Loading...
Skyline Geo LetterCOBALT G E 0 S C I E N C E S March 2, 2020 Corry Morris corrymorris a gmail. com RE: Geotechnical Evaluation Existing Construction 219 Skyline Drive Edmonds, Washington Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 We have prepared this letter to discuss aspects of the recently constructed project at the referenced site with regard to relevant sections of the Edmonds City Code and Community Development Code. Specifically, Comment No. 1 from a City of Edmonds letter dated June lo, 2019 discusses the location of a hot tub within 50 feet of a mapped steep slope area. Site and Project Description The site is located at 219 Skyline Drive in Edmonds, Washington. The property consists of one irregularly shaped parcel (No. 00423000005200) with a total area of about 0.27 acres. The east -central portion of the property is developed with a multi -story single-family residence with attached garage and driveway. The remainder of the property is undeveloped and vegetated with grasses, bushes/shrubs, and sparse trees. There is a hot tub on a concrete slab located in the western portion of the property. The property slopes downward from east to west at magnitudes ranging from about 5 to 45 percent and total relief of about 30 feet. There are localized areas with magnitudes of 30 to 45 percent west of the residence extending further west. These slopes are up to 10 feet in height and developed with graded benches, landscaped areas with boulders, and other landscaping features. There are 2 to 8 feet tall rockery walls along the south and west property lines, facing apparent cut native soils. The hot tub and slab are located in a landscaped area just above a moderately steep slope and short rockery wall. Area Geology The site is underlain by Vashon Glacial Till. Vashon Glacial Till consists of a heterogeneous mixture of silt, sand, clay, and gravel. These deposits are typically dense to very dense below a thin weathered zone. Glacial till is typically impermeable and resistant to global instability. Geologic Hazards We understand that the site contains areas considered to be steep slope hazards by the City of Edmonds municipal code. The slope areas west of the residence have local relief of about 10 feet and have slope magnitudes Of up to 45 percent. These areas are fully vegetated with herbs, bushes, and trees; and locally have rockery walls facing previous cuts. www.cobaltgeo.com (2o6) 331-1097 March 2, 2020 Page 2 of 5 Geotechnical Evaluation Code Items and Discussion 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. The site is situated within a moderately steep slope area that was created over a time through natural erosion along with grading associated with construction of the residence, backyard areas, and adjacent properties. The site is not situated within an active or historic landslide and the risk of landslide activi , is very low. Also, the site and adjacent areas are underlain by Vashon Glacial Till which is resistant to local and global instability. The proposed hot tub and supporting slab will not increase the threat of geologic hazards on adjacent properties, will not impact other critical areas, and are safe as designed under anticipated conditions. The slab is located in a nearly level area that was created through previous legal grading. 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; Based on our observations at the site, the slab is essentially set back about 3 feet from the edge of a moderately steep slope extending downward to the west. The existing location is suitable and will not affect slope stability. Local maintenance of landscaping surface materials and vegetation may be necessary: however, this is a typical situation and not unique to this property or local conditions. 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; www.cobaltgeo.com (2o6) 331-1097 March 2, 2020 Page 3 of 5 Geotechnical Evaluation The site currently fully developed. No specific buffer (in addition to what is present currently) is required from a geotechnical standpoint. We recommend that all sloped areas remain vegetated and protected from erosion through continued maintenance of existing surface landscaping materials/vegetation. 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; Based on our observations of the finished landscaping and site development, the development will not and has not decreased slope stability on adjacent properties, will not and has not increased surface water discharge or sedimentation beyond current levels, and will not/does not impact other critical areas. 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 www.cobaltgeo.com (2o6) 331-1097 March 2, 2020 Page 4 of 5 Geotechnical Evaluation g. Development shall be designed to minimize impervious lot coverage; Based on site topography, underl,g soil conditions, and our experience with similar projects, the estimated static and seismic factors of safM are well above minimum requirements. The factor of safety for landslide activity do not appear to have been or will not be affected by the existing slab/hot tub development. The development does not pose a risk to critical areas or the need to increase buffers on adjacent properties. 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 ist 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. Conclusions & Recommendations It is our opinion that the hot tub and supporting slab are located within a zone of stable soils with no additional setback or buffer from slope or erosion hazard areas. The construction will not and does not adversely affect slope stability or geologic hazards within 200 feet of the property. Closure The information presented herein is based upon professional interpretation utilizing standard practices and a degree of conservatism deemed proper for this project. We emphasize that this report is valid for this project as outlined above and for the current site conditions and should not be used for any other site. This letter does not discuss the potential or likelihood for soil settlement below the slab, if fill is present. Soil settlement due to poor compaction or loose soil removal is not part of our analysis and would be the responsibility of the builder/contractor and/or owner. www.cobaltgeo.com (206) 331-1097 March 2, 2020 Page 5 of 5 Geotechnical Evaluation Sincerely, Cobalt Geosciences, LLC NONY,y OF W ASy�y\� CA. '� y 54896 <<v� 0,�S,s -T ERA `����v ��NAt Exp. 6/26/2020 Phil Haberman, PE, LG, LEG Principal PH/sc 445 2513 22..,. ed Gel', AL I fate 1 1 f lli 1 www.cobaltgeo.com (2o6) 331-1097