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20160108131905.pdfroLWIW woommw January 6, 2016 Northwest, nc Keith Kemper 7417 Meadowdale Beach Rd Edmonds, WA 98026 l Subject: INFILTRATION EVALUATION ADDENDUM PROPOSED NEW RESIDENCE 9303 OLYMPIC VIEW DR EDMONDS, WASHINGTON Geotechnical Engineers, Geologists & Environmental Scientists 7 2016 ak� r,g'J'Y (IF gbMONDS G-3978 Ref: "Infiltration Evaluation, Proposed New Residence, 9303 Olympic View Dr, Edmonds, Washington, G-3978", GEO Group Northwest, Inc., Nov 4, 2015. "Engineering Plan Review Comments, December 7, 2015, BLD20150239", City of Edmonds, Dec. 7, 2015 Dear Mr. Kemper: Per the request of Mr. Santhosh Moolayil of Insight Engineering we have prepared the following addendum letter to our referenced Infiltration Evaluation to address the referenced comments by the City of Edmonds. CITY OF EDMONDS COMMENTS The referenced City of Edmonds comment letter includes the following relevant comment: STORM 1) 12/7/15 - Comment was addressed; however, footing drains, sump pump, retaining wall and rockery drainage, typically do not discharge to the onsite stormwater management system unless the system is designed to account for the additional water. a. Please revise the drainage report to address the addition of the footing drains, sump pump, retaining wall and rockery drainage to the proposed system. b. Or the footing drains, sump pump, retaining wall rockery drainage may discharge to a separate system (i.e. drywell). If you choose this option, please provide a 13240 NE 20th Street, Suite 12 Bellevue, Washington 98005 Phone 4251649-8757 FAX 425/649-8758 January 6, 2016 G-3978 9303 Olympic View Dr, Edmonds, Washington Page 2 memo from the Geotechnical Engineer of record approving the location of the drywell. PROJECT DESCRIPTION Based upon information provided by Insight Engineering and Craig Chaney Architect we understand that the proposed development at the site is to include a new single family residence. We understand that the majority of site stormwater (roof collection) will be discharged to an existing stormwater system at the existing driveway. A driveway which includes permeable pavers is proposed for the northeast corner of the property. About 750 square feet of impervious runoff from the new driveway is proposed to be infiltrated into the existing site soils at a new infiltration trench to be located at the east side of the property. Additionally rockery and wall drains at the west side of the property as well as footing drains will be routed to a sump pump in the basement/cellar where this water can then be pumped to the infiltration trench at the east side of the lot. The sump pump is proposed to have an inlet at an elevation of 171 which is roughly 9 - feet below the existing grade at the building area. INFILTRATION EVALUATION REVIEW Recall that our subsurface investigation as presented in the referenced report at the east side of the property encountered dense to very dense gravelly silty SAND with occasional cobbles underlying the site to an explored depth of 36 -inches below ground surface. These soils have been interpreted to be the Vashon Advance Outwash. These soils are silty but will allow for some infiltration of stormwater. We recommended a design long-term infiltration rate of 0.32 in/hr for the infiltration system. Our subsurface investigation was limited to the upper 3 -feet below the ground surface due to hand -auger refusal. Groundwater seepage was not encountered within this zone and also it could not be ascertained from viewing the high bank at the west side of the property. Based upon our review of available online investigations by others we suggested in our report that the groundwater seepage level may be at a deeper level such as 9.5 -feet below ground surface which was reported for one property in the site vicinity. Based upon geologic mapping for the site it is anticipated that the Transitional Beds (Qtb) which consist of silty and clayey soils underlie the Advance Outwash soil deposit. GEO Group Northwest, Inc. January 6, 2016 G-3978 9303 Olympic View Dr, Edmonds, Washington Page 3 RESPONSE TO CITY COMMENT Our investigation at the site was not deep enough to determine whether or not groundwater seepage will be encountered at the excavation for the proposed basement/cellar. If the Advance Outwash soils extend deeper than the bottom of the basement/cellar excavation then it is unlikely that seepage will be encountered which will require collection by footing drains, rockery wall drains and the sump pump. We recommend that we are on-site at the time of construction to verify the lack of a seepage zone within the excavated depth. Alternatively we can return to the site and perform a deeper investigation of the site soils via a boring or test pits. Accordingly, it is our opinion that the anticipated groundwater seepage at the site is anticipated to be negligible and the footing drain, rockery wall drainage and sump pump can be connected to the proposed infiltration system If you have any questions, or if we may be of further service, please do not hesitate to contact us. S incerely, GEO GROUP NORTHWEST, INC. Adam Gaston Project Engineer William Chang, P.E. Principal cc: Mr. Santhosh Moolayil - santhosh@iusi htqWjnggli� Mr. Craig Chaney - chaney.cs@gmail.com GEO Group Northwest, Inc.