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GEOTECH RESPONSE - ENG.pdfGOC CONSULTANTS, INC.NOV 0 Classico Homes Inc. PO Box 5012 Lynnwood, Washington 98046 Attention: Joe Schmaus Subject: Supplemental Letter Proposed Two Single -Family Residences 532 — 71^ Avenue South Edmonds, Washington Dear Mr. Schmaus: 13256 Northeast 206 Street, Suite 16 5 2015 Bellevue, Washington 98005 (425) 747-5618 FAX(425)747-8561 CITY OF E0100NE) August 20, 2015 JN 15193 Pf'y COPY via email., classicohomes@gmail.com We prepared a geotechnical engineering study for this project dated May 20, 2015. We have been provided with a plan review comment letter prepared by the City of Edmonds dated July 27, 2015. That letter states that the subject site contains and is adjacent to a slope that is steep enough to be considered both a Landslide Hazard Area and Erosion Hazard Area, and that a geotechnical engineer must address applicable sections of the Edmonds Code. Our previous report did not address the code sections because we understood that previous geotechnical studies by others had satisfied the city's requirements. We have prepared this letter to supplement our previous study and address the code sections. The current house layout plan for the western house (dated 8118/15) indicates a setback of about 52 feet from the northern property line and 5 feet from the western property line. The layout for the eastern house (dated 8/19/15) indicates a main house setback of 20.5 feet from the northern property line. These layouts were used in our analysis. SLOPE STABILITYANALYSIS Based on the current building layouts provided, it is our understanding that foundations for the western proposed residences will be set back about 5 feet from the top of the taller steep slopes in the northern and western portions of the site. Decks will extend, north of the residences, but will not extend over the top of the steep northern slope, The eastern structure will be at the crest of the shorter eastern) section of the northern slope. We have conducted a slope stability analysis of the steep slopes and their relationship to the residences using' the computer program SLOPE/W. We used a topographic survey to develop a cross- section of the ground surface. Soil parameters were needed for the analysis, with most significant being the defining strength parameters of the site soils, including angle of internal friction. For the upper loose sands found in the borings, we selected an internal friction value of 32 degrees. For the underlying medium -dense sand, we chose a representative angle of internal friction of 36 degrees, Using the existing slope configuration and appropriate soil parameters, the stability of the slopes was analyzed for both static and dynamic loading conditions. For the dynamic analysis, a peak ground coefficient of acceleration of 0.238 was used. Factors of safety against failures of the northern slope were found to be 1.08 and 0.72 for static and seismic conditions, respectively, Factors of safety against failures of the western slope were found to be 1.21 and 0,74 for static and seismic conditions, respectively. Loads from the piles which will support the portions of the residences and decks adjacent to the northern and western slopes will extend at least 15 feet below the ground surface and will not impact stability of the slopes. Our analyses found that the pile loads did not decrease stability of the steep northern and western slopes. GEOTECH CONSULTANTS, INC. Classico Homes August 20, 2015 CONCLUSIONS AND RECOMMENDATIONS JN15193 Page 2 The proposed building setbacks described above are acceptable provided that the recommendations presented in our geotechnical study are followed. As stated: in our geotechnical report, pipe piles will be used to support the portions of the residences close to the steep northern and western slopes to reduce the potential for potential slope failures to impact the residences,. The pipe piles will be driven at least 15 feet into the underlying soils, and will be connected with structural grade beams, The dense soils that will support the piles are not subject to slope instability due to their density and improved geometry. No fill soils will be placed between the proposed residences and the steep northern and western slopes, and no Stormwater runoff will be directed toward the slopes, Z. -I F111@M1Z=1,=5n; 4 9 6 0 1. 1011141110111"NiM ------ -- i 11, I a IN [I'm tZT6 I IM I The slopes within the northern and western portions of the site are considered Landslide Hazard Areas and Erosion Hazard Areas per Edmonds Code because of their inclination, height, and composition, The code requires a buffer from such areas, equal to the height of the slope or 50 feet, whichever is greater. However, Section 23.80.070.1(b) states that the buffer may be reduced to a minimum of 10 feet when a qualified professional demonstrates that the reduction will adequately protect the proposed development, adjacent developments and users, and the subject critical area. Section 23.80,070.2 states that an erosion or landslide hazard area and/or buffer may only, occur for activities for which a hazards analysis is submitted that shows the proposed development will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions, the development will not decrease slope stability on adjacent properties, and the alterations will not adversely impact other critical areas. In our opinion, these conditions are satisfied by the proposed development plan, Edmonds Code Section 23.80.070.3 states that development within an erosion or landslide hazard area and/or buffer cannot decrease the factor of safety for landslides below 1.5 for static conditions and 1.2 for dynamic conditions. If stability is below, these limits, the proposed development shall provide practicable approaches to reduce risk to human safety and improve, the factor of safety for landsliding. In no case shall the existing factor of safety be reduced for the subject property or adjacent properties. Based on our slope stability analyses and our experience with similar projects in the Puget Sound region, it is our opinion that the proposed residences can be constructed close to the top of the steep northern and western slopes without detrimentally impacting those slopes. Although the existing factors of safety against static and dynamic landslides are below 1.5 and! 1.2, respectively, the proposed development will not decrease those factors of safety. In fact, because a reduced amount of surface water will reach the steep slopes following development, the potential for slope failures will be decreased, In our opinion the proposed development satisfies the requirements of Section 23.40 and 23.80 of Edmonds Code, provided the recommendations in our report are followed. We appreciate the opportunity to be of service on this project. If you have any questions, or if we may be of further service, please do not hesitate to contact us. TRC/JHS:at Respectfully submitted, VP r�or �T GEO ECH CONSULTANTS, INC. James H. Strange, Jr., P.E. Associate GEOTECH CONSULTANTS, INC,