2320 RAILWAY AVE INTERWEST CONSTRUCTION INC Geotech Report 2023-02-17

I I Interwest Construction Co.-Railway Avenue Improvements,Everett,WA Feb. 14,2019 I Revised Feb. 7,2020 Geotechnical Engineering Report P1486-T18 I I 1 I I I I I I I I I I typically in a loose to medium dense condition. It should be expected that occasional organics and pieces of timber could be present based on the depositional history and might be difficult to excavate where encountered. The contractor should be prepared to deal with these conditions. • Pavement Sections: Native, in-situ soil conditions are amenable to the use of soil-supported pavements. We recommend a conventional pavement section comprising an asphalt concrete pavement over a crushed rock base course over a I properly prepared (compacted) subgrade or a granular subbase, depending on subgrade conditions during pavement subgrade preparation. All soil subgrades should be thoroughly compacted,then proof-rolled with a loaded dump truck or heavy compactor. Any localized zones of yielding subgrade disclosed during this proof-rolling operation should be over-excavated to a depth of 12 inches and replaced with a suitable structural fill material. The following sections of this report present our specific geotechnical conclusions and recommendations concerning site preparation, asphalt pavement, and structural fill. The Washington State Department of Transportation(WSDOT)Standard Specifications and Standard Plans cited herein refer to WSDOT publications M41-10, Standard Specifications for Road, Bridge,and Municipal Construction, and M21-01, Standard Plans for Road, Bridge, and Municipal Construction, respectively. 4.1 Site Preparation Preparation of the project site should involve erosion control, temporary drainage, clearing, stripping, excavations, cutting, subgrade compaction, and filling. Erosion Control: Before new construction begins,an appropriate erosion control system should beY through installed. This system should collect and filter all surface water runoff silt fencing. We anticipate a system of berms and drainage ditches around construction areas will provide an adequate collection system. Silt fencing fabric should meet the requirements of WSDOT Standard Specification 9-33.2 Table 3. In addition, silt fencing should embed a minimum of 6 inches below existing grade. An erosion control system requires occasional observation and maintenance. Specifically,holes in the filter and areas where the filter has shifted above ground surface should be replaced or repaired as soon as they are identified. Temporary Drainage: We recommend intercepting and diverting any potential sources of surface or near-surface water within the construction zones before stripping begins. Because the selection of an appropriate drainage system will depend on the water quantity, season, weather conditions, construction sequence, and contractor's methods, final decisions regarding drainage systems are best made in the field at the time of construction. Based on our current understanding of the construction plans, surface and subsurface conditions, we anticipate that curbs,berms, or ditches placed around the work areas will adequately intercept surface water runoff. I Migizi Group, Inc. Page 6 of 13