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Updated Geotech Report for Planning Comments.pdf2401 loth Ave E O E O T E C H Seattle, Washington 98102 CONSULTANTS, INC. Q c to (425) 747-5618 FAY (425) 747-8561 JUN 0 7 2018 June 7, 2018 BUILDING DEPARTMENT JN 17538 Monique Tran CITY OF EDMONDS P.O. Box 831 Edmonds, Washington 98020 Subject: Seasonal Grading Concerns - Proposed Residence 23016 - 94th Avenue West, Edmonds, Washington Reference: "Geotechnical Engineering Study," subject site (JN06013) , Geotech Consultants, February 22, 2006 "Geotechnical Engineering Study Addendum - Infiltration Considerations", subject site, GCI, November 3, 2017 "Review of Plans", subject site, GCI, May 3, 2018 Plans: Adair Custom Homes — Sheets CS, All-8, and D1-7, Dated November 20, 2017. Western Engineers— Sheets 1-4, Dated May 2, 2018. Dear Ms. Tran - via email: monique0526@yahoo.com We understand that the City of Edmonds would like for us to specifically address the Seasonal Grading Restrictions EDC 23.80,70.6 for the site. 6. Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year; We concur with this, but we would reserve the position that a conditional extension from the director be a possibility if finishing grading at the site would lead to increased site stability over suspending grading. If you have any questions, or if we may be of further service, please do not hesitate to contact us. Respectfully submitted, G James H. Associate LTANTS, INC. 8 Jr., P.E. JHS:jhs OEOTECH CONSULTANTS, INC. Monique Tran P.O. Box 831 Edmonds, Washington 98020 Subject: Review Comments Response - Proposed Residence 23016 - 94th Avenue West, Edmonds, Washington 2401 1Oth Ave E Seattle, Washington 98102 (425) 747-5618 FAX (425) 747-8561 June 7, 2018 JN 17538 Reference: "Geotechnical Engineering Study," subject site (JN06013) , Geotech Consultants, February 22, 2006 "Geotechnical Engineering Study Addendum - Infiltration Considerations", subject site, GCI, November 3, 2017 "Review of Plans", subject site, GCI, May 3, 2018 Plans: Adair Custom Homes — Sheets CS, A1-8, and D1-7, Dated November 20, 2017. Western Engineers— Sheets 1-4, Dated May 2, 2018. Dear Ms. Tran - via email: monique0526@yahoo.com As previously stated, our recent recommendations (2017-18) relate to the current plan of a single family residence on the site rather than the original (2006) plan of a four lot subplat. We understand that the City of Edmonds would like for us to address more of the code sections for the current plan with regards to our original (2006) report and our recent review of plans. From EDC 23.80.70 with responses in blue: 1. Minimum Building Setback. The site soils are very dense glacial till and the proposed building setback of 30 feet from the northern property line will provide 40 to 50 feet of setback from the top of the steep slope area. This setback is sufficient to protect the structure from landsliding for the life of the structure, and will not cause an increased risk of landslides taking place on or off the site. 2. Buffer Requirements. We recommend a grading buffer of 10 feet off the north property line, this will equate to a buffer of 20 to 30 feet from the top of the steep slope. Since the slope is 10-20 feet on the neighboring property, a vegetation buffer on the subject site is not necessary. Based on the plans, these buffers are already incorporated into the plans (No grading is planned on the northern edge of the site, and all drainage is to be directed away from the slope). 3. Alterations. No alterations are proposed for the hazard area. Erosion hazard on the site is essentially non- existent as the steep slope is 10 to 20 feet on the neighboring property to the north. Standard erosion control BMPs are recommended. 4. Design Standards within Erosion and Landslide Hazard Areas. a. The structure is 40 to 50 feet from the steep slope and will be supported on dense glacial till, which is stable at the site inclinations; in our opinion, the proposed development will 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 redirection of surface water (from the roof) away from the slope would be expected to improve slope stability at the proposed development. b. The proposed setbacks avoid geologically hazardous areas and other critical areas; c. No alterations to the hazard areas are proposed. d. In our opinion, the buffers locate the structures and improvements in order to preserve the most critical portion of the site and its natural landforms and vegetation; e. In our opinion, the proposed development will not result in greater risk or a need for increased buffers on neighboring properties; Monique Tran June 7, 2018 JN 17538 Page 2 f. No alterations to the slopes are proposed. g. Development shall be designed to minimize impervious lot coverage; No input. 5. Vegetation Retention. N/A; 6. Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year; We concur with this, but we would reserve the position that a conditional extension be a possibility if finishing grading at the site would lead to increased site stability over suspending grading. 7. Point Discharges. N/A We have completed a general review of the geotechnical aspects of the referenced plans and specifications for the proposed residence. In our judgment, the proposed plans conform to the recommendations in our geotechnical engineering report from 2006. Based on our recent visit to the site, the surrounding site conditions have not changed, and in our opinion, even though the proposed land use has changed (from a four -lot plat to a single family lot) the referenced study is still applicable. The original study did not recommended a minimum setback from the steep slope on the northern adjacent property (as very stable glacial till soils were encountered and normal backyard setbacks from the property line would have provided an adequate setback from the slope); but based on the soil information and the height/inclination of the slope, we find that the currently proposed structural setback of 30 feet from the north property line (which equates to an approximate 40-50 foot setback from the top of the steep slope is acceptable from a stability standpoint. The water from the development is being infiltrated away from the steep slope area in a hydraulically separate area, so would not affect the slope stability. Pursuant to EDC 23.80.060 Development standards — General requirements: If the recommendations and conditions of the geotechnical engineering report are satisfied during construction and use of the project, the proposed project: 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. We recommend the following geotechnical inspections for the project: • Excavation observation • Structural fill observation and testing • Verify foundation bearing • Foundation drainage observation We acknowledge appointment as Special Inspector for this project, and by copy of this letter, request that we be kept informed of the progress of construction so we are able to make the necessary observations, as required by the City of Edmonds, in a timely manner. If you have any questions, or if we may be of further service, please do not hesitate to contact us. Respectfully submitted, GEO James H. Associate WTS, INC. 711 r.. E. JHS:jhs GEOTECH CONSULTANTS, INC.