3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 stockpiling and the southern settlement plates were used to back calculate consolidation parameters associated with areas without past debris stockpiling. Based on the assumption of raising the grade by approximately 3 feet, outside of the preload area, our calculations suggest that between 0 to 4-inches of primary settlement should be expected. Our analyses also indicate that up to 6 inches of secondary settlement could occur outside the surcharge area over 50 years. In general, we expect that 6 to 10 inches of total settlement could occur outside the preload area over 50 years. Differential settlements as large as 4 inches could occur across the site due to past debris stockpiling. Utility conveyance and structures outside the surcharged areas should be designed to account for up to 10 inches of long term settlement and up to 4 inches of differential settlement. Anticipated Differential Settlement Across Surcharged to Non-Surcharged Area We anticipate significant differential between the surcharged and non-surcharged area due to geometry of the preload surcharge. The geometry of the preload surcharge is presented in Figure 17. We have indicated 4 settlement zones within the preload surcharge area. Zone 1 represent the top of the preload surcharge. We assume that zone 1 has completed primary consolidation and only recompression settlement and secondary settlement is anticipated within zone 1. Zone 2 represents the 1.5H:1 V side slopes of the preload surcharge area. As the load applied to the underlying compressible soils varies significantly across Zone 2, we expect that future differential settlements across this zone will be significant. Zone 3 represents gradual slope at the north end of the preload surcharge. The load applied to the underlying compressible soil across Zone 3 varies somewhat, therefore, we would expect some differential settlement to occur across this area. Zone 4 represent the area where an ecology block wall was used the limit the footprint of the preload area within the project site. Because of the abrupt change of loading applied to the underlying soils across Zone 4, we anticipate large differential settlements across very short distances in this area. Based on our calculations, we estimate that utilities, structures and flatwork extending from surcharged areas to the outside of surcharged areas through zones 2 and 3 can expect to undergo as much as 5- to 8-inches of differential settlement over a length of 30 feet. The differential settlement across zone 3 is expected to be close to the lower bounds the calculated differential settlement. We expect that differential settlements as much as 5- to 8-inches of differential settlement over a length of 10 feet could occur across Zone4. We recommend that underground utilities be designed with adequate fall and using flexible pipe material, such as fused HDPE pipe. Utility connections should be flexible and capable of tolerating as much as 8- to 5-inches of differential settlement over a length of 30 feet. Utilities corridors should be located in areas not overlain by hardscapes, to allow for future reconstruction if needed. We recommend that no utilities be designed to cross Zone 4. We recommend that all proposed utilities founded above or within the potentially compressible organic silt layer be designed to tolerate up to 6 inches of differential settlement as a result of Final Geotechnical Report-3-Acre Park.docx 15 HWA GeoSciences Inc.