monitoring: U.S. Geological Survey Open-File Report 2016–1082, 28 p., http://dx.doi.org/10.3133/ofr20161082.

ISSN: 2331-1258 (online)

Study Area

Table of Contents

  • Preface
  • Acknowledgments
  • Abstract
  • Introduction
  • Geologic Site Conditions
  • Field Instrumentation
  • Laboratory Analyses of Colluvium and Glacial Sediments
  • Discussion
  • Summary
  • References

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Assessing landslide potential on coastal bluffs near Mukilteo, Washington—Geologic site characterization for hydrologic monitoring
Series title Open-File Report
Series number 2016-1082
DOI 10.3133/ofr20161082
Year Published 2016
Language English
Publisher U.S. Geological Survey
Publisher location Reston VA
Contributing office(s) Geologic Hazards Science Center
Description Report: vi, 34 p. HTML Document: Data Release
First page 1
Last page 28
Country United States
State Washington
Other Geospatial Puget Sound
Online Only (Y/N) Y
Additional Online Files (Y/N) N