8407 BROADWAY SECURE SPACE STORAGE Geotech Report 2025-04-24

Zi pperGeo Broadway Everett Expansion Geoprofessional Consultants Project No. 2601.01 July 7, 2022 The eastern two thirds of the proposed south building line is located about 3 to 8 feet from the existing vault. Using option A would require excavations below footings ranging from about 4 to 14 feet below existing site grades. Excavations for option A would need to be sloped and wide enough to allow worker access to construct the footings and foundation stem walls. Option B would require similar excavation depths, however would only require an excavation width equal to the footing width plus two feet. Worker access would not be required as once the excavation is completed, it is simply backfilled with CDF or lean mix concrete and footings are constructed at typical depths for frost protection. Option C would require no excavations below footings. Bearing Subgrade Preparation Building foundations located in close proximity to the existing stormwater vault and possibly other areas are expected to be supported on very dense glacial till soils. However soil conditions at expected foundation subgrade elevations in other areas of the site will likely consist of previously placed structural fill. Based on borings, the previously placed structural fill has marginal support characteristics. In order to limit differential settlement between footings founded on dense to very dense glacial till soils and marginal existing structural fill, some improvement of the existing fill is recommended. In areas where existing fill soils are encountered at foundation subgrade elevation, we recommend over-excavating the fill a minimum of two feet below foundation subgrade elevation. The width of the over-excavation should be equal to the footing width plus two feet. The subgrade at this elevation should then be compacted to a firm and unyielding condition and to at least 95 percent of the modified proctor(ASTM D1557)maximum dry density. The over-excavation should then be backfilled up to design footing subgrade elevation with the excavated material placed and compacted in accordance with the recommendations presented in the Structural Fill section of this report. Allowable Bearing Pressure and Settlement For footings founded as recommended above using option A or B we recommend an allowable bearing pressure of 2,500 psf. A one-third increase of the bearing pressure may be used for short-term dynamic loads such as wind and seismic forces. We estimate footing settlement for footings founded as recommended in this report will be about 1 inch total and % inch over 40 feet differential. If option C is selected for foundation support of footings located in close proximity to the existing stormwater vault, we recommend deep foundations consisting of pin piles. The table below provides recommended axial compressive capacities for a variety of pin pile diameters. 12