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bld20130919 - HWA response - 03.06.2014.pdfHWA GEOSCIENCES INC �g,. March 6, 2014 HWA Project No. 2012-089-21 Task 700 City of Edmonds Public Narks: Engineering Department 121 Fifth Avenue North Edmonds, Washington 98020 Attention: Mr. Leif Bjorback Building Official Subject: GEOTE cHmcA,L R vlEw ESLHA _ 2ND REsrONSE RE, Ni w Proposed 5 --Lot Short Plat-Meadowview Estates 1562072 nd Avenue West Edmonds, Washington City Project No. 001.000.62.524.20.41.00 Grading Permit 20130919 Dear Mr. Bjorback, As requested, HWA GeoSciences Inc. (HWA) previously undertook and completed a completeness review of the submittal prepared. by Mr. Dent Halvorson regarding the proposed construction of 4 new residences and driveway on an. existing 3.22 acre lot within Edmonds, Washington. The subj ect property is' located at 15620 72'd 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 are not included in this submittal. This letter acknowledges the responses submitted on February 12t". 2014 to our most recent response review dated December 12, 2013 on portions of the submittal package concerned with geology or geotechnical engineering. DOCUMENT REVIEW The following documents were submitted for use during our response review: Geotechnical Engineering Study, Proposed Five -Lot. short Plat, 15620-72naAvenue Nrest, Edmonds, YYashington (Originally Prepared by Geotech Consultants, July 12, 2013/Revised 1019/13) (1 copy). Civil Construction Plan Sheets Including: CS -01 Co ler sheet, ER -01 2] 312 30th Drize sE. TESL_ Plan, ER -0.2 TESL Notes and Details, SP -01 Site Plan, , RD -01 Suits I It) Grading, Road and Utility) Placa, RD -02 Load and Utility Plan, Road Botholl,'A'A 98021.701a 'tit. 42531 4,0106 Tax. 425.774.271 4 FV1MIMINNageoxom March 6, 2014 HWA Project No. 2012-089-21 Task 700 and Utility Profiles RD -03 and RD -04, DT -01 and DT -02 Notes and 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 (Prepared by LDC, November 14, 2103) that replaces the originally submitted Small Site Stores Drainage Report (Prepared by LDC, July 16, 2013) (1 copy). ® Geotechnical Hazard Identification/Declaration and Mitigation Statement of Risk - Proposed Five -lot Short Plat; Meadowview Estates, 15620 -72nd 4venue 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. Plus three new documents entitled,- Large ntitled: Large Site Storm Drainage Report (Revised and re -submitted by LDC, February 12, 2014) that replaces the originally submitted Large Site Storm Drainage Report prepared by LDC on November 14, 2013(1 Copy). Storrnwater Review prepared by LDC on January 30, 2014, in response to comments as received from. the City of Edmonds review on January 14, 2014. Update Letter - Proposed Five Lot -Short Plat, Meadowview Estates, 15620 72"d Avenue West, Edmonds, Washington, Prepared by Geotech Consultants Inc., on January 20, 2014. GEOTECHNICAL REVIEW COMMENT RESPONSE In general, the submitted responses prepared by LDC (January 30, 2014) and Geotech Consultants (January 20, 2014) corresponding to our first set of review comments made on December 12, 2013, were satisfactory without exception except where: outlined below. HWA Comment Checklist Item 8 Geotechnical Report: In general, the report appears to meet the generally accepted engineering practices; however the following omissions are noted: 1) 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 5". (HWA - from thg 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 Halvorson Geotechnical Review ESLHA 2 HWA GeoSciences Inc. March 6, 2014 HWA Project No. 2012-0$9-21 Task 700 site in 1990. " (HWA - 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 25 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" (HWA - 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). Applicant response: Contained in Update Letter prepared by Geotech Consultants on January 20, 2013: "HWA noted they read our study as indicating the average slope retreat - back will be about 3 to 4 feet over a period of .125 years, but wanted to make sure that is correct. We stated in our study that a 3- to 4 -foot deep debris slide could occur in the next 125 years. We suppose there could be some minor sloughing of the slope due to other forces such as freeze/thaw and wetting.and drying, but it is inconsequential. Therefore, yes, we believe that the average slope retreat -back will be approximately 3- to 4 feet in a period of 125 years". (HWA-Thanks for the Clarification -No Exception). HWA 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). Does the geotechnical engineer have any concerns or require special provisions to be observed when construction encroaches into this area? Applicant Response: Please see attached 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 dtainage 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.) Halvorson Geotechnical Review FSLI-IA 3 11WA GeoSciences Inc. March 6, 2014 HWA Project No. 2012-089-21 Task 700 Geotechnical Engineer Response: Contained in Update Letter prepared by Geoteeh Consultants on January 20, 2013: "We still believe that minor disturbance is suitable where construction will extend into the buffer; but we also believe that putting a note on the plans stating that, "the spoils from the trench excavation should be placed on the upslope, eastern side of the trench " should be done; this will minimize the disturbance nearer the top- of-slope. op- ofslope. Tn addition, we do not believe that full-time observation is necessary; periodic observation of the storm drainage system that is installed within the slope buffer is suitable in our opinion. A note regarding periodic observations should also be placed on the plans. " (HWA-We concur with the incorporation of additional plan notes, with the exception that the notes include provision for daily observation when work is undertaken during periods of wet weather). 2) Sheet RD -02: The proposed grade of the storm drain section between CB -3 and C13-2 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?) Geotechnical Engineer Response: Contained in Update Letter prepared by Geotech Consultants on January 20, 2013: "HWA asked if we had concerns regards to the storm drainage trench acting as an interceptor trench of shallow groundwater, and whether an "under drainpipe" should be tied into CB -2 because of this. We have observed the excavation of several test pits and test borings on the site, and very sandy soils were found in all of these. It is our professional opinion that the likelihood of there being any groundwater is very low because of the sandy soils, and thus the probability of the storm drainage trench acting as an infiltration trench is also very low. Therefore we believe that the storm sewer plan for the project is suitable as -is, and no under drainpipe is necessary. " (HWA-response noted.) Stormwater Engineer Response: Contained in the Stormwater Review letter dated January 30"', 2014, prepared by LDC. "Bentonite or concrete check dams have been added on the proposed steep pipes to prevent the possibility of acting as an interceptor for shallow groundwater" (HWA-We consider the addition of check dams to be a suitable means of minimizing the potential risk of excessive within -trench downslope flow. The type and location of the check dams should be noted on the plans.) Halvorson Geotechnical Review ESLHA 4 HWA GeoScienees Inc. March 6, 2014 HWA Project No. 2012-089-21 Task 700 Miscellaneous Plan Sheet Comments Sheet ER -01: Note No. 2 states -"Cut slopes and soil stockpiles should not be covered each day with plastic during wet weather" (I WA -We think this is a typo and the word not should be deleted.) Sheet RIS -01: Please note the spacing required and/or locations of the check dams. Sheet DT -02: A detail for a 6" Perf, Pipe and Rock Interflow Collection Trench is shown on this sheet. (HWA-Where is it used on site? We could not find it delineated on any of the pian sheets.) CLOSURE Within the limitations of scope, schedule and budget, our review of geoteclniical 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. 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) 7740106. Sincerely, �{ VY a sbi HWA GE;oSmNcEs INN. 1704 STEVE! ELLIOTT GREENE Steven E. Grecne, L.G., L.E.G. Principal Engineering Geologist Vice -President Halvorson Geotechnical Review ESLHA 5 HWA GeoScicrices lnc.