4027 4TH ST SE WWTP STAIR LANDING 2021-12-20

Geotechnical Engineering Letter NGA File No. 900014 Everett Water Pollution Control Facility July 30, 2014 Everett, Washington Page 2 • Photos of one of the staircases that has experienced settlement. • A report titled "Geotechnical Report — Water Pollutions Control Facility Improvements — Everett, Washington," prepared by HWA GeoSciences, Inc., dated April 1, 2011. SCOPE The purpose of this study is to review the subsurface conditions within the vicinity of the two staircases and provide our opinion regarding the settlement and recommendations for the proposed foundation underpinning support. Specifically, our scope of services includes the following: Review available soil and geologic maps of the area as well as the provided geotechnical report. 2. Provide our recommendations for pin pile installation. 3. Provide a written letter with our opinions and recommendations. REPORT REVEIW A geotechnical engineering study by HWA GeoSciences, Inc. (HWA) was completed in 2011 for the expansion of the waste water facility. In their report, HWA researched several other existing geotechnical reports which included 54 soil borings in the treatment plant area. The explorations were drilled and sampled to depths of 104 feet below the ground surface. Smith Island is believed to have been created by old dredged material. The explorations encountered up to 10 feet of fill material underlain by about 95 feet of very soft to soft organic soils, silt, and clay. The soft alluvial deposit was underlain by medium dense sand to course sand at depths below 100 feet deep. Shallow groundwater was also present in the explorations. Subsurface Conditions Geology: The geologic units for this site are shown on Geologic Map of the Everett 7.5-Minute Quadrangle — Snohomish County, Washington, by James P. Minard, (USGS, April, 1985). The site is mapped as Qyal (Younger Alluvium). The Younger Alluvium is described as unconsolidated, stratified gray to brown clay, silt, and very fine sand to fine with abundant organic material. NELSON GEOTECHNICAL ASSOCIATES, INC.