7100 HARDESON RD SITE DEVELOPMENT Geotech Report 2025-03-04

Geotechnical Engineering Report lierracon Merrill Creek Operations Base Expansion • Everett, Washington December 14, 2020 • Terracon Project No. 81205012 GeoReport west, which appears to include a combination of advance outwash soils and the overlying glacial till. These reclamation and grading activities occurred episodically, and part time monitoring of fill placement and compaction was performed for some of the fill. Notably, soft,wet clay and silt associated with two settling ponds was buried during site reclamation and grading— the footprint of one of these ponds appears to correspond with the southern portion of the proposed building expansion based on historical drawings. Previous Explorations Numerous explorations have been advanced elsewhere on this Community Transit campus, including borings advanced for the design of the current maintenance and operations building. Based on our review of the existing geotechnical reports throughout the campus (including southwest of the proposed expansion), boring logs at most locations indicate the presence of typically medium dense to dense fill above the very dense native soil. However, where settling ponds were situated during mining, previous borings encountered on the order of 15 to 20 ft of soft sediment beneath 15 to 20 feet of the more competent reclamation fill. Based upon our current exploration, such a condition does not appear to exist beneath the proposed building expansion. Groundwater was encountered in a minority of previous borings, typically at depths between 10 and 18 feet. Where encountered, groundwater appeared to be perched above soil lenses of low permeability. The majority of previous borings did not encounter groundwater. Existing Maintenance and Operations Building Foundations We reviewed the geotechnical report prepared for design and construction of the existing building, titled `Subsurface Exploration and Geotechnical Engineering Report' and prepared by Agra Earth & Environmental, Inc., (AEE), dated 04/12/1996. This report recommended support of the existing building through the use of conventional shallow foundations, including strip (continuous) footings and isolated spread footings, with bearing pressures of 3,000 psf or less. Floors consisting of concrete slabs-on grade were also recommended. It is our understanding that the existing building is supported as recommended in the AEE 1996 report. In the AEE report, the presence of soft settling pond sediments was noted along the central portion of the southern MCOB building footprint. AEE recommended the placement of a pre-load fill to induce a monitored settlement of the soft soils, followed by removal of the pre-load fill. We were unable to discover whether pre-loading actually occurred prior to building construction. According to Community Transit, there have not been significant problems with building performance (such as cracked floors, cracked wall finishes, sticking doors or windows) that would indicate appreciable post-construction settlement of the building foundations or floors. However, according to Community Transit, there has been notable cracking and distress Responsive • Resourceful • Reliable 3