8102 EVERGREEN WAY Geotech Report 2025-08-04

DevCo, Inc. ES-6516 April 1, 2019 Page 6 Excavations and Slopes Excavation activities are likely to expose existing fill and dense to very dense glacial till deposits. Based on the soil conditions observed at the boring locations, the following allowable temporary slope inclinations, as a function of horizontal to vertical (H:V) inclination, may be used. The applicable Federal Occupation Safety and Health Administration (OSHA) and Washington Industrial Safety and Health Act (WISHA) soil classifications are also provided: • Areas exposing existing fill 1.5H:1V (Type C) • Areas containing groundwater seepage 1.5H:1V (Type C) • Dense to very dense glacial till 0.75H:1 V (Type A) Steeper temporary slope inclinations within undisturbed, very dense native deposits may be feasible based on the soil and groundwater conditions exposed within the excavations. If pursued, ESNW can assist in evaluating the feasibility of utilizing oversteepened slopes at the time of construction. If the recommended temporary slope inclinations cannot be achieved, temporary shoring may be necessary to support excavations. The presence of perched groundwater may cause localized sloughing of temporary slopes due to excess seepage forces. An ESNW representative should observe temporary and permanent slopes to confirm slope inclinations are suitable for the exposed soil conditions and to provide additional excavation and slope recommendations, as necessary. Permanent slopes should be planted with vegetation to enhance stability and to minimize erosion, and should maintain a gradient of 2H:1V or flatter. In-situ and Imported Soils On-site soils are moisture sensitive, and successful use of on-site soils as structural fill will largely be dictated by the moisture content at the time of placement and compaction. Remedial measures, such as soil aeration may be necessary as part of site grading and earthwork activities. If the on-site soils cannot be successfully compacted, the use of an imported soil may be necessary. In our opinion, a contingency should be provided in the project budget for export of soil that cannot be successfully compacted as structural fill if grading activities take place during periods of extended rainfall activity. Soils with fines contents greater than 5 percent typically degrade rapidly when exposed to periods of rainfall. Imported soil intended for use as structural fill should consist of a well-graded, granular soil with a moisture content that is at (or slightly above) the optimum level. The fines content of the imported granular soil should be 5 percent or less during wet-weather conditions (where the fines content is defined as the percent passing the Number 200 sieve, based on the minus three- quarter-inch fraction). Earth Solutions NK LLC