3800 RIVERFRONT BLVD Geotech Report 2023-05-04

March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 4.9.3 Existing Soil Stockpile In 2016 a stockpile of Common Borrow soil was placed along the northern edge of the landfill I site. HWA observed placement of this stockpile. The stockpile was placed during the wet winter months. The Common Borrow was imported to the site at a moisture content significantly above optimum moisture content. During placement of the material, the increased moisture content of the soil did not allow for achievement of 90 percent compaction or for the planned side slopes to be achieved. The top of this stockpile was removed and spread across the site in the summer of 2018. However, we expect that the lower reaches of the stockpile are still present below grade at the northern end of the landfill site. We expect the remaining stockpile soil to still be significantly over optimum moisture content. We expect that proof rolling of the subgrade soils in this area will require increased over-excavation and replacement within the roadway prism. I 4.9.4 Wet Weather Earthwork IGeneral recommendations relative to earthwork performed in wet weather or in wet conditions are presented below. These recommendations should be incorporated into the contract specifications. I • Earthwork should be performed in small areas to minimize exposure to wet weather. Excavation of unsuitable and/or softened soil should be followed promptly by Iplacement and compaction of clean structural fill. The size and type of construction equipment used may need to be limited to prevent soil disturbance. Under some I circumstances, it may be necessary to excavate soils with a backhoe to minimize subgrade disturbance caused by equipment traffic. • For wet weather conditions, the allowable fines content of the structural fill should be reduced to no more than 5 percent by weight of the portion of the fill material passing the 3/-inch sieve. The fines should be non-plastic. It should be noted this is an Iadditional restriction on the structural fill materials specified. • The ground surface within the construction area should be graded to promote surface Iwater run-off and to prevent ponding. • Within the construction area, the ground surface should be sealed on completion of I each shift by a smooth drum vibratory roller, or equivalent, and under no circumstances should soil be left uncompacted and exposed to moisture infiltration. • Bales of straw and/or geotextile silt fences should be strategically located to control erosion and the movement of soil. I4.9.5 Temporary Excavations Maintenance of safe working conditions, including temporary excavation stability, is the responsibility of the contractor. In accordance with Part N of Washington Administrative Code IIIGeotechnical Report Revision 3 26 2019.doc 25 HWA GEOSCIENCES INC.