Sitewide Soil Capping Interim Action

Location: Spokane Valley, WA
Client: Confidential Manufacturing and Fabrication Company
Contract Value: $1,524,568

AEC was chosen by the Owner and Design Engineering to provide all labor, equipment, and materials to perform an Ecology-approved Sitewide Soil Capping Interim Action at the Trentwood Facility located in Spokane Valley, Washington. The purpose of the project was to place several multi-layer caps over excavated areas of the site in an effort to reduce and/or eliminate contaminants of concern from infiltrating groundwater.

AEC performed site preparation, backfill, grading, lining, and capping activities in the following areas of the site:

Oil House Area
Truck Shop Area
Wastewater Area
Rail Car Unloading Area
Field Constructed Tanks Area
Oil Reclamation Building Area
Remelt/Hot Line Area
West Landfill Area
Cold Mill/Finishing Area
Former West Discharge Ravine Area
Former South Discharge Ravine Area
Site preparation work included installing temporary erosion and sediment controls and performing extensive clearing and grubbing activities including removing trees, grass and other vegetation. All clean overburden that was removed during site preparation was stockpiled and used for backfill.

Excavation and soil removal activities were performed under a previous contract to address impacted near surface soils and deep vadose zone soils. For the purpose of this contract, AEC was only required to remove a small amount of contaminated soil which was hauled to an existing onsite Soil Management Area for stockpiling prior to cap installation

The majority of AEC’s scope involved installing multi-layered caps in each area consisting of a suitable subgrade, a base course, an impervious layer, and a protective surface layer. All areas were property backfilled and graded prior to installation of the caps. The multi-layered caps were constructed with two low-permeability layers consisting of an HDPE geomembrane and geosynthetic clay layer (GCL), a drainage layer to promote runoff, and a 1 foot thick surface layer (pavement, gravel, or topsoil) that will be determined by facility operation at each cap location. In several areas, the existing grading was modified to ensure the proper drainage of stormwater.

AEC also constructed an infiltration swale for collected stormwater. There was one area on the property know as the West Discharge Ravine Area that required site restoration and revegation activities.

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