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

Interwest Construction Co.-Railway Avenue Improvements,Everett,WA Feb. 14,2019/Revised Feb. 7,2020 Geotechnical Engineering Report P1486-T18 4.2.3 Lateral Pressures on Shoring Walls The lateral soil pressures acting on shoring walls will depend on the ground surface configuration behind the wall, and the amount of lateral wall movement which can occur as the excavation is made. For drained shoring that is free to yield at the top at least one-thousandth of the height of the wall, soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing. The following recommendations apply to the case of shoring with up to one level of bracing; MGI should be consulted if multiple bracing is planned. We recommend that yielding Iwalls be designed using an equivalent fluid density of 40 and 60 pcf for horizontal backfill surfaces and backfill or cut slope surfaces inclined at 2:1 (horizontal to vertical) above the horizontal, respectively. For undrained conditions, we recommend that yielding walls be designed using an equivalent fluid density of 80 and 100 pcf for horizontal backfill surfaces and backfill or cut surfaces inclined at 21, respectively. 4.2.4 Pipe Bedding and Manhole Support Based on our test borings, we expect soils over the length of the project area to generally consist of silty sand with gravel and silt in a medium-dense to dense condition at the base of the trench excavation. We recommend that all manholes be supported on dense soils or on at least 2 feet of granular fill compacted to 95 percent of the maximum dry density defined below. Utility pipes should be bedded in sand or smooth rounded gravel, such as specified in the City of Everett Typical Trench Schematic Details, WSDOT Standard Specifications, Section 9-03.12(3) as modified for sanitary sewers, or as recommended by the manufacturer. We recommend that the pipe be covered with bedding material to at least one foot above the pipe. This bedding material should be lightly tamped into place. Backfill placed above the bedding material shall consist of structural fill compacted as discussed below. Shallow or perched groundwater may infiltrate through the utility trench pipe bedding. We recommend manufactured water stops or plugs of controlled density fill(CDF)or both be placed in the pipe trench to control seepage along the pipe. We expect settlement of the proposed pipeline will be negligible provided the soils at the base of the excavation are not unduly disturbed during installation. Pipe bedding should be placed according to the manufacturers recommendations. 4.2.5 Backfill All fill material should be free of debris, organic contaminants and rock fragments larger than 6 inches. We recommend that backfill be placed in lifts not exceeding 10 inches in loose thickness and mechanically compacted to at least 90 percent of the maximum dry density determined by Test Method ASTM D-1557. The upper 5 feet under roadways or road shoulders should be compacted to 95 percent. The moisture content of the backfill material should be adjusted as necessary to achieve this compaction. Based upon our auger boring explorations,and field observations,we anticipate that the majority of soils encountered during trenchwork activities will be comprised of relatively clean (less than Migizi Group, Inc. Page 9 of 13 •