405 RIVERSIDE RD Geotech Report 2022-10-25

r ' December 8, 2016 Project No. T-7388-1 Potential long-term settlement due to secondary compression of the soft silt and clay layer may also occur. These secondary settlements cannot be fully mitigated during the surcharge program. The magnitude of this settlement will be greater in the early years diminishing with time. Estimated total settlement due to secondary compression rrange up to a maximum of one -inch in 20 years. Approximately 50 percent of the total secondary settlement will be differential in nature due to the varying thickness and compressibility characteristics of the silt and clay layer. 5.3 Excavations All excavations at the site associated with confined spaces, such as utility trenches and lower building levels, must be completed in accordance with local, state, or federal requirements. Based on current Washington Industrial Safety and Health Act (WISHA) regulations, the upper fills and medium dense, native sands at the project site would be classified as Group B soils. The soft silts and clays would be classified as Group C soils. Accordingly, temporary excavations in Type C and Type B soils should have their slopes laid back at inclinations no steeper than 1.5:1 (Horizontal:Vertical) and 1:1 (Horizontal:Vertical), respectively, from the toe to the crest of the slope. If there is insufficient room to complete the excavations in this manner, or if excavations greater than 20 feet in depth are planned, temporary shoring to support the excavations may be required. Properly designed and installed shoring trench boxes can be used to support utility trench excavations where required. Groundwater should be anticipated within excavations extending below the existing fill into the native sands. If the excavation only extends two to three feet below the groundwater table, the volume of water and rate of flow into the excavation is expected to be moderate and are not expected to impact the stability of the excavations when completed, as described. Conventional sump pumping procedures, along with a system of collection trenches, if necessary should be capable of maintaining a relatively dry excavation for construction purposes. Deeper excavations that will require worker entry will likely require pre -draining using deep pump wells or closely spaced well points. This information is provided solely for the benefit of the owner and other design consultants, and should not be construed to imply that Terra Associates, Inc. assumes responsibility for job site safety. It is understood that job site safety is the sole responsibility of the project contractor. 5.4 Foundations The building may be supported on conventional spread footing foundations bearing on foundation subgrade prepared as recommended in Section 5.2 of this report. Perimeter foundations exposed to the weather should bear at a minimum depth of 1.5 feet below final exterior grades for frost protection. Interior foundations can be constructed at any convenient depth below the floor slab. ' We recommend designing foundations for a net allowable bearing capacity of 2,500 pounds per square foot (psf). For short-term loads, such as wind and seismic, a one-third increase in this allowable capacity can be used. As noted earlier, following successful completion of the surcharge program, estimated primary consolidation ' settlement due to building loads are less than '/z-inch. Total long-term settlement due to continued creep compression of the soft silt/clay layer is estimated to range up to a maximum of one -inch over a 20-year span, with approximately 50 percent of the long-term settlement being differential in nature. Page No. 7