3620 W MUKILTEO BLVD Geotech Report 2025-08-13

Geotechnical Engineering Evacuation Mukilteo Boulevard Development Everett,Washington March 31,2003 NGA File No.366503 Page 6 spread footings. However,foundations should extend through any topsoil,fill, or loose soils and bear on the underlying firm native soils or structural fill supported on these soils. Perched ground water was encountered in several of our subsurface explorations. We expect the amount of this perched water to decrease during the dry season. However,project budget and scheduling should allow for additional expenses to control ground water if encountered in structure, vault, or utility excavations. We have provided preliminary recommendations for ground water control in the Site Drainage subsection of this report. Based on the composition and high moisture contents of the material encountered in our explorations, using the majority of the on-site soils as structural fill would be extremely difficult, even during dry weather. The contractor would likely need to moisture-condition these soils to near-optimum moisture content prior to using this material as structural fill. This may entail spreading the material in thin lifts over a Iarge area during extended periods of dry weather and working the material until the desired moisture content is achieved. Another alternative would be to mix the on-site material with cement or kiln dust for moisture conditioning purposes. For planning purposes, the use of the on-site material for structural fill should be limited to selected areas. The contractor should be prepared to import material to the site for use as structural fill. We understand that an excavation up to 12 feet deep is planned for detention vault installation. Recommendations for worker safety and maximum excavation inclinations can be found in the Temporary and Permanent Slopes subsection of this report. If the excavation approaches existing structures or site boundaries, it may be necessary to install temporary shoring for all or portions of the excavation. We are available to provide shoring design for the excavation. Shallow perched ground water was encountered in our subsurface exploration located in the area of the proposed vault. It is likely that ground water seepage will be encountered in the vault excavation during construction, which could cause erosion and instability of the excavation. If seepage is encountered, sump pumps and/or cutoff drains should be installed around the vault excavation to reduce the amount of seepage entering the excavation. This recommendation is further discussed in the Subsurface Drainage subsection of this report. NELSON GEOTECHNICAL ASSOCIATES INC. EXHIBIT it /4) PAGE...IS:1F41f