400 RIVERSIDE RD SEPA 016 - 039 - RIVERSIDE BUSINESS PARK Geotech Report 2025-04-14

December 8, 2016 Project No. T-7388-1 Based on the soil conditions encountered and the local geology, the 2012 International Building Code (IBC) indicates that site class "D" should be used in structural design. ' Seismic Design Parameters (IBC 2012/2015) Spectral response acceleration (Short Period), SMs 1.239 ' Spectral response acceleration (1 —Second Period), SMI 0.719 Five percent damped 0.2-second period, SDs 0.826 Five percent damped 1-second period, SDJ 0.479 The above values are for Latitude 48.00842N and Longitude -122.18405W, and were obtained from United States Geological Survey(USGS)Ground Motion Parameter Calculator(accessed November 22,2016). 5.0 DISCUSSION AND RECOMMENDATIONS ' 5.1 General Based on our study, in our opinion, development of the site as proposed is feasible from a geotechnical ' engineering standpoint. The primary geotechnical concern at the site is the presence of compressible soil strata susceptible to consolidation under the planned building loads. The compressible soil consists of a 4- to 11-foot thick layer of soft to medium stiff silt and clay underlying the site's upper fills. These soils are highly compressible and if not mitigated, unacceptable levels of differential building settlement would likely occur under proposed project loads. Given the depth to the very soft soil layer, in our opinion, the potential post ' construction building settlements can be mitigated by implementing a fill surcharge program. The existing fills at the site consist mostly of silty sand soils and variably thick layers of sandy silt. The CPT data indicate the fill to be predominantly in a medium dense to dense condition; however, CPT-4 and CPT-6 show that some of the fills are loose. Based on the data, in most site areas the medium dense fills appeared suitable for support of new fill, pavements, or building elements. Where loose soils are present, it will be ' necessary to remove the soils to a minimum depth of two feet below base-of-footing elevation and replace with compacted structural fill. The need for overexcavation will require confirmation by geotechnical observation and testing at the time of grading as outlined in the following Site Preparation and Grading Section. The existing fill and underlying native soils are moisture sensitive and close moisture control will be required to facilitate structural compaction of the soils if used as fill from site excavations. Detailed recommendations regarding these issues and other geotechnical design considerations are provided in the following sections. These recommendations should be incorporated into the final design drawings and construction specifications. 1 Page No. 4