3400 RIVERFRONT BLVD Geotech Report 2022-10-25 (2)

RESPONSE TO REVIEW COMMENT 8 The estimated compression index value was based on the numerous consolidation tests performed in the similar organic silt soil layer at the nearby Polygon Simpson site. The estimated over-consolidation ratio value was based on the site geologic history. The estimated coefficient of consolidation value was based on laboratory testing performed by ESNW in March 2015, which we will include in an appendix in our comprehensive geotechnical report for the project. The maximum drainage path was estimated based on the average thickness of compressible, fine- grained soil being 15 feet. We then assumed double-drainage conditions,which results in a maximum drainage path of 7.5 feet. The estimated secondary compression index value was based on correlations with moisture content developed by Mesri (1973)2. REVIEW COMMENT 9 Table 1 on Page 3 reports using a drainage path of 7.5 feet(i.e. a 15-foot-thick layer that is drained at the top and the bottom by the presence of a significantly more permeable layer). Boring B-101 presents an approximately 30-foot-thick silt layer, indicating a 15-foot maximum drainage path. That would double the drainage path from 7.5 to 15 feet thick which has a significant effect on the time rate of settlement results. Please clarify. RESPONSE TO REVIEW COMMENT 9 Based on the 14 borings advanced at the site, the average thickness of the silt layer is 15 feet. We then assumed double-drainage conditions, which results in a maximum drainage path of 7.5 feet. We agree that there are some locations where the drainage path will be less than or greater than the average value we used in our analysis. Our objective was not to consider all possible variables but to provide a reasonable estimate of the typical amount of time required for the soil to consolidate. Ultimately, settlement monitoring will be used to determine when an adequate amount of consolidation has taken place at various locations across the site. REVIEW COMMENT 10 Page 3 of the report refers to the desire to achieve 90 percent of primary and secondary consolidation with the preload and surcharge. The basis for choosing 90 percent should be discussed in the report. Table 2 on Page 4 suggests that at a location with 20 feet of compressible silt, the total settlement was predicted to be upwards of 18 inches under a 10 foot surcharge. Therefore 90 percent of 18 inches leaves almost 2 inches of settlement remaining long-term due to static settlement. This is consistent with your stated goal (Page 5) of limiting post-construction secondary settlement to 2 inches after removing the surcharge load; however, potential settlement caused by degradation of organic materials and induced by seismic liquefaction have not been discussed in the report. There will likely be more than 2 inches of long-term settlement once additional settlement caused by decomposition of organics and liquefaction are factored into the settlement risk at the site. In addition, if the silt layer is 30 feet thick as noted in Boring B-101, then more than 2 inches of long-term secondary settlement alone 2 Mesri,G.(1973). "Coefficient of Secondary Compression,"Journal of the Soil Mechanics and Foundations Division, American Society of Civil Engineers,Vol.99, No.SM1, 122-137. G EODESIGN= 5 Polygon-128-01:0811 15