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Meadowview HWA.pdfDecember 12, 2013 HWA Project No. 2012-089-21 Task 700 City of Edmonds Public Works: Engineering Department 121 Fifth Avenue North Edmonds, Washington 98020 Attention: Mr. Leif Bjorback Building Official Subject: GEOTECHNICAL REVIEW ESLHA-RESPONSE REVIEW Proposed 5 —Lot Short Plat-Meadowview Estates 15620 72nd Avenue West Edmonds, Washington City Project No. 001.000.62.524.20.41.00 Dear Mr. Bjorback, As requested, HWA GeoSciences Inc. (HWA) previously undertook and completed a completeness review of the submittal prepared by Mr. Kent Halvorson regarding the proposed construction of 4 new residences and driveway on an existing 3.22 acre lot within Edmonds, Washington. The subject property is located at 15620 72nd Avenue West and is within the Earth Subsidence and Landslide Hazard Area of North Edmonds. We understand that the current application is for drainage, road, and utility installation with associated grading for lot line adjustment only. Building construction plans and structural design calculations were not included in this submittal. This letter acknowledges the responses submitted on November 15th, 2013 to our review and comments on portions of the submittal package concerned with geology or geotechnical engineering. The following documents were submitted for use during our response review: Geotechnical Engineering Study, Proposed Five -Lot short Plat, 15620-72" d Avenue West, Edmonds, Washington (Originally Prepared by Geotech Consultants, July 12, 2013/Revised 10/9/13) (1 copy). Civil Construction Plan Sheets Including: CS -01 Cover sheet, ER -01 21312 30th Dave sE;' TESC Plan, ER -02 TESC Notes and Details, SP -01 Site Plan, , RD -01 suite 110 Grading, Road and Utility Plan, RD -02 Road and Utility Plan, Road Bothell, WA 98021.7010 and Utility Profiles RD -03 and RD -04, DT -01 and DT -02 Notes and Tel: 425.774.0106 Fax: 425.774.2714 '> www.hwageoxora December 12, 2013 HWA Project No. 2012-089-21 Task 700 Details,and Tree Plan TR -01 (Originally Prepared by LDC on July 17,2013 -Revised on October 23, 2013) (1 copy). ® Large Site Storm Drainage Plan (Pxepared by LI)C, �tovember 14, 2103) that replaces the originally submitted Small Site Storm Drainage Report (Prepared by LDC, July 16, 2013) (1 copy). ® Geotechnical Hazard Identifzcation/Declaration and Mitigation Statement of Risk - Proposed Five -lot Short Plat, Meadowview Estates, 15620 -72nd Avenue West, Edmonds, Washington. (Originally Prepared by Geotech Consultants, July 19, 2013 -Revised November 13, 2013) (1 copy). ® Record of Easement -Recorded May 21, 1998 -For conveyance of stormwater. ® Review Comment Response Letter, Prepared by LDC on November 14, 2013, by Mr. Dan Carmody, P.E. In general, the submitted responses prepared by LDC (November 14, 2013) corresponding to our review comments are satisfactory without exception except where outlined below. HMA Comment Checklist Item 6 Topographic Map: Proposed structures are not delineated. Map to be updated when structural plans are submitted for review in the future. Applicant Response: "Noted. Topographical map will be.updated with the proposed structure locations once structural plans are submitted to the City for review ". A - No Exception). A Comment Checklist Item 7 Land Clearing/Tree Cutting Plan: Sheet -"TR -01 in current submittal package. This plan appears to meet the general requirements per COE Handout P#46. Tree removal appears to be restricted to the footprint for the new driveway and adjacent slope cut. However, it is not clear if the geotechnical engineer has reviewed the proposed tree cutting plan and determined whether, in his opinion, it creates any slope stability issues or if a landscape/restoration plan should be devised to mitigate potential long term erosion impacts. Applicant Response: "Please see attached certification that the Geotechnical Engineer has reviewed and approved the current plan layout." (HWA- The revised certification states "In preparation of this letter, we reviewed the project civil plans, which include sheets I through 9; these were prepared by LDC dated November 13, 2013. The last sheet is a tree removal plan." (It appears that the Geotechnical Engineer has now reviewed sheet TR -01 and affirms in his declaration that "the civil plans and specification for the project conform to the recommendations in our study" A - No Exception). A Comment Checklist Item 8 Geotechnical Report: In general, the report appears to meet the generally accepted engineering practices; however the following omissions are noted: Halvorson Geotechnical Review ESLHA 2 HWA GeoSciences Inc. December 12, 2013 HWA Project No. 2012-089-21 Task 700 1) On page 1, Site Conditions Chapter, Surface section of the report, a 40 -foot wide landslide that occurred in 1997 is discussed. If known it would be useful to have the location of the slide marked in the Site Plan. Applicant Response: The landslide area has been added to the Site Exploration Plan, Plate 2 in the revised Geotechnical Engineering Study. (11WA - Noted, leo Exception). 2) It appears that geologic interpretations were made, however, there is no indication that the report was reviewed or approved by a geologist as required in COE Chapter 19.10.030G (4). Applicant Response: The report has now been reviewed by a licensed geologist, as signed on page 15. (HWA -Noted, No Exception). 3) The report discusses static and seismic stability of the slope with regard to deep-seated movement, but fails to provide an estimate of the rate of episodic bluff retreat expected (potentially caused by isolated slides such as occurred in 1997 and/or soil softening due to freeze -thaw, wetting and drying, etc) for periods of 25 and 125 years as required in COE Chapter 19.10.030G (6). Applicant Response: "The discussion episodic bluff retreat for periods of 25- and 125 - years is included on page S". (HWA - from the revised Geotechnical Engineering Study, Page 5, 2nd paragraph" "we estimate that the very steep slope could retreat 3 to 4 feet in the next 125 years due to a debris (surface soil) slide (which is what essentially occurred on the site in 1997). " A - We read that to mean average slope retreat -back from current top of slope will be about 3 to 4 feet over a period of 125 years) and "it is possible that a 3 —to -4 foot deep debris slide could occur in the next 2S years in portions of the very steep slope, but an average movement of 3 to 4 feet is possible at any one area on the very steep slope in 125 years" A - We read that to mean that a 3 to 4 foot deep debris slide can occur anywhere along the very steep slope in 25 years. In summary, do we understand this section to mean that the slope is expected -to retreat by an "average" distance of 3 to 4 feet in 125 years and that the mechanism for this retreat will be the coalescence of several isolated debris flows with an expected reoccurrence period of 25 years, caused by heavy precipitation, and that no sloughing is expected from soil softening induced by freeze -thaw and wetting and drying. Is that correct? -Please Clarify). A Checklist Item 9 Civil Construction Plan Sheets: In general, the plan sheets are complete as to form. The following comments are provided for consideration 1) Sheet ER -01: It appears that the clearing limits extend into the area that the geotechnical engineer recommended not be disturbed (see Page 6, 1 st Par. of the geotechnical report). Halvorson Geotechnical Review ESLHA 3 HWA GeoSciences Inc. December 12, 2013 HWA Project No. 2012-089-21 Task 700 Does the geotechnical engineer have any concerns or require special provisions to be observed when construction encroaches into this area? " Applicant Response: Please see attdched letter from the Geotechnical Engineer regarding the compliance of the civil plans to the recommendations in the Geotechnical report.(HWA- The last paragraph (top of page 2) of the revised Geotechnical Hazard Identification/Declaration and Mitigation Statement of Risk letter prepared by Geotech Consultants Discusses that "one minor area of disturbance is proposed within the 25 foot buffer for the installation of the stormwater system just north of the catch basin" and concludes that "this disturbance is suitable because it will occur near where the past slope repair was done, which is an area where the top of the steep slope is most stable; and no significant trees will be removed" (HWA-we concur that some disturbance is unavoidable to install the drainage system connection in this area. however, construction methods that will minimize disturbance need to be employed while under the continuous observation of the geotechnical engineer.) 2) Sheet RD -02: The proposed grade of the storm drain section between CB -3 and CB -2 is extremely steep (25.25%). It is likely that the trench backfill will act as a French drain and collect and convey infiltrating/shallow ground water seepage down toward CB -2 where the inclination and direction of the trench changes abruptly creating the potential for localized wet soil conditions that spread towards the steep slope. At this location consideration might be given to inclusion of perforated trench drain tied into CB -2. Applicant Response: The proposed slope between the catch basins in question has been reduced to below 20%. (HWA- we note that the gradient has been reduced to 15.39% which is still relatively high. We still consider, it likely that the trench will act as. an interceptor for shallow groundwater derived from locally infiltrated seasonal runoff and consideration should be given to installing an under drain pipe in,the trench that is tied into CB -2.' Does the geotechnical engineer have any concerns with this scenario?) Within the limitations of scope, schedule and budget, our review of geotechnical and critical areas elements of the submittal package was completed in accordance with generally accepted principles and practices in this area at the time this letter was prepared. We make no other warranty either express or implied. .0 Halvorson Geotechnical Review ESLHA 4 HWA GeoSciences Inc. December 12, 2013 HWA Project No. 2012-089-21 Task 700 We appreciate the opportunity to be of service. If you have any questions regarding this report, please do not hesitate to contact either of the undersigned at (425) 774-0106. Sincerely, HWA GEoSCIENCEs INC. PM 0 I STEVEN ELLIOTT GREENE I Steven E. Greene, L.G., L.E.G. Principal Engineering Geologist Vice -President Halvorson Geotechnical Review ESLHA 5 HWA GeoSciences Inc.