Shallow Soil Excavations: • Total area: Approximately 460 square feet (sf.) • Excavate to a depth of 3 feet below ground surface (bgs) • Temporarily stockpile soil identified for off-site disposal for characterization. • Load into trucks and dispose of as hazardous waste at a permitted landfill. • Backfill (outside the footprint of Deep Soil Excavation Area): − Reuse stockpiled overburden soil from 3 feet bgs to 0.5 feet bgs, if available. If stockpiled soil is unavailable, imported backfill will be needed. − 95% compaction requirement for backfilled material in lifts no greater than 1-foot-thick − Crushed surfacing top course (per WSDOT Standard Specification 9-03.9(3)) from 0.5 bgs to surface. Deep Soil Excavation, Shoring, and Dewatering: • Total area: Approximately 2,220 sf. • Provide shoring system to allow excavation to up to 14.5 feet bgs around the outer perimeter of the limits shown on the Drawings. • Excavation Area A: − Approximately 1,940 sf. − Excavate upper 10 feet, temporarily stockpile on the Site for potential reuse − Excavate soil from 10 to a maximum of 14.5 feet bgs or when clay is encountered. Stockpile as necessary for characterization and load into truck and dispose of as hazardous waste at a permitted landfill • Excavation Area B: − Approximately 190 sf. − Excavate upper 8 feet, temporarily stockpile on the Site for potential reuse − Excavate soil from 8 feet to a maximum of 14.5 feet bgs or when clay is encountered. Stockpile as necessary for characterization and load into truck and dispose of as hazardous waste at a permitted landfill • Excavation Area C: − Approximately 90 sf. − Excavate upper 4 feet, temporarily stockpile on the Site for potential reuse − Excavate soil from 4 feet to a maximum of 14.5 feet bgs or 1 foot below the top of the clay layer (whichever is shallower). Stockpile soil identified for offsite disposal for characterization − Load into trucks and dispose of as hazardous waste at a permitted landfill. • Pump, temporarily contain, and treat groundwater in the excavation during excavation and backfill activities. Well points located outside the excavation footprint will be allowed as an alternative to dewatering within the excavation footprint. Groundwater must be treated to remove PCE and degradation products to concentrations below detection limits. Turbidity must not be more than 25 nephelometric turbidity units at the point of discharge. • Backfill: − Reuse stockpiled overburden soil, amended with Daramend (see Evonik proposal included as Attachment D) from base of excavation to 10 feet bgs. − Reuse stockpiled overburden soil from 10 feet bgs to 0.5 feet bgs, until exhausted. − Once stockpiled overburden soil has been exhausted, backfill to 0.5 feet bgs with select borrow (per WSDOT Standard Specification 9-03.14(2)). − Crushed surfacing top course (per WSDOT Standard Specification 9-03.9(3)) from 0.5 feet bgs to surface.
