3800 RIVERFRONT BLVD Geotech Report 2023-05-04

March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 Degradation of the existing fill soils is expected during periods of wet weather. The importationI and placement of crushed rock, to act as a working surface, should be expected for all construction activities that are to take place during wet weather. Onsite infiltration will not be allowed due to restrictions associated with the landfill site. Stormwater management will consist of treatment and discharge to appropriate locations. 111 4.2 SEISMIC DESIGN CONSIDERATIONS 4.2.1 Seismic Design ParametersI Earthquake loading for the subject retaining wall structures was developed in accordance with Section 3.4 of the AASHTO Guide Specifications for LRFD Bridge Design, 2nd Edition, I 2011(AASHTO, 2011 with 2012, 2014 and 2015 Interim Revisions) and the WSDOT amendments to the AASHTO Guide Specifications provided in the Bridge Design Manual (WSDOT, 2017). For seismic analysis, the Site Class is required to be established and is determined based on the average soil properties in the upper 100 feet below the ground surface. AASHTO accounts for the effects of ground conditions on structures and slopes based on site classes. Site classes are defined by the average density and stiffness of the soil profile underlying the site and the presence or absence of soft soils. The Site Class can be correlated to the average shear wave velocity in the upper 100 feet of the soil profile. Measurements of the average shear wave velocity have been performed for this site. The average value is estimated to be 740 feet per second (fps), which corresponds to Site Class D. Site Class is further modified when 10 or more feet of soft silt, clay, peat and/or liquefiable materials underlie the site. Due to the presence of over 10 feet of peat and organic silt along the alignment, the site technically classifies as Site 111 Class F. For Site Class F, AASHTO requires that a site-specific ground response analysis be performed to account for the presence of the soft material and determine how it influences the ground motions for various structural periods. Generally, the presence of a soft layer dampens the ground motions for structures with short periods (less than 0.5 seconds) and amplifies ground motions for structures with longer periods. As the Riverfront Boulevard project does not contain any structures and slope stability is dependent on the anticipated PGA (period less than 0.5 seconds) we do not believe that conducting a site specific seismic response analysis is warranted for this project. Therefore, we recommend the use of seismic site class D for design of the Riverfront Boulevard. 1 Table 2 presents recommended seismic coefficients for use with the General Procedure described in AASHTO (2011), which is based upon a design event with a 7 percent probability ofI exceedance in 75 years (equal to a return period of 1,033 years). Ground motions for the site are based on probabilistic earthquake hazard mapping efforts including those conducted by the United States Geological Survey(Frankel, et al., 2002) and (Peterson, et al., 2014). TheI parameters provided include the 2014 update and were based on the design memorandum from WSDOT, dated January 8, 2017, which have since been incorporated into the Bridge Design Geotechnical Report Revision 3 26 2019.doc 8 HWA GEOSCIENCES INC.