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 can temporarily remain in place and provide limited temporary support until the soldier pile wall is constructed behind it. The rockery wall would be removed progressively as the retaining wall is constructed. A permanent easement will be required within the Glacier Lane right-of-way in order to construct the soldier pile wall and associated tieback anchors. We estimate that loose to dense cohesionless soils exist in the upper 23 feet, which is nearly the full height of the rockery. The contact of the loose to dense disturbed advance outwash and the underlying dense to very dense advance outwash approximately coincides with the base of the rockery. Portions of the tieback bond zone will be within both soil units, while soldier piles will be founded almost entirely in the dense to very dense advance outwash sand. Construction sequencing will be critical for deconstructing the existing rockery in a manner that will not result in further ground displacement or erosion of the right-of-way. Furthermore, construction vibrations and surcharging could negatively impact the rockery, which is currently considered unstable. Geotechnical engineering recommendations for the retaining wall system and other earth- "! connected phases of the project are outlined below. The recommendations contained in this report are based upon the results of data presented herein, engineering analyses, and on our current understanding of the proposed project. 4.2 Earthwork Based on the subsurface conditions encountered in our exploration, we expect that the on-site soil within the limits of construction can be removed with conventional excavation and augering/drilling equipment. Earthwork should be limited to only that necessary to construct the retaining wall to avoid surcharging the rockery, which in its present state should be considered unstable. Earthwork activities such as proof-rolling and compaction should be performed after retaining wall construction is complete. Recommendations for site preparation, structural fill, and temporary slopes are presented below. 4.2.1 Site Preparation Prior to equipment arriving onsite, clearing and grading limits should be established and marked. Silt fences should be constructed along the downslope side of all areas planned for clearing and grading. Preparation for site grading and construction should begin with procedures intended to control surface water runoff. The sandy soil on site is susceptible to erosion by flowing water. We anticipate that the use of silt fences, straw waddles, and wood slash will be adequate for surface water control during wet weather and wet site conditions. After construction of the retaining wall, areas that are to be stripped, excavated, regraded, or are to receive structural fill, should be proof-rolled with heavy compaction equipment. Any soft, loose, or otherwise unsuitable areas identified during proof-rolling should be recompacted if practical or Responsive • Resourceful • Reliable 6