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

REVIEW COMMENT 3 Additional deep borings should be drilled at the site, particularly on the eastern side of the site where the development will be close to the river bank. The available borings located east of Eclipse Mill Road (B-102 and B-103) terminate prematurely in soils that are not confirmed to be increasing in density, and which may be potentially liquefiable. The standard of practice is not to terminate borings until consistently higher blow counts are observed in non-liquefiable soils, and to a depth of at least 50 feet. Both borings B-102 and B-103 were terminated in potentially loose to medium dense sand below the water table. We recommend additional borings be performed at each building location (or at least every other building if soils show some consistency)east of Eclipse Mill Road given the heterogeneous nature of the fill materials and the underlying native compressible silt and liquefiable sand layers. Borings should extend at least 50 feet deep or well into non-liquefiable sand deposits, whichever is deeper. RESPONSE TO REVIEW COMMENT 3 We drilled three additional borings (B-1 through B-3) along the eastern edge of the site. The borings were drilled to depths greater than 50 feet until dense, non-liquefiable soil was encountered. The soil conditions encountered in the borings are relatively uniform. In our opinion, the five borings and four test pits along the eastern edge of the development are adequate for characterizing the soil conditions in this area. The locations of our additional borings are shown on the site plan on Figure 1 and copies of the boring logs are presented in the Attachment of this letter. REVIEW COMMENT 4 Page 2 of the report describes a 0 to 20-foot thickness of the silt and organic silt layer, which is not consistent with the historical explorations available. Boring B-101 indicates that the silt layer was observed to be as thick as 30 feet in some places. This should be clarified. RESPONSE TO REVIEW COMMENT 4 The site is typically underlain by a soft to medium stiff layer of silt and organic silt that varies in thickness from 0 to 20 feet. An additional 10-to 15-foot-thick layer of hard silt was observed in 2 of the 14 borings advanced on the site (ESNW B-101 and ESNW B-2). It was not discussed on our draft Summary of Settlement and Surcharge Analysis letter because this hard silt layer is generally absent and, where present, is too dense to be susceptible to consolidation settlement. Our comprehensive geotechnical report will identify that there is a hard silt layer present at some locations. REVIEW COMMENT 5 On Page 2 it states that a comprehensive geotechnical report for this project will be prepared at a later date that"addresses additional geotechnical aspects of this project." The comprehensive report must evaluate the potential impacts of seismic shaking, including liquefaction and lateral spreading, on the site and planned structures. Liquefaction, lateral spreading, and post- liquefaction settlement all appear to be significant issues with significant potential impacts to the development and the planned structures. Under an IBC level seismic event, the impacts of MIDESIGN.W 3 Polygon-128-01:081115