3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 6 settlement plates appear to have completed primary consolidation and are now tracking along their associated secondary consolidation curves. Based on this analysis the preload surcharge can be removed at any time. Anticipated Future Settlement Under the Preload Surcharge Area Upon removal of the preload we expect that the underlying compressible soils will undergo some amount of rebound. This rebound will result in what appears to be negative settlement at each settlement plate. Rebound generally occurs quickly upon removal of the preload fill. We expect 1 to 2-inches of recompression settlement could occur once the surcharge is removed and the restroom structure is constructed(assuming bearing pressure of 2,000 psf for the structure). Recompression settlement is expected to occur quickly upon completion of the proposed structures. After rebound has occurred, the subsurface soils will continue to undergo long-term secondary and biodegradation settlements. The anticipated secondary settlements under the preload surcharge area will have been reduced by the preload surcharge. However, secondary settlements are still expected to occur. These long terms settlements are expected to result in differential settlement of the proposed restroom and associated flatwork over the design life of the improvements. To quantify the magnitude of anticipated secondary settlements and biodegradation over the design life of the proposed improvements, HWA has evaluated extensometer data collected across the adjacent landfill site. This data allowed HWA to back calculate secondary consolidation parameters for the peat and organic silt deposits observed across the 3-Acre Park and landfill site. We estimate that between 1 to 3-inches of long term secondary settlement could occur across the surcharge area over a design life of 50 year. Accountingfor both recompression and secondarysettlement, we estimate that the proposed P p P surcharge area could experience a total of 3 to 5 inches of settlement over 50 years with possible differential settlements as large as 2 inches across the surcharge area. Proposed improvements within the preload area should be designed to tolerate these anticipated settlement magnitudes. Anticipated Future Settlement Outside of the Preload Surcharge Area Outside of the preloaded area we expect that the proposed grade increases will result in both primary and secondary consolidation settlements. As we know that various undocumented debris stockpiles have been placed and removed from the 3-Acre Park site over time, it is difficult to determine appropriate consolidation parameters for the underlying compressible soils. We expect compressibility parameters of the underlying compressible soils to vary significantly due to the presence of past debris stockpiling. As this variability was observed in settlement plate data from the preload surcharge area, HWA used the settlement plate data to back calculate a range of primary consolidation parameters for the site. The northern settlement plate readings were used to back calculate consolidation parameters associated with areas of past debris Final Geotechnical Report-3-Acre Park.docx 14 HWA GeoSciences Inc.