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GEOTECH REPORT ADDENDUM.pdfOctober 12, 2015 File No. 13-207.200 Mr. Mark Loewen Loewen Homes LLC 17201-72 d Avenue West Edmonds, WA 98026 o mio NOV 1115 CITY"o.; k i Subject: Geotechnical Report Addendum Geologic Hazards Assessment Proposed Single -Family Residence 9303 Olympic View Drive, Edmonds, WA Dear Mr. Loewen, I NJmGE@ C O R P O R A T E D Geotechnical & Earthquake Engineering Consultants As requested, PanGEO Inc. conducted further assessment of the geologic hazards at the subject site and its potential impacts of the proposed development. The following sections present results of our assessment, and opinions and conclusions. EROSION HAZARDS Based on soil conditions encountered in the test pits and our field observations, it is our opinion that the soils in the western portion of the site, where is a steep slope, will likely exhibit moderate to high erosion potential. However, in our opinion, the eastern portion of the site, where the proposed development will be located, should have low erosion potential. It is our further opinion that the potential erosion hazards at the site can be effectively mitigated with the best management practice during construction and with properly designed and implemented landscaping for permanent erosion control. During construction, the temporary erosion hazard can be effectively managed with an appropriate erosion and sediment control plan, including but not limited to the following: • Where practical, maintain vegetation buffers around construction areas; 32113 Eastlake AVetllle Ste B SeatCle, AV -A 98102 Tel (206) 262-0370 Fax (206) 262-0374 Geotechnical Report Addendum 9303 Olympic View Drive, Edmonds, WA October 12, 2015 Silt fences should be installed along the perimeter of construction areas to control erosion and the movement of soil; 0 Site stripping, excavation and subgrade preparation should be followed promptly by the placement and compaction of clean structural fill or CDF; + The ground surface within the construction area should be graded slightly toward to east to prevent surface water run-off flowing onto the steep slope; The size and type of construction equipment used may have to be limited to prevent soil disturbance; • Adequately cover soil stockpiles and temporary excavations with plastic sheeting in the raining season; • Hydroseed or place straw in areas where grading is completed; • Construct shallow, upgrade perimeter ditches or low earthen berms to divert water away from the top of slopes; and * If possible, stage construction such that the amount of exposed soil and exposure time is minimized; Permanent erosion control measures should include establishing vegetation, landscape plants, and hardscape established at the end of project, and reducing surface runoff to the minimum extent possible. Water from roof drains and other impervious areas should be properly collected and discharged into an appropriate outlet, and should not be discharged on to the steep slope areas to the west. POTENTIAL LANDSLIDE HAZARDS Based on our review of the City GIS and Google maps, the west -facing slope at the site is approximately 150 feet high, and the average of the slope gradient is estimated to be about 80 to 90 percent. As such, the west -facing slope is considered as potential landslide hazard area. The relatively flat, eastern portion of the site is not considered as Potential Landslide Hazard area. Review of LiDAR image indicated that there is no obvious past landslides in the immediately vicinity of the subject site. As part of our study, we conducted a site reconnaissance to observe 13-207 9303 Olympic View Dr - Report Addendum 2 PanGEO, Inc. Geotechnical Report Addendum 9303 Olympic View Drive, Edmonds, WA October 12, 2015 the surface conditions of the site including the upper portion of the west -facing steep slope. No obvious evidence of previous slides or instabilities was observed in the upper portion of the west -facing steep slope near the proposed flat building area. Furthermore, based on our review of the existing maps, our field observations, as well as the results of our subsurface explorations which encountered dense native soil at shallow depths, it is our opinion that the site is globally stable in its current configuration, and the proposed development will not decrease the site stability, provided that the recommendations presented in our report and this letter are properly incorporated into the design and construction of the project. OPINIONS AND CONCLUSIONS Based on results of our study and evaluation, it is our opinion that the site meets Edmond City Code and Community Development Code (ECDC) 23.80.060 A for the proposed development. ECDC section 23.80.70A.1 requires a Buffer Setback of 50 feet or the height of the slope within the project area, whichever is greater. Based on the subsurface conditions encountered at the site and our understanding of the proposed development, in our opinion, the buffer setback may be reduced to 10 feet and the building setback may be reduced to 20 feet from the slope crest (top of steep slope). ECDC section 23.80.70A.2 states that alterations of an erosion or landslide hazard area and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The development will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; b. The development will not decrease slope stability on adjacent properties; and c. Such alterations will not adversely impact other critical areas; It is also our opinion that the above conditions are satisfied for the proposed development plans. ECDC section 23.80.70A.2 states that development within an erosion or landslide hazard area and/or buffer shall be designed to meet the basic requirements A through G 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. 13-207 9303 Olympic View Dr - Report Addendum 3 PanGEO, Inc. Geotechnical Report Addendum 9303 Olympic View Drive, Edmonds, WA October 12, 2015 In our opinion, the site is currently stable and meet the required slope stability safety factors. Additionally, it is our opinion that the proposed development will not decrease the site stability. In fact, the site stability will likely be improved since the surface drainage at the site will be improved due to the proposed development. In summary, based on results of our field exploration and geotechnical evaluation, in our opinion, the proposed development satisfies the requirements of ECDC Sections 23.40 and 23.80, provided that our recommendations are incorporated and good construction practice are implemented. We appreciate the opportunity to be of service. Please feel free to contact us if you have questions or need additional information. Sincerely, Michael H. Xue, P.E. Senior Geotechnical Engineer 13-207 9303 Olympic View Dr - Report Addendum 4 PanGEO, Inc.