3400 RIVERFRONT BLVD Geotech Report 2022-10-25

• • • • groundwater beneath the site likely corresponds to the elevation of the river. However, it is • possible that perched groundwater is present at shallower depths. • 5.0 CONCLUSIONS • • 5.1 SUMMARY • Based on our review of the available information and the results of our explorations, it is our • opinion that the site can be developed as proposed. The following items will have an impact on design and construction of the proposed project. Our specific recommendations for site • development and design are provided in the following sections of this report. • • A 5-foot or 10-foot-high surcharge fill should be installed across part of the site to accelerate primary and secondary settlement. We recommend monitoring of settlement plates to • determine when the surcharge fill has adequately consolidated the site. • • The proposed townhomes can be supported on mat foundations. The mat foundations will mitigate settlement that might result from the decomposition of the wood debris observed in • the undocumented fill layer. The upper 24 inches of foundation subgrade should be evaluated during construction to verify that it has been adequately compacted. • • We recommend that the upper 24 inches of roadway subgrade be properly compacted. • Alternatively, we recommend cement amending the upper 16 inches of subgrade. • • Our analysis indicates that liquefaction-induced seismic settlement will likely occur during a design-level earthquake. The structural engineer should determine if the magnitude of • seismic settlement is within acceptable limits. • • Our analysis indicates that lateral spreading will likely occur during a design-level earthquake that will impact the row of townhomes along the eastern edge of the site that are closest to • the riverbank. We recommend that the townhomes be supported on mat foundations,which will mitigate the risk of damage from lateral spreading. • • 5.2 LONG-TERM SETTLEMENT CAUSED BY FILLING • 5.2.1 Existing Utilities and Grading Plans • The site includes several existing utilities, some of which will remain in service and some will be replaced or abandoned as part of the project. The locations of the utilities that will remain in • place are shown on Figure 2. We understand that the following utilities are present at the site: • • A 48-inch-diameter pressurized sanitary sewer line constructed in 1985 of prestressed concrete cylinder pipe. The pipe invert elevation is approximately-2.5 feet. This sanitary • sewer line will be covered by up to 10 feet of new fill (typically around 8 feet of fill) and will • remain in service. This pipeline is currently located beneath the drainage swale. • • A 72-inch-diameter pressurized sanitary line constructed in 1995 of reinforced concrete pipe • that is supported on piles. The pipe invert elevation is approximately 0.5 foot. This sanitary sewer line will be covered by up to 5 feet of new fill (typically around 4 to 5 feet of fill) and • will remain in service. This pipeline is currently located beneath the eastern side of the • drainage swale. • • • • • G EODESIGN= 6 Polygon-128-01:091615 • •