9512 EVERGREEN WAY 2022-08-29 - Laserfiche WebLink

May 4, 2017 HWA Project No. 2017-036-21 should not be allowed within a distance from the top of any excavation equal to 1/2 of the slope height. 4.8 DEWATERING CONSIDERATIONS Perched groundwater was encountered in most of our hand borings at the project location; particularly on the western portion of the property. Prospective contractors should be prepared to encounter and manage water intrusion from perched water zones above the till and water -bearing sand lenses within the till layer. We expect localized pump and sump systems will likely be the most effective dewatering method. 4.9 DRAINAGE CONSIDERATIONS Portions of the site immediately adjacent to proposed new building footprint or parking area should be sloped so that surface water can drain away from the excavations and/or foundation areas proposed for the new building expansion and the exposed subgrade for the new parking area. Roadways, parking and sidewalk areas should be inclined or crowned to facilitate drainage and direct runoff into gutters and catch basins as appropriate. 4.10 STORMWATER MANAGEMENT The increased stormwater associated with the proposed project will necessitate upgraded stormwater management facilities. The entire footprint is predominantly underlain by glacial till soil, present at shallow depths. Glacial till soils possess high fines contents and are generally not conducive to onsite infiltration. Based on our extensive experience of conducting saturated hydraulic conductivity tests and pilot infiltration tests (PIT) on glacial till soils throughout the region, we expect that the infiltration rate of the native glacial till soils underlying the proposed library site will be significantly lower than the 0.3 in/hr required for infiltration per Section 3.1 of the Washington Department of Ecology, 2014 Stormwater Management Manual for Western Washington, Volume 1. We expect that the very dense glacial till deposits will act as a low permeability soil barrier preventing infiltration. In our opinion, the use on onsite infiltration as a means of stormwater management is not feasible for this project. However, should the option of on -site infiltration require continued consideration, a Pilot Infiltration Test (PIT) must be conducted to determine an appropriate design infiltration rate for facility sizing. 5 CONDITIONS AND LIMITATIONS We have prepared this report for the Everett Department of Facilities and Property Management and their architect for use in design of this project. This report should be provided in its entirety to prospective contractors for bidding and estimating purposes; however, the conclusions and interpretations presented herein should not be construed as our warranty of the subsurface conditions. Experience shows that soil and ground water conditions can vary significantly over Final Geologic Assessment 11 HWA GeoSciences Inc.