3400 RIVERFRONT BLVD Geotech Report 2022-10-25

S S S S We observed that a large amount of debris is present along the base of the riverbank. The debris • consists of large pieces of concrete, metal, brick,wood, scattered timber piles, and scattered • riprap. Although some minor erosion appears to have occurred along the riverbank, it appears that the debris is helping to minimize the amount of erosion that is likely to occur. We S understand that the Polygon property line extends to roughly the top of the riverbank and that • the State of Washington owns the riverbank between the Polygon property and the river. • According to conversations with Snohomish County and City planning department personnel, the riverbank is not classified as a dike or levee. 1111 S 6.0 RECOMMENDATIONS • 6.1 SURCHARGE FILL • We anticipate that the magnitude of consolidation settlement will generally be more than the new • buildings, roadways, and infrastructure can tolerate and will occur too slowly to accommodate • the proposed construction schedule. As a result,we recommend that a surcharge be placed to • accelerate the settlement. The surcharge thickness and length of time left in place should be • sufficient to complete approximately 90 percent of primary consolidation and 90 percent of secondary compression settlement. We recommend that this surcharge only be placed at S locations that have not previously been surcharged. Based on our analysis, we anticipate that the surcharge will generally need to remain in place for • three to eight months for a 5-foot surcharge and one to three months for a 10-foot surcharge. • Although we have provided estimates of the time required to achieve the necessary amount of S settlement, the actual duration of the surcharge should be based on the results of a settlement • monitoring program. The settlement monitoring program should monitor the rate of settlement • at the site and confirm when the desired amount of settlement has been achieved. • In our opinion, it will be most economical to use two surcharge heights for this project. We S recommend a 5-foot surcharge where new fill heights will be 5 feet or less and a 10-foot • surcharge where new fill heights will be greater than 5 feet. These recommended surcharge • heights are based on heights above final grades. The surcharge should extend at least 10 feet beyond building and pavement areas. The surcharge should have an in-place unit weight of at S least 115 pcf. Temporary fill slopes for the surcharge can be built to a maximum slope of • 11/2H:1 V. All fill placed below finish grade elevations should be placed and compacted as • structural fill. The surcharge fill should be placed in 2-foot horizontal lifts and compacted with . tracked equipment. We understand that the City does not want surcharge placed above their existing utilities in Eclipse Mill Road. S S After the surcharge is removed, some additional minor secondary settlement could still occur • gradually over time. We anticipate that the magnitude of additional secondary settlement will be less than 2 inches over the life of the project and that long-term differential settlement will • generally be 1 inch or less across the maximum dimension of a typical townhome of • approximately 100 feet, provided the recommendations included in this report are followed. The • organics in the soft to medium stiff silt and organic silt layer are located below the groundwater . S S S G EODESIGN= 12 Polygon-128-01:091615 • S