Loading...
REVIEWED PLN_BLD BLD2020-0750+Geotech Report+1.4.2021_10.35.02_PM+1973171NELSON GEOTECHNICAL ASSOCIATES. INC. 17311-1351h Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com MEMORANDUM DATE: November 10, 2020 TO: Mr. Chris Foster FROM: Khaled M. Shawish, PE Katelyn S. Brower, GIT RE: Geotechnical Comment Response Memorandum - REVISED Foster Deck Rebuild 16211— 70th Place West Edmonds, Washington NGA File No. 1151OB20 This memorandum presents the results of our geotechnical engineering review of the proposed deck rebuild project located 16211 — 701h Place West, Edmonds, Washington. We previously prepared a geotechnical engineering letter for the project and supplemental memo dated February 27, 2020 and April 1, 2020, respectively. The project includes the replacement of an existing double deck and walkway situated above the residence located at 16211— 70th Place West in Edmonds, Washington. In the following section, we summarize the geotechnical comment raised by the City of Edmonds followed by our response to the concern. CITY OF EDMONDS REVIEW COMMENTS AND RESPONSES "Address... the proposal (including the replacement of the existing walkway) and ECDC 23.80.060 and ECDC 23.80.070.A.1-7. Since we have entered into October, the geotech will also need to specifically address item #6 seasonal restrictions. Pursuant to ECDC 23.80.070.A.7 point discharge onto a steep slope is prohibited, please address." Response: Based on our understanding, the project proposal includes the replacement of an existing double deck and walkway located at the north-northeast side of the residence, situated above a steep slope in the backyard of the site. The existing deck structure has experienced settlement and is in poor condition. It will be replaced with a new double -deck structure placed on drilled, sonotube foundations, with lateral support provided by battered piles. Geotechnical Comment Response Memo — REVISED NGA File No. 1151OB20 Foster Deck Rebuild November 10, 2020 Edmonds, Washington Page 2 Responses to the Edmonds Community Development Code (ECDC) Section 23.80.060: 1. Activities... will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions. The structure is being replaced within the same footprint of the existing structure, and loading conditions do not pose additional exposure of slope stability hazards to adjacent properties. 2. Activities... will not adversely impact other critical areas. The structure is being replaced within the same footprint and drainage patterns will be minimally altered as a result of the project. The project will not adversely impact other critical areas in the vicinity of the site. 3. Activities... are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions. In our previous report, we did not recommend a minimum setback from the top of slope to the deck. We were informed the deck was to be rebuilt similar to the existing structure and walkway, generally maintaining the existing setback. Due to limited site configuration, and to provide an effective setback from the face of the slope, we recommended a minimum embedment of the deck foundations into native soils. The recommended deeper foundation support systems as outlined in our previous letter should provide adequate support for the deck and allow for normal slope recession during a reasonable life span of the structures provided that the site slopes are not disturbed during or after construction. Along with the lateral support provided by the planned battered piles, we believe this should be sufficient. 4. Activities... are certified as safe as designed and under anticipated conditions by a qualified engineer. 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. Responses to subsections of the ECDC Section 23.80.070 (A): 1. 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. Setbacks and effective setback recommendations have been outlined in the geotechnical report, and have been incorporated into the project design. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Memo — REVISED NGA File No. 11510B20 Foster Deck Rebuild November 10, 2020 Edmonds, Washington Page 3 2. Buffer Requirements. A buffer may be established with specific requirements and limitations, including but not limited to, drainage, grading, irrigation, and vegetation. Buffer widths have not been altered by the project design, since the proposed development will be within the same footprint of the existing structures. Drainage patterns and existing vegetation will not be significantly altered by the proposed development. 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. The proposed development will not alter erosion or landslide hazard areas, or buffers, but the proposed development with the effective setback recommendations 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. The proposed development will not substantially change the slope stability for the site or adjacent properties from existing conditions. c. Such alterations will not adversely impact other critical areas. The structure is being replaced within the same footprint and drainage patterns will be minimally altered as a result of the project. The project will not adversely impact other critical areas in the vicinity of 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. b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas. 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. d. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation. Development has been designed within the same footprint as the existing deck, and will be founded on the native soils that are interpreted to form the core of the steep slopes at depth. Therefore, the factor of safety for landslide occurrences should not decrease below 1.5 for static conditions and 1.2 for dynamic conditions. The proposed deck redevelopment will not affect the existing topography within the steeply sloping areas of the site which meet the criteria for designation as a hazard area. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Memo — REVISED NGA File No. 1151OB20 Foster Deck Rebuild November 10, 2020 Edmonds, Washington Page 4 e. 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. f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes; and g. Development shall be designed to minimize impervious lot coverage. Retaining walls will not be utilized in the design, and existing slopes will not be altered by the proposed development. Furthermore, proposed development will not significantly increase impervious lot coverage compared to preexisting conditions. 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. No vegetation will be removed as part of the proposed development. 6. Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year... If possible, construction should take place during the dry season. We anticipate that ground -disturbing activities will occur after May 1'. However, the project will not result in substantial grading. If permitted by the Director, a seasonal grading extension to stabilize 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. 7. 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: a. Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge,- b. . 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 c. 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. Although it is our opinion that the drainage patterns on the site will be minimally altered as a result of the planned project, we recommended all runoff generated within the site including roof downspouts, yard areas, and hard surfaces should be collected into catch basins and yard drains and tightlined into an approved stormwater management system or to discharge at the base of the slope. Under no circumstances should runoff be allowed to concentrate on the slope either during construction or after construction has been completed. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Memo — REVISED Foster Deck Rebuild Edmonds, Washington NGA File No. 11510B20 November 10, 2020 Page 5 CONSTRUCTION MONITORING We should be retained to provide construction monitoring services during the earthwork phase of the project, specifically to observe any wet season work, sonotube and pile installation, to evaluate embedment for effective setbacks, monitor slope conditions, excavations, and drainage system installation during construction. We trust this memorandum should satisfy your needs at this time. Please contact us if you have any questions or require additional services. # NELSON GEOTECHNICAL ASSOCIATES, INC.