On December 2, 1977, during a period of unusually heavy rainfall, a landslide involving an area of about 3-1/2 acres occurred near the community of Lexington in southwest Washington. The head scarp of the landslide came within 25 feet of a recently constructed 1 M.G. steel reservoir. The site area, located north of Longview, is underlain by the upper Eocene age Cowlitz Formation. The lithology of the Cowlitz Formation is complex, containing interbedded shallow marine and non-marine sedimentary, volcanic, and pyroclastic rock types. Geotechnical evaluation of the landslide included geologic reconnaissance, drilling and sampling, installation of inclinometer instrumentation, survey hub line monitoring, and office studies. By the time the inclinometer instrumentation had been installed, the slide movement had ceased, even though maximum total vertical displacements in excess of 5 feet had been observed. As a consequence, the location of the failure plane was not confirmed by the inclinometer. The groundwater levels at the time of failure were also unknown. The analysis of the landslide and possible remedial measures required the interpretive skills and experienced professional judgement of both the engineering geologist and geotechnical engineer. Quantitative data collected in the field and laboratory had to be compared, through analytical procedures, with the geology and observed subsurface conditions. The interpretive and analytical processes led to the design of an environmentally compatible key trench buttress to stabilize the landslide, and afford protection to the 1 M.G. water tank from further retrogression of the landslide. Moreover, the buttress had to be located and constructed so as not to adversely affect further residential development. During construction of the key trench buttress, further ground movements occurred. These, combined with other field observations disclosed a complex failure mode; but the validity of the conclusions drawn during the evaluation and design phases of the landslide investigation were confirmed. As important as the sophisticated tools, instruments, and refined analytical techniques are in any landslide investigation, the combined judgement of the engineering geologist and geotechnical engineer, fine tuned by experience, is just as vital to the successful solution of difficult geotechnical problems. (Author)

  • Supplemental Notes:
    • Proceedings of the Eighteenth Annual Engineering Geology and Soils Engineering Symposium, held at the Red Lion Inn, Boise, Idaho.
  • Corporate Authors:

    Idaho Transportation Department

    3311 W State Street, P. O. Box 7129
    Boise, ID  United States  83707-1129

    University of Idaho, Moscow

    Department of Geology and Geological Engineering
    Moscow, ID  United States  83844-3025

    Boise State University

    Department of Geology and Geophysics
    Boise, ID  United States 

    Idaho State University, Pocatello

    Department of Geology
    Pocatello, ID  United States  83201

    Idaho State University, Pocatello

    Department of Geology
    Pocatello, ID  United States  83201
  • Authors:
    • Rippe, A H
    • Kent, M D
  • Conference:
  • Publication Date: 1980-4

Media Info

  • Features: Figures; References; Tables;
  • Pagination: p. 15-31

Subject/Index Terms

Filing Info

  • Accession Number: 00334056
  • Record Type: Publication
  • Created Date: Sep 16 1981 12:00AM