3315 BROADWAY 2017 HOPEWORKS STATION 2 Geotech Report 2022-10-25

HopeWorks Social Enterprises JN 17298 July 12, 2017 Page 4 58 to 59 feet at the eastern edge of the site, which corresponds to depths of 8 to 9 feet below the proposed bottom-of-excavation along the proposed building's eastern perimeter. However, medium-dense silt was exposed at an elevation of approximately 67 feet along the eastern property line, which is near the proposed excavation depth. Considering the varying soil conditions across the property, there are several foundation options: • The proposed building can be supported on conventional footing foundations bearing directly on the dense underlying slightly silty sand or on structural fill placed on top of these competent native soils. Overexcavation should be anticipated beneath approximately the eastern three-quarters of the building footprint to reach the competent dense slightly silty sand for this option. The overexcavation could extend as deep as 9 to 10 feet below the existing ground surface along the eastern property line. We recommend structural fill placed beneath foundations consist of 2- to 4-inch quarry or concrete spalls, 2-inch ballast rock, or lean-mix concrete. An allowable bearing pressure of 5,000 pounds per square foot(psf) can be used for this application. • Use the 5,000 psf bearing capacity for footings on the western third of the building where dense soil will likely be exposed at shallow depths, but for the eastern three-quarters of the building footprint, use footing foundations designed for an allowable bearing pressure of 2,000 psf — these footings would just bear on the upper native medium-dense silt encountered near the existing ground surface. The exact line of demarcation between bearing pressures will likely need to be determined by a qualified geotechnical engineer after the excavation for the building has begun. • Again use a 5,000 psf bearing capacity for the footings on the western third of the building, but support the eastern end of the building on a deep foundation system consisting of driven pipe piles or drilled piers. Additional recommendations can be found in the Pipe Piles section of this report. The silty nature of the soils encountered underlying the site makes them very moisture-sensitive. When wet, the silty sand and silt soils can become softened from equipment and foot traffic. Therefore, if foundations are constructed during wet weather, or when the soils are wet, it will likely be necessary to protect bearing surfaces with a layer on imported, clean crushed rock to prevent disturbance of the footing subgrades. The high fines content of the onsite soils make them very difficult to adequately compact; therefore, we recommend the onsite soils not be used as structural fill. Some amount of overexcavation should be anticipated beneath the proposed foundations to reach the competent dense native silty sand. Open excavations in the loose upper soils should be inclined no steeper than a 1:1 (Horizontal:Vertical) continuously from the top to the bottom of the cut. Temporary excavation easements onto the adjacent properties/right-of-ways may be necessary depending on the depth of the overexcavation and the proximity to the property lines. No open excavation should extend below a 1.5:1 (H:V) line extending downward from the base on any adjacent building footings. It would be prudent to determine an accurate bottom-of-footing elevation of the northern perimeter foundation of the existing HopeWorks building adjacent to the south to ensure that the building is not undermined by the excavation for the new building. In addition, as previously discussed, the northern adjacent YWCA building has an offset of 0.59 to 0.65 feet from the subject site and based on our observations, the bottom of the YWCA building's southern perimeter footing is approximately 2 to 3 feet above the existing ground surface of the subject site. An ecology block wall currently follows the northern perimeter of the subject site, providing lateral support to the YWCA building. Considering this, it appears the southern perimeter foundation of the YWCA building will need to be underpinned in order to complete the excavation for the proposed building. Underpinning can be completed with small-diameter pipe piles driven into the underlying GEOTECH CONSULTANTS,INC.