2312 W CASINO RD COMMUNITY TRANSIT Geotech Report 2022-10-25

4.6.4.Utility Trenches We recommend trench excavation, pipe bedding, and trench backfill be completed using the general procedures described in the 2018 WSDOT Standard Specifications or other suitable procedures specified by the project civil engineer. The native glacial deposits and fill derived from the till are generally of low corrosivity based on our experience in the Puget Sound area. Utility trench backfill should consist of structural fill and should be placed in lifts of 12 inches or less(loose thickness)such that adequate compaction can be achieved throughout the lift.Thinner loose lifts may be needed depending on the backfill materials, equipment, and/or compaction test results. Each lift must be compacted prior to placing the subsequent lift. Prior to compaction, the backfill should be moisture conditioned to within 2 percent of the optimum moisture content, if necessary. The backfill should be compacted in accordance with the criteria discussed above. Figure 4 illustrates recommended trench compaction criteria under pavement and non-structural areas. 4.6.5.Dewatering Considerations A variable perched groundwater condition should be anticipated at the site. We recommend the groundwater level be maintained a minimum of 1 foot below the bottom excavations during construction or that level necessary to stabilize the shoring or temporary excavations. The level will depend upon the dewatering method,the size of the excavation and other factors. Based on the soil consistency and fines content, and depth to very dense glacial till, we anticipate that groundwater recharge will occur at a slow to moderate rate following an initial influx of groundwater. Permeability rates for the loose to medium dense silty sand fill soils are typically in the range of 10-2 to 10-a centimeters per second (cm/sec), and the rate for native till soils are typically in the range of 10-4 to 10-6 cm/sec. In areas where the groundwater levels need to be lowered by less than about 3 feet, it should be possible to dewater by pumping from sumps augmented with gravel-lined trenches.The excavation for the sump and the drainage trenches should be backfilled with clean gravel or crushed rock to reduce the amount of sediment in the water pumped from the sump (i.e.,to serve as a filter). 4.6.6.Temporary Slopes We recommend temporary cut slopes be inclined at 1H:1V or flatter depending on localized sloughing. Steeper inclinations up to 3/4H:1V are acceptable in the lower very dense glacial deposits. In addition, we recommend the following procedures for open cuts at the site: • No traffic,construction equipment,or supplies should be allowed at the top of cut slopes for a distance of at least 5 feet from the top of the cut; • Exposed soil along temporary cut slopes should be protected from surface erosion using waterproof tarps or Visqueen, when appropriate; • Construction should be scheduled so that the length of time the temporary cut is left open is minimized; • Erosion control measures should be constructed as appropriate to reduce runoff from the site; • Surface water flow should be diverted away from all excavations; and • The general conditions of the temporary cut slopes should be observed periodically by a geotechnical engineer to identify potential problems. GEOENGINEERS� November 14,2019 Page 9 File No.2095-019-07