1815 13TH ST SEATTLE CHILDRENS NORTH CLINIC 2019-07-08

CENTRAL UTILITY PLANT PARKING LOT Everett,Washington • The soils below the bottom of the planned infiltration facilities are stiff to very hard and contain a significant percentage of fines, which limits the infiltration capacity. Infiltration is generally not considered feasible for soils with an infiltration capacity less than 0.25 inches per hour. Based on our analysis, it is our opinion that the on-site soils are generally suitable for only limited stormwater infiltration. Earthwork Based on the subsurface soil conditions encountered in the explorations,we expect the soils at the site may be excavated using conventional heavy duty construction equipment. The materials we encountered include fill and glacially consolidated soils. The glacial soils commonly contain cobbles and boulders that may be encountered during excavation. Asphalt, concrete, and debris from the previous development on the site may also be encountered. The on-site soils contain significant fines (material passing the U.S. standard No. 200 sieve) and will be highly moisture-sensitive and susceptible to disturbance, especially when wet. It is our understanding that the work may be completed during winter. Ideally, earthwork should be undertaken during extended periods of dry weather when the surficial soils will be less susceptible to disturbance and provide better support for construction equipment. If subgrade preparation is undertaken during periods of wet weather,the contractor should be prepared to address additional subgrade disturbance and over-excavation of soft areas. Trafficability on the site may be difficult, even during dry weather conditions, due to the loose, silty fill that will be exposed during grading. If exposed, the soils will be especially susceptible to disturbance from construction equipment during wet weather conditions and pumping and rutting of the exposed soils under equipment loads may occur. The contractor should be prepared to protect the site and prevent subgrade soils from deteriorating in wet weather conditions. Clearing and Site Preparation The crushed gravel surfacing currently covering a majority of the project site can be reused provided the crushed gravel can be effectively segregated from the underlying fill and native soils. A filter fabric is present in many areas which will help to segregate the crushed rock soils. The crushed gravel surfacing can be stockpiled and reused as structural fill for areas of over-excavation or as a subbase material for the parking lot, with the requirement that it is adequately segregated from the underlying fill and native soils and not contaminated with fines from these soils. Construction of the proposed improvements will require clearing and site preparation. Concrete and asphalt, where present, may be recycled and reused as structural fill in limited areas; otherwise it should be removed from the site along with other construction debris. All existing utilities should be removed from the parking lot footprint and rerouted if needed. Areas to be developed or graded should be cleared of surface and subsurface deleterious matter including any debris, shrubs, trees and associated stumps and roots. Graded areas should be stripped of organic soils. Organic soils are likely only associated with the existing landscape areas. Page 4 October 17,2012 GeoEngineers,Inc. He No.10738-010-00