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1141 Viewland Geotech Addendum 20190294E001-6-La s s o c i a t e d earth sciences i n c o r p o r a t e d April 3, 2020 Project No. 20190294EO01 Terri Maupin and Pete Crouch 1141 Vineland Way Edmonds, Washington 98020 Subject: Response to City of Edmonds Review Comments Maupin-Crouch Residence 1141 Vineland Way Edmonds, Washington BLD2020-0349 Reference: "Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report, Maupin-Crouch Residence," prepared by Associated Earth Sciences, Inc. (AESI) for Terri Maupin and Pete Crouch, dated January 10, 2020. Dear Terri Maupin and Pete Crouch: This letter has been prepared in response to a comment received via email from Denise Nelson with the City of Edmonds on March 26, 2020, regarding the virtual intake appointment for the subject project. Our response is based on the findings of our previous work and subsequent email correspondence with Denise Nelson and Kernen Lien of the City of Edmonds. Comment Geotech does not address all of the criteria in ECDC 23.80.060 and 23.80.070. A geotech addendum specifically addressing the criteria in ECDC 23.80.060 and 23.80.070 will be required. Response to Comment Each item in Edmonds Community Development Code (ECDC) 23.80.060 and 23.80.070 is listed below (in bold type), along with our comments addressing each listed code item. Kirkland i Tacoma i Mount Vernon 425-827-7701 i www.aesgeo.com ECDC 23.80.060 Development Standards — General Requirements A. Alterations of geologically hazardous areas or associated buffers may only occur for activities that: 1. Will not increase the threat of geological hazard to adjacent properties beyond pre -development conditions. An existing home is located on the property. The project consists of demolition of the existing home and construction of a new home. The footprint of the new home will overlie the footprint of the existing home. Slope stability analyses completed for our previous study indicate that low factors of safety exist under seismic conditions for the steep slope located below the existing and proposed homes at the west end of the site. The setback of the new home from the top of this steep slope will be identical to the top of slope setback of the existing home. The existing home is founded on shallow spread footings. The new home will be founded on small -diameter pipe piles that will transfer building loads to the competent, glacially consolidated sediments located at least 13 to 29 feet below the ground surface. Given the comparable top of slope setbacks for the existing and proposed homes, and the use of a pipe pile foundation for the new home, it is our opinion that the project will not increase landslide or other geologic hazard risk on the steep slope at the west end of the property, or in other areas of the site beyond the pre-existing condition. 2. Will not adversely impact other critical areas. The inclinations of the slopes located at the western and eastern ends of the property exceed 40 percent over a height of at least 10 feet and, therefore, classify as Landslide Hazard Areas under the ECDC. These slopes also classify as Erosion Hazard Areas under the ECDC. Construction for the proposed project will be limited to the relatively flat -lying portion of the site between these two slopes. Construction access will be from the south, resulting in no disturbance of the steep slopes. Because the steep slopes will not be altered, the project will not adversely impact erosion hazard risk in the critical areas. Landslide hazard risk will be mitigated by building setbacks, drainage practices which will avoid discharge of water on or above the steep slope areas, and the use of a pipe pile foundation that will transfer building loads to the competent, glacially consolidated sediments at depth. To our knowledge, the Landslide and Erosion Hazard Areas at the west and east ends of the parcel are the only critical areas on the site. 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions. As described in Item Nos. 1 and 2 (above), landslide and erosion hazards for the project will be mitigated by avoiding alteration/disturbance of the steep slopes through building setbacks, drainage practices, and use of a deep foundation system. Erosion and N landslide hazard risks will be mitigated to levels equal to or less than the existing conditions. 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist licensed in the state of Washington. It is our opinion that the proposed project mitigates landslide and erosion hazard risks by avoiding alteration/disturbance of the steep slopes, establishing suitable building setbacks, using prudent drainage practices that avoid discharge of water on or above steep slopes, and the use of a deep foundation system. Provided that the recommendations presented in our January 2020 geotechnical report are properly followed, it is our opinion that the project is safe as designed. B. Critical Facilities Prohibited. Critical facilities shall not be sited within geologically hazardous areas unless there is not other practical alternative. The project consists of construction of a new single-family home. It does not involve construction of a critical facility. The new home will be located outside of the geologically hazardous areas located at the east and west ends of the site, and disturbance of these critical areas will be avoided during construction. ECDC 23.80.070 Development Standards — Specific Hazards A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.060 and the specific following requirements: 1. Minimum Building Setbacks. The minimum setback shall be the distance required to ensure the proposed structure will not be at risk from landslides for the life of the structure, considered to be 120 years, and will not cause an increased risk of landslides taking place on or off the site. A setback shall be established from all edges of landslide hazard areas. The size of the setback shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by the development, based upon review of and concurrence with a critical areas report prepared by a qualified professional. Slope stability analyses completed for our January 2020 geotechnical report indicated that during a strong seismic event, theoretical failure surfaces on the western slope extend under the footprint of the existing and proposed house with low factors of safety (less than 1.2). Details regarding the slope stability analysis were provided in our January 2020 geotechnical report. In order to mitigate landslide hazards associated with the western slope, we recommend a minimum building setback from the top of this 91 slope of 10 feet, and support of the proposed building on small -diameter pipe piles. Use of a pipe pile foundation will avoid surcharging the slope with building loads by transferring the loads to the competent, glacially consolidated sediments encountered in our borings at depths of approximately 13 to 29 feet. Review of the civil plans for the project indicate that the top of slope setback for the proposed building will be approximately 19 feet. 2. Buffer Requirements. A buffer may be established with specific requirements and limitations, including but not limited to drainage, grading, irrigation, and vegetation. Buffer requirements shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by activities within the buffer area, based upon review of and concurrence with a critical areas report prepared by a qualified professional. Provided the recommendations in our January 2020 geotechnical report are properly followed, it is our opinion that no adverse impact will result from elimination of buffers associated with the steep slopes at the eastern and western ends of the site. These steep slopes lie outside of the disturbance limits for the project. 3. Alterations. Alterations of an erosion or landslide hazard area, minimum building setback and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. b. The alteration will not decrease slope stability on adjacent properties; and, c. Such alterations will not adversely impact other critical areas. The project will not result in alteration of the Landslide and Erosion Hazard Areas on the site. 4. Design Standards within Erosion and Landslide Hazard Areas a. The proposed development shall not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. If stability at the proposed development site 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. 21 Slope stability analysis of the steep slopes at the eastern and western ends of the parcel were completed for our January 2020 report. The analyses indicated that minimum factors of safety exceeded 1.5 for static conditions in both slope areas and the minimum factor of safety for the eastern slope under seismic conditions exceeded 1.2. The minimum calculated factor of safety for the western slope was below 1.2 under seismic conditions. In order to mitigate landslide hazards associated with the western slope under seismic conditions, we recommend that the proposed house be founded on small -diameter pipe piles. Use of a pipe pile foundation will transfer building loads to the competent, glacially consolidated sediments located below the elevation of the toe of the western steep slope, thereby avoiding surcharging the slope with building loads. Further mitigation of landslide hazards in this area will be provided by setting back the proposed home approximately 19 feet from the top of the steep slope, and by constructing a stormwater drainage system that avoids discharge of runoff on or above the steep slopes. b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas. The project will be limited to the construction of a single home, which will be located in the most naturally stable portion of the site. c. Structures and improvements shall minimize alterations to the natural contour of the slope, and foundations shall be tiered where possible to conform to existing topography. The proposed home will be located in a relatively flat -lying area located between the upper (eastern) and lower (western) steep slopes. Earthwork for construction of the home is anticipated to be limited to shallow excavation for construction of building foundation elements, driving pipe piles, installation of below grade utilities, and subgrade preparation for patio/pavement areas. d. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation. The Landslide and Erosion Hazard Areas lie outside of the disturbance limits depicted on the civil plans. e. The proposed project shall not result in greater risk or a need for increased buffers on neighboring properties. The existing home is founded on a shallow, spread footing foundation. The proposed project will reduce landslide hazard risk by supporting the new home on driven pipe piles that will transfer building loads to the competent sediments at depth, thereby avoiding surcharge loads on the steep slope at the west end of the site. Disturbance A of the steep slopes on the site will be avoided during construction. For these reasons, the proposed project will not result in increased landslide or erosion hazard risk or a need for increased buffers on or off the subject site. f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes. The project will not include alteration of the steep slopes on the site. Construction will be limited to the relatively flat -lying area in the central portion of the site and grade changes associated with the project are anticipated to be minimal. g. Development shall be designed to minimize impervious lot coverage. The project will utilize pervious pavers to minimize impervious lot coverage. S. Vegetation Retention. Unless otherwise provided or as part of an approved alteration, removal of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited. The project will not involve removal of vegetation from the Landslide or Erosion Hazard Areas at the site. 6. Seasonal Restriction. No work is planned in the Landslide or Erosion Hazard Area. 7. Point Discharges Review of the civil plans indicates that water from footing drains and roof downspouts will be discharged to the municipal storm drain system through a perforated stub -out connection. The perforated stub -out will be located east of the proposed house, approximately 140 feet from the top of the western steep slope. Given the high silt content of the soils encountered in the upper portion of boring EB-1, infiltration rates achievable in these sediments are anticipated to be low. For this reason, and the distance between the perforated stub -out and the top of the steep slope, it is our opinion that landslide risk associated with the use a perforated stub -out connection in this area is low. Runoff collected in area drains and the patio underdrain system will bypass the perforated stub -out connection and discharge to the municipal storm drain system. No stormwater discharge is planned on or above the steep slopes. 8. Prohibited Development. On -site sewage disposal systems, including drain fields, shall be prohibited within Erosion and Landslide Hazard Areas and related buffers. The proposed home will be served by the municipal sewer system. [1 B. Earth Subsidence and Landslide Hazard Area. In addition to the requirements of this chapter, development proposals for lands located within the earth subsidence and landslide hazard area indicated on the critical areas inventory shall be subject to the provisions of Chapter 19.10 ECDC. The proposed development is not located in the Earth Subsidence and Landslide Hazard Area. C. Seismic Hazard Areas The site is not located in a Seismic Hazard Area. We appreciate this opportunity to have been of service to you with your project. Should you have any questions, or require additional information, please do not hesitate to call. Sincerely, ASSOCIATED EARTH SCIENCES, INC. Kirkland, Washington l sT/y_ Timo..fiy J.�in!i er,L.E.G., L.Hg. Senior Enbeering Geologist TJ P/ms - 20190294E001-6 BL I'T �oF\N AS H,ti 1SJP�� 0/0 ON Bruce L. Blyton, P.E. Senior Principal Engineer VA