4920 EVERGREEN WAY STAPLES Geotech Report 2022-10-25

Geotechnical Engineering Report lrerracon Staples Rockery Failure■ Everett,Washington March 8, 2019 • Terracon Project No. 81175122 The area of subsidence extends to the south beyond the location where the building wall type becomes a retaining wall (i.e. approximately 90 feet from the north end). 3.5 Perceived Risks of Existing Southern Portion of Rockery Further rockery movement, or collapse, in the southern section of rockery should, by design, not adversely impact the building as this section of building wall has been designed as a retaining wall. However, collapse of the rockery for this section may result in loss of ground within the City right-of-way. Additionally, the structural integrity of the building wall, which was designed as retaining wall, is not known. Further evaluation of both the subsurface condition behind the rockery for this section of wall and the structural integrity of the building wall would need to be performed to fully assess the impacts of a rockery failure. 4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION 4.1 Geotechnical Considerations Based on the results of our visual reconnaissance, subsurface exploration, laboratory testing,and our analyses, it is our opinion that the rockery is failing and could be expected to continue to fail. As the rockery continues to fail, the large rocks and the soil behind the rockery will displace and accumulate behind the building wall. As noted earlier in this report, the original design drawings appear to indicate that the northernmost approximate 90 feet of the building wall was originally designed as an exterior building wall with no provision for lateral earth load. As such, continued loading from rock and soil on this wall could result in damage to the building wall and ultimately impact the integrity of the building. Continued failure of the rockery and lateral displacement of the rockery and soil behind the rockery will also impact the Glacier Lane right-of-way resulting in loss of ground, ground cracking and settlement, and hazards to the public. For these reasons,we recommend the retaining wall construction and rockery demolition occur as soon as practical. In the meantime, we also recommend that the building wall be regularly monitored for any signs of movement or structural distress and the condition be evaluated on a regular basis by a structural engineer. We also recommend that the area immediately behind the rockery within the Glacier Lane right-of-way be cordoned off and access to the area by the public be restricted. We recommend that the rockery be replaced with a newly constructed retaining wall. Since the rockery is located approximately on the property line / right-of-way line and the existing building is located very close to the rockery, consideration of wall type and constructability issues are key. Conventional construction techniques that would remove the rockery wall followed by construction of a retaining wall would require significant temporary excavations into the Glacier Lane right-of- way that would impact local traffic and utilities in the right-of-way. To limit the impacts to Glacier Lane, we recommend construction of a soldier pile wall with a single row of tieback anchors. A soldier pile wall could be constructed top-down behind the existing rockery so that the rockery Responsive• Resourceful • Reliable 5