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1996 Landau post-landslide geotech report.pdfLANDAU ASSOCIATES, INC. Environmental and Geotechnical Services February 22, 1996 City of Edmonds, Community Services Department 250 Fifth Avenue North Edmonds, WA 98020 Attn: Mr. Jim Walker, P.E., City Engineer RE: SUMMARY LETTER REPORT SLOPE FAILURE AT 9207 OLYMPIC VIEW DRIVE EDMONDS, WASHINGTON Dear Mr. Walker: This letter presents the results of our observations and opinions regarding the slope failure at the subject site. This letter has been prepared based on our general knowledge of soil conditions in the area and as further described in the report by Nelson-Couverette and Associates, Inc. titled Geotechnical Evaluation Report for Vista Del Mar Subdivision, Edmonds, Washington, dated April 26, 1993, and our recent observations of the site. SITE CONDITIONS The undersigned representatives of Landau Associates Inc. viewed conditions on February 16, 1996. We were accompanied by Jim Walker and Jeannine Graf from the City of Edmonds. The topography of the subject property consists of a steep slope that forms a bluff overlooking Puget Sound and an upland area of relatively gentle to moderate slopes. The predominant soil type in the area consists of dense, fine to medium sand with occasional gravelly and coarser sand layers. This soil is classified as advance glacial outwash and is referred to as Esperance Sand. The sand was overridden and compacted by the weight of thick glacial ice. A small single family residence is located on the upland portion and is set back from the edge of the slope by about 30 ft. It is our understanding that an 8-inch diameter sanitary sewer line servicing the houses along the bluff is typically located about 10 ft back from the top of the slope. The slide is located on the face of the bluff. In the area of the slide, the bluff has a relatively uniform steep slope and has a slight hollow shape. The top of the slide extends up 02/22/96 JA074\059\JWALKER.LET P.O. 13OX 1029 • EDMONDS, WA 98020-9129 • (206) 778-0907 • FAX (206) 778-6409 SPOKAM!: 009) 327 9737 • FAX (509) 327-9691 /'IA('OMA: (2061 926 2-193 • I-AX (206) 926-2531 to, or very near, the crest of the slope. The material that slid appears to have been limited to a zone of less competent surface soil located on the bluff face. The depth of the material that slid appears to be typically on the order of one to two feet below the previously existing slope face. Except for the slide area, the ground surface is covered with dense, natural vegetation. The general nature and apparent age of the adjacent vegetation suggests that sliding has not occurred in this area in recent years. According to comments made by a local resident to a member of our firm, this portion of the bluff has apparently not experienced landsliding for at least the last 30 years. Clearing for a new residential subdivision is currently in progress on land located east and up slope from the subject site. A potential concern was raised that the activity on this adjacent site may have contributed to the slope failure by increasing or concentrating overland flow or subsurface seepage toward the slide area. However, no signs of overland flow (such as continuing seepage, mud or silt deposits, etc.) from the property onto the adjacent road surface was observed at the time of our visit. Sheet flow pattern across the pavement was identified, but it was observed that a small asphalt berm along the west edge of the road appears to direct the runoff into a landscaped area where the water appears to readily infiltrate into the ground. This infiltration point was located north of the slide area, a sufficient distance that is unlikely to contribute water to the slide area. CONCLUSIONS Based on our observations, the slide was most likely triggered by the combination of recent freezing temperatures followed by substantial rainfall. This weather pattern had the combined effect of weakening and loosening the surficial material while at the same time increasing its weight. Similar slope failures occurred through out the Puget Sound area as a result of the recent near -record rainfall. The failure was located within a zone of loosened surface soil. This type of slide failure can be attributed to natural erosion and weathering processes and is typical of the material and steep slopes found in the area. We did not observe any indication that surface runoff, either from the site itself or from the recently cleared land to the east, directly contributed to the failure. No signs of surface water erosion on the face of the slide scarp were evident. Similarly, signs of subsurface seepage within the upper slide zone were also not apparent. The underlying dense sand appears to be stable; signs of ongoing movement were not observed. However, an accelerated rate of surface erosion and sloughing in the slide area will continue until a vegetative cover is re-established. 02/22/96 J:\074\059\JWALKER.LET 2 LANDAU ASSOCIATES, INC. At this time, it does not appear that City of Edmonds' sewer line is impacted or threatened by the slide. Periodically, this area should be visually monitored until stabilizing vegetation is re-established so that progressive erosion does not continue to move back the face of the slope and threaten the sewer line. USE OF THIS REPORT Landau Associates, Inc. has prepared this letter report for the exclusive use of City of Edmonds for specific application to the recent surface slide located at 9207 Olympic View Drive. The conclusions contained in this letter are based, in part, upon our observations of the slide, our general knowledge of soil conditions in the area, and data from nearby explorations completed by others. There may be variations in subsurface conditions at the site beyond those described. The precise behavior of the bluff is difficult to accurately judge because it is dependent on a number of factors that are difficult to predict. Because the slopes forming the bluff along Puget Sound are only marginally stable, Landau Associates cannot provide assurance that additional sliding and/or instability will not occur in the area. Our services have been conducted in accordance with generally accepted practices of the geotechnical engineering profession; no other warranty, express or implied, is made as to the professional advice included in this report. We appreciate the opportunity to provide these services and look forward to assisting you in the future. Please contact us if you have any questions regarding the information contained in this letter. LANDAU ASSOCIATES, INC. By: Dennis R. Stettler, P.E. Principal and Timothy D. Huntting, P.E. Project Engineer DRS/TDH/skd No. 74059.10 cc: Ms. Jeannine Graf, City of Edmonds, Building Official 02/22/96 JA074\059\JWALKER.LET 3 LANDAU ASSOCIATES. INC.