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20080627103003.pdfRY LIU & ASSOCIATES, INC. C Geotechnical Engineering Engineering Geology Earth Science April 14, 2008 Mr. John Cooper 7306 - 164"' Place SW Edmonds, WA 98037 Dear Mr. Cooper: Subject: Responses to Comments in Landau Associates 4/8/08 Memo Landslide Investigation and Stabilization Recommendations John Cooper Residence 7306 - 164' Place SW Edmonds, Washington L&A Job No. 8A010 APR 1 Z 2Hj. LIWILDING DEPARTMENT CM/ OF EDMONDS Regarding the comments in Landau Associates 4/8/08 Technical Memo, we have the following responses: 1. Slope Stability The goal of our 3/4/08 geotechnical report and our slide repair design is to determine the causes of the 12/3/07 slide, recommend a slide repair that can be implemented on the steep slope, and design an economically and safe repair plan to stabilize the slide stricken slope. We believe we have achieved that goal. As presented in our 3/4/08 geotechnical report, the slope behind the house is underlain at shallow depth by fresh advance outwash deposits at shallow depth. These deposits are of high shear strength and high resistance against slope failure. Therefore, the potential for deep-seated landslides cutting into these competent basal soils on the slope should be minimal. The 12/3/07 slide on the slope was more of a mudflow occurred in the surflcial weak topsoil and weathered soils from erosion by a large amount of roof runoff discharged during the 100 -year 12/3/07 rainstorm from a tightline that was open-ended on the slope at a 19213 Kenlake Place NE Seattle, WA 98155-3242 Phone (425) 483-9134 Fax (425) 486-2746 April 14, 2008 Mr. John Cooper L&A Job No. 8A010 Page 2 point within the slide zone. To the best of our knowledge there had been no known slides on the slope prior to the 12/3/07 slide, although creeping of surficial soils on the slope caused by release of roof runoff has continued for years. It is our opinion that the slope will be stabilized after the proposed slide repair is completed. 2. Design Earth Pressure The walls are to be backfilled with geofoam blocks to within about two feet of the finish grade of the wall backslope. The idea of using geofoam blocks was to reduce pressure on the walls and the loading on the slope, as geofoam weighs at about one-fourth to one-fifth of compacted structural fill. Also, the backslopes of the walls will be at about 4H:1V. Therefore, the design soil pressure of 35 pcf EFD is more than justified. The design of the walls was based on equilibrium analysis, with the safety factors included in the use of allowable bending stress of steel pipes (0.6 f,,) and adding 40% to pile embedment based on equilibrium analysis. Based on the analysis for the tallest wall, pile embedment at equilibrium is 4 feet, with 40% added is 5.6 feet. Minimum pile embedment of 8 feet is specified on Drawing SS -1. 3. Helical Anchors The retaining walls are only up to 5 to 6 feet tall, and the free length required for the helical anchors should be no more than 4 to 5 feet. The free length of helical anchors is already shown on Drawing SS -1 to be 8 feet minimum. 4. Roof Drain Tightline The roof drain tightline may consist of 4 -inch, solid -wall, Schedule 80, PVC pipes with glued water -tight joints. It should go under the walls then lie on the surface of the slope below the walls, and should be anchored to the slope per the sketch attached shown on revised Drawing SS -1. The tightline should be tied to an existing storm sewer line under the street below the LIU & ASSOCIATES, INC. April 14, 2008 Mr. John Cooper L&A Job No. 8A010 Page 3 slope if available. Otherwise, it should be discharged onto a rock spall pad at the base of the slope to dissipate energy and minimize erosion. 5. Re -vegetation Already covered on Page 8 of our 314108 geotechnical report. 6. Construction Inspection We agree that construction of slide repair work should be inspected by a geotechnical engineer. The scope and items of inspection should be stipulated and outlined by City of Edmonds. Please call if you have questions. r:x?iRFs 7117! Very truly yours, LIU kian) C J. S. (JLiu,Ph.D., P.E. Consulting Geotechnical Engineer LIU & ASSOCIATES, INC.