4920 EVERGREEN WAY STAPLES Geotech Report 2022-10-25

Geotechnical Engineering Report lierracon Staples Rockery Failure ■ Everett,Washington March 8, 2019 ■Terracon Project No. 81175122 EXECUTIVE SUMMARY A geotechnical exploration program has been performed atop the existing rockery to investigate the rockery failure that occurred in November, 2017 between Glacier Lane and the west side of the building at 4920 Evergreen Way(i.e. the Staples building). The results of the exploration were used to develop recommendations for designing and constructing a retaining wall to replace the existing rockery. Terracon's geotechnical scope of services included the advancement of a boring behind the rockery to a depth of 56% feet below the existing ground surface (bgs). The retaining wall is planned for permitting within the City of Everett right-of-way. The proposed design is a soldier pile and tieback retaining system. The following geotechnical considerations were identified: • The failed section of rockery and soils behind it have sloughed and are resting against the building wall. This sloughed material is creating a lateral pressure of which the building wall was not designed to resist. • We encountered undocumented fill up to 8 feet and loose to medium dense sand up to 23 feet bgs. Dense to very dense sands and gravels were observed below about 23 feet bgs. The failed section of rockery likely occurred due to the presence of loose, cohesionless soil. Seasonal rainfall may have contributed to the failure; however, evidence of groundwater was not observed at the time of the field investigation. • We believe the most efficient retaining wall type considering the physical constraints at the site is a soldier pile and tieback system. The retaining wall will need to be permitted for permanent use in the right-of-way.We estimate an allowable anchor pullout strength of 1,000 and 2,000 psf for soil above and below 23 feet bgs, respectively (i.e. approximately elevation 305 ft). Pullout strengths of up two times can be achieved through secondary pressure grouting. • The seismic site classification for this site is D, based on the conditions encountered. Close monitoring of the rockery deconstruction and retaining wall construction operations discussed herein is critical. We recommend that Terracon be retained to monitor this portion of the work. This summary should be used in conjunction with the entire report for design purposes. It should be recognized that details were not included or fully developed in this section, and the report must be read in its entirety for a comprehensive understanding of the items contained herein. The section titled GENERAL COMMENTS should be read for an understanding of the report limitations. Responsive ■ Resourceful ■ Reliable iii