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APPROVED PLN RESUB 1-BLD2022-0926+Geotechnical_Report+3.7.2023_12.59.35_PM+3406572COBALT G E 0 S C I E N C E S November 28, 2022 Landsverk Quality Homes Attn: Mr. Joseph Rowett josephPlgh-inc.com RE: Geotechnical Addendum Proposed Development 950019oth Street SW Parcel No.'s 004346000006io2,104, 105, & 1o6 Edmonds, Washington RESUB Mar07 2023 CITV OF EDMONDS DEVELOPMENTSERVICES DEPARTMENT BLD2022-0926 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 In accordance with your authorization, Cobalt Geosciences, LLC has prepared an addendum letter for the project. Steep Slope/Landslide Hazards There are local slopes near the property that have magnitudes of at least 40 percent and relief of more than 10 feet. Per the City of Edmonds GIS maps, areas that have magnitudes of 40 percent or more and relief of at least 10 feet are located near the north property line and elsewhere on properties to the northwest and northeast of the subject property (within 50 feet of the subject property). Areas within the property do not meet the criteria of steep slope or landslide hazards since their vertical relief is less than 10 feet. These areas can be safely benched and graded as part of site development. Overall, the steep slope areas on adjacent parcels appear stable at this time with no evidence of instability or erosion. Note that we observed these areas from the subject site and aerial photographs. The total relief is about 15 feet for slopes to the northwest of the site and relief of about 40 feet for slopes to the north and northeast of the site. While these areas consist of steep slope hazards, they do not exhibit evidence or characteristics of landslide hazard areas. Code Information 23.80.o6o 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 the geological hazard to adjacent properties beyond predevelopment conditions; 2. Will not adversely impact other critical areas; 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. APPROVED BY PLANNING www.cobaltgeo.com (2o6) 331-1097 November 28, 2022 Page 2 of 4 Geotechnical Addendum The project will include local grading and lot creation within moderately steep slope areas and for the north lot, near local steep slope hazard areas. It is our opinion that this work will not affect global or local stability provided work is monitored by the geotechnical engineer and erosion control measures are in place during construction. It will be necessary to bench and key anX existing slope areas with new structural fills (where proposed) and construction of properlX designed retaining walls where proposed and necessary. Similarly._ grading can be completed near the mapped steep slope hazard areas near the north propeM line provided any temporary excavations are protected from erosion and are created per our recommendations. These areas, if proposed to be graded, may be faced with retaining walls if necessarylproposed. The steep slope hazard areas appear to terminate at the north property line. The slope areas are cross gradient (do not slope toward the property) and it is our opinion that no buffer or setback is required from this area. However, we must review the final plans to verify that site grading and development in these areas do not require shoring or other design considerations, such as shoring. The proposed construction can be completed with no increase of the threat of geologic hazards on adjacent properties, will not impact other critical areas, and can be considered safe as designed under anticipated conditions. Again, we should be provided with the grading and development plans for this area to confirm suitability. 23.80.07o 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.o6o, Development standards — General requirements, and the specific following requirements: 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. 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; No specific setback is required from steep slope areas. Properly designed walls or graded areas are generally suitable near the north property line and closest steep slope areas. 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; No buffer is required from the localized steep slope hazard area near the north property line. We anticipate that new homes will be located at least.-, feet from the north property line and hazard areas. Proper erosion control, temporary cuts, and permanent walls or graded areas are anticipated and required. www.cobaltgeo.com (2o6) 331-1097 November 28, 2022 Page 3 of 4 Geotechnical Addendum 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; Provided earthwork activities are performed in accordance with the approved plans, all runoff is fully controlled, and periodic geotechnical oversight is performed, the development will not decrease slope stability on adjacent properties, will not increase surface water discharge or sedimentation beyond current levels, and will not impact other critical areas. Temporary and permanent erosion and sediment control devices should be in at all times during construction. The steep slope hazard areas themselves will not be disturbed or graded. 4. Design Standards within Erosion and Landslide Hazard Areas. Development within an erosion or landslide hazard area and/or buffer shall be designed to meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides greater long-term slope stability while meeting all other provisions of this title. The requirement for long-term slope stability shall exclude designs that require regular and periodic maintenance to maintain their level of function. The basic development design standards are: 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; 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; e. The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties; 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; The very limited project scope does not pose a risk to critical areas or the need to increase buffers on adjacent properties. Essentially, the current stability will not be affected by the proposed construction since the proposed grading and slope stability will be equal to what is currently present once completed (or more stable). Factors of safety against landslide movements are well above minimum values based on the soil types, topography, and locations of the development. www.cobaltgeo.com (2o6) 331-1097 November 28, 2022 Page 4 of 4 Geotechnical Addendum 5. 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; 6. 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; 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; and We concur with the above code items and have no additional comments at this time. We should be on site to verify aspects of the construction. These include but are not limited to soil bearing for footings, vault excavation and backfill placement, footing drainage, fill compaction, slab on grade and road preparation, temporary excavations, erosion control, and drainage systems. Sincerely, Cobalt Geosciences, LLC HONry9 06 r •1 y`. �� 54896 ��FssCISTER���\�� ZONAL�- 11/28/2022 Phil Haberman, PE, LG, LEG Principal www.cobaltgeo.com (2o6) 331-1097