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REVIEWED PLN BLD2020-1409+GEO MEMO+3.8.2021_1.06.28_PM+2081123NELSON GEOTECHNICAL ASSOCIATES. INC. 17311-1351" Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com MEMORANDUM DATE: March 8, 2021 TO: Chris and Railin Santiago FROM: Khaled M. Shawish, PE Alex B. Rinaldi, LG RE: Geotechnical Comment Response Memorandum Santiago Deck Addition 18308 — 84t" Place West Edmonds, Washington NGA File No. 1225420 This memorandum presents the results of our geotechnical engineering review of the proposed deck addition project located 18308 — 84t" Place West, Edmonds, Washington. We previously prepared a geotechnical engineering letter for the project dated January 6, 2021. The project includes addition of an elevated deck structure along the northwestern portion of the existing residence. We have been requested to respond to the geotechnical comments furnished in a letter prepared by the City of Edmonds, dated January 21, 2021. In the following sections, we summarize the geotechnical comments raised by the City of Edmonds followed by our response. CITY OF EDMONDS REVIEW COMMENTS AND RESPONSES "Please update the January 6, 2021 geotech report to address how the proposed deck complies with the standards in ECDC 23.80.060 and 23.80.070" Edmonds Community Development Code (ECDC) 23.80.060: ECDC 23.80.060 (A) states, "Alterations of geologically hazardous areas or associated buffers may only occur for activities that:" Geotechnical Comment Response Memo NGA File No. 1225420 Santiago Deck Addition March 8, 2021 Edmonds, Washington Page 2 • "Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions." The proposed deck will consist of an approximately 30- by 16-foot rectangular -shaped surface elevated above an existing tool shed and partially attached to the north -central to northwestern portion of the existing residence. The deck will extend approximately 16-feet north of the existing residence and approximately 10-feet west of the western edge of the existing house. The downhill western deck support foundations will be situated along gently sloping terrain east of the steeper slopes designated as landslide hazard areas 40 percent by City of Edmonds GIS map. Based on our understanding of the project plans the proposed deck will require minimal earth disturbance or alteration to existing drainage patterns, therefore it is our opinion that the proposal should not increase the threat of geological hazard to adjacent properties beyond predevelopment conditions, provided the recommendations in our previous report are strictly followed. • Will not adversely impact other critical areas. As we anticipate minimal grading and earthwork will be required during construction, it is our opinion that the proposal will not adversely impact other critical areas in the vicinity of the site, including but not limited to landslide hazard areas and erosion hazard areas. • Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions. Our soil explorations indicated competent native glacial soils at depth beneath surficial fill soils and/or topsoil. Our previous recommendations indicated the proposed deck should be supported on sonotube foundations embedded a minimum of 1-foot into competent glacial material at depth. Provided recommendations for foundation support, drainage, and erosion control measures, as documented in our previous report are followed closely it is our opinion that potential hazard to the project will be minimized to a level equal to or less than predevelopment conditions. • Are certified as safe as designed and under anticipated conditions by a qualified engineer. Based on our understanding of the plans and prevailing geologic conditions, it is our opinion that construction of the planned infrastructure as part of this project should not increase the threat of erosion or landsliding to adjacent properties beyond predevelopment conditions, should not adversely impact other critical areas, is designed so that the hazard to the project is eliminated or mitigated to a level equal or less than predevelopment conditions, and is safe as designed under anticipated conditions. Recommendations provided in our previous report as well as this memorandum should be strictly followed. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Memo NGA File No. 1225420 Santiago Deck Addition March 8, 2021 Edmonds, Washington Page 3 Edmonds Community Development Code (ECDC) 23.80.070: ECDC 23.80.070 (A) states, "Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.060, Development standards — General requirements, and the specific following requirements:" • Minimum Building Setback. 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. According to City of Edmonds GIS webpage the western edge of the proposed deck, extending about 10- feet from the western edge of the house will be setback approximately 5-feet from the mapped landslide hazard area and associated buffer. However, provided the recommendations for foundation support documented in our previous report are followed the effective setback from the base of sonotube foundations to the face of the slope will be approximately 20-feet. This is assuming the shallowest anticipated sonotube depth along the downhill western edge of the deck, which based on our explorations should be approximately 4.5- to 5.0 feet below the existing ground surface to achieve 1-foot of embedment into competent native soils. Along with the previously recommended 1-foot embedment of sonotube foundation into native material, we also recommend the downhill western sonotube foundations extend a minimum of 5-feet below the existing ground surface to ensure adequate effective structure setback from the steeper slopes below are achieved. In our opinion the current structure setback is adequate provided the downhill sonotube foundations are embedded a minimum of 1-foot into competent material, but no less than 5-feet below the existing ground surface and that this setback/layout should provide adequate separation from the steeper slopes below. Furthermore, the deck and deck foundation configuration recommended above and described in our previous report should not put the structure at risk from landslides for the life of the structure and should not increase the risk of landslides taking place on or off the site. • Buffer Requirements. A buffer may be established with specific requirements and limitations, including but not limited to, drainage, grading, irrigation, and vegetation. The proposed structure and associated construction should not significantly alter the slope conditions, and anticipated earthwork disturbance will be minimal and limited to localized excavations for sonotube/deck pier foundations. Drainage patterns and existing vegetation should not be significantly altered by the proposed development. At this time, no additional buffer requirements are recommended provided areas outside and specifically below the proposed deck construction are not disturbed through any clearing, grading, or placement of stockpiles. Any disturbed soils should be promptly vegetated or otherwise protected from erosion as soon as feasible. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Memo Santiago Deck Addition Edmonds, Washington NGA File No. 1225420 March 8, 2021 Page 4 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: The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. We do not anticipate an increase in surface water discharge or sedimentation to adjacent properties as a result of the proposal. Adequately spaced decking along joist framing as well as installation of temporary and permanent erosion protection BMP's should prevent alterations to existing drainage patterns and sedimentation transport to adjacent properties both during and after construction. • The alteration will not decrease slope stability on adjacent properties. Provided the recommendations in our previous report and this memorandum are closely followed it is our opinion that the proposal will not decrease the existing slope stability conditions within the site and on adjacent properties. • Such alterations will not adversely impact other critical areas. Provided the recommendations in our previous report and this memorandum are closely followed it is our opinion that the proposal will not adversely impact other critical areas. • Design Standards within Erosion and Landslide Hazard Areas. 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. The planned deck will include deep sonotube foundations embedded into the native glacial soils along the downhill western side founded within the core of the steep slopes. It is our opinion that if the project is constructed according to plans and specifications, the factor of safety for landslide occurrences should not decrease below 1.5 for static conditions and 1.2 for dynamic conditions. • Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas. The proposed deck structure will be located directly adjacent to the northwestern portion of the house and above steep slope landslide hazard areas within the very western portion of the site to avoid disturbing slopes within and to the west of the property. • 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. No cuts or fills are proposed to the existing topography. Deck post and pier foundations should reduce alterations to the contour of the existing ground surface. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Memo Santiago Deck Addition Edmonds, Washington NGA File No. 1225420 March 8, 2021 Page 5 • Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation. The proposed deck will be situated on gently sloping to relatively level ground surface in order to preserve the critical areas within and in the vicinity of the site and reduce impact on geological and other critical areas in the vicinity. Also, no significant trees are situated within the footprint of the proposed deck and we do not anticipate any tree or significant vegetation removal as part of this project. • The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties. Increased buffers are unnecessary for adjacent property due to the extent of the steep slopes on the site and proposed development as part of this project. • The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes; and No grading is proposed to be completed for the proposed deck addition. • Development shall be designed to minimize impervious lot coverage. We understand that the proposed deck does not contribute to impervious lot coverage. Project construction drawings show the areas beneath the deck to remain as an existing attached tool shed structure and otherwise existing vegetated surfaces. No bare soil areas are to remain after construction is complete. • 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. No vegetation will be removed as part of the proposed development. • Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year; provided, that the director may extend or shorten the dry season on a case -by -case basis depending on actual weather conditions, except that timber harvest, not including brush clearing or stump removal, may be allowed pursuant to an approved forest practice permit issued by the city of Edmonds or the Washington State Department of Natural Resources; If possible, construction should take place during the dry season. We anticipate that ground -disturbing activities will occur after May 15Y. However, the project will not result in substantial grading. If permitted by the Director, a seasonal grading extension to complete the deck may be warranted such that the minor earthwork activities include Best Management Practices as outlined in the previous geotechnical documentation to control erosion and prevent sedimentation during earthwork activities. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Memo Santiago Deck Addition Edmonds, Washington NGA File No. 1225420 March 8, 2021 Page 6 • Point Discharges. Point discharges from surface water facilities and roof drains onto or upstream from an erosion or landslide hazard area shall be prohibited except as follows: • Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge; • Discharged at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels that previously conveyed storm water runoff in the predeveloped state; or • Dispersed discharge upslope of the steep slope onto a low -gradient, undisturbed buffer demonstrated to be adequate to infiltrate all surface and storm water runoff, and where it can be demonstrated that such discharge will not increase the saturation of the slope. The proposal does not include point discharges; adequately spaced decking will ensure drainage patterns within the site are not significantly altered and will not adversely impact existing site stability conditions. MINIMUM RISK STATEMENT Pursuant to ECDC 23.80.060, it is our opinion that construction will not increase the threat of erosion or landsliding to adjacent properties beyond predevelopment conditions, will not adversely impact other critical areas, is designed so that the hazard to the project is eliminated or mitigated to a level equal or less than predevelopment conditions, and is safe as designed under anticipated conditions. We trust that this memorandum should satisfy your needs at this time. Please do not hesitate to contact us should you require anything further. NELSON GEOTECHNICAL ASSOCIATES, INC.