600 RIVERSIDE RD Geotech Report 2022-10-25

1 Landau Associates I 3.0 CONCLUSIONS AND RECOMMENDATIONS Based upon evaluation of the data collected during this investigation, it is our opinion that subsurface conditions at the project site are suitable for the proposed improvements, provided the recommendations contained herein are incorporated into the project design. The following sections present conclusions and recommendations regarding environmental considerations, contaminated soil handling and disposal, health and safety considerations, site preparation and earthwork, fill and compaction, utility construction, below grade pump stations, and pavement design. 3.1 Environmental Considerations Twenty specific areas on the site were previously identified as exceeding soil cleanup action levels. Most of the contaminated soil above cleanup action levels at the twenty identified areas of the site was excavated between April 15 and May 9, 1997; one remediation area was excavated in September and October of 1996. The cleanup areas were excavated to a depth where contaminant levels were below the cleanup action levels or to the groundwater level, whichever occurred first, then backfilled with clean dredged sand from the Snohomish River, which Weyerhaeuser had stockpiled onsite. Confirmation sampling and analysis was conducted in the excavations at the time of remediation. It is possible that contaminated soil that exceeds site cleanup action levels and that was not discovered or removed during the previous site cleanup could be encountered during construction of the site infrastructure. If encountered, this soil would require offsite disposal at an approved landfill facility. Soil below the site cleanup action level could remain onsite consistent with the Consent Decree. Excavations for utilities extending below the existing grade and deeper excavations required for the proposed pump stations and storm drain lines may encounter contaminated soil and groundwater that was not discovered or removed during previous site cleanup. Consequently, site excavations extending into existing site soil should be monitored for the presence of contamination and the work should be conducted by health and safety trained workers. Monitoring should include visual and odor indications of contamination as well as health and safety equipment monitoring by appropriate field monitoring equipment. It is intended that all of the soil presently onsite remain onsite and be largely contained beneath future building and paved areas. Therefore, environmental testing need not be performed on soil excavated during site development, unless the soil is intended for removal from the site or distinct visual evidence of contamination is identified. Specific recommendations related to the handling and disposal of contaminated soil is provided in Section 3.2 of this report. The proposed site development also has the potential to change the exposure conditions and contaminant transport mechanisms for the site. For example, construction excavations for the proposed deeper utilities and pump stations will extend into the upper sand unit and the upper Geotechnical Engineering Report 0121030.010.011 Riverside Business Park March 22, 2018 Public Access Trails and Roadway Improvements 3-1 r� 1 1 1 D 1 1