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COBALT <br />GEOSCIENCES <br />June t3, zozz <br />Updated August 18, zozz <br />Rick Hardt <br />rvhardt(a)email.com <br />RE: Geotechnical Letter <br />Proposed Residential Development <br />953o Holly Drive <br />Everett, Washington <br />Cobalt Geosciences, LLC <br />P.O. Box 82243 <br />Kenmore, Washington 98028 <br />In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to <br />discuss recent comments regarding proposed shallow stormwater systems. We have reviewed the <br />updated drainage plan by Omega Engineering, Inc. dated August tt, zozz. This plan shows a <br />bioretention cell near the southeast property corner and another in the northern portion of the <br />site. <br />Our previous recommendations for cell placement and cutoff drain placement remain valid for <br />these systems. This is reflected in our updated response to previous comments. <br />Comment: Will lateral flow from the bioretention cells impact adjacent properties due to a <br />restrictive layer or high groundwater? <br />Response: <br />There will be lateral migration of infiltrating runoff. The impact to adjacent properties and <br />proposed residences can be minimized by locating the perforated collection drains downgradient <br />of the systems, extending at least 6 inches into the dense glacial till at depth. For cutoff drains to <br />function, they must be emplaced at a depth suitable to intercept the lateral flow. In this case, that <br />would be into the dense and nearly impermeable glacial till. These elevations are likely to vary <br />across this site. We must be on site to verify cutoff drain placement. Footing drains should be set <br />at the base of the foundation elevations or lower. The use of 6 inch diameter perforated PVC pipes <br />is recommended along with angular rock backfill at least z feet above the pipe. Mirafi 14oN fabric <br />should be placed between the rock and structural fill up to grade. <br />Based on the updated site plan, a cutoff drain should be placed along the east and south sides of <br />the northerly bioretention cell. Other cutoff drains do not appear warranted at this time for the <br />other bioretention cell. <br />Comment: Are the cutoff drains as designed sufficiently protective of downgradient and/or <br />adjacent properties? Do they need a liner? <br />Response: <br />If the drains extend at least 6 inches into dense glacial till, they should adequately intercept and <br />convey the lateral flow without the need of a downslope liner. <br />www.cobaltgeo.com (2o6) 331-1097 <br />