3800 RIVERFRONT BLVD Geotech Report 2023-05-04

March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 the east of the landfill site. However, our analysis suggests that neither flow sliding or lateral spreading is expected to extend to the alignment of Riverfront Boulevard. As such we do not expect flow sliding or lateral spreading to be a design concern for the Riverfront Boulevard project. Lateral spread evaluations were completed using Washington State Department of Transportation (WSDOT)procedures utilizing the computer program Slide 5.0. The geometry of the of liquefiable soil along cross sections C-C' and D-D', utilized in the lateral spread analysis, are shown in Figures 4C and 4D. As indicated in Section 4.2.2, these cross sections assume that the refuse is not liquefiable during the design earthquake. These analyses indicate that lateral spreading and flow sliding are limited to areas east of the existing leachate collection trench. For a previously planned development at the site, HWA completed additional lateral spread analysis along cross sections C-C' and D-D', assuming saturated portions of the refuse liquefy during the design earthquake, to estimate maximum anticipated lateral spread displacements. Anticipated levels of displacement were evaluated using equations outlined in Bray and Travasarou(2007). These analyses indicated that even with portions of the refuse undergoing liquefaction, anticipated displacements along Riverfront Boulevard would be limited to less than 2-inches under the 1033 year design earthquake. Hence, even with the conservative assumption that portions of the refuse will liquefy during the design earthquake, the anticipated levels of displacement(less than 2-inches) are below a level that would result in a threat to life safety of the traveling public. It should also be noted that these analyses did not take into account the increased resistance to lateral spreading that would occur due to the driving of pile foundations associated with the future phases of development east of Riverfront Boulevard. The presence of future pile foundations would be expected to further reduce anticipated displacements. Therefore, based on our analysis, we do not expect lateral spreading to be a design concern for Riverfront Boulevard. 4.2.5 Near Fault Hazards The site is located 5 miles from the northernmost fault in the Southern Whidbey Island Fault Zone. The Southern Whidbey Island Fault Zone consists of late Quaternary Reverse Faults with slip rates on the order of 0.2 to 1 mm per year. Due to the site's proximity to these active faults, the site is likely to experience large accelerations. The large ground motions are included in the design parameters and therefore will be incorporated into the design. Near fault ground motions also have the potential to exhibit forward directivity, which results in a strong pulse of acceleration at longer structural periods. As the Riverfront Boulevard project does not include any structures, forward directivity is not a design concern. 111 4.2.6 Ground Rupture Ground rupture can occur when a fault extends up to the ground surface. Based on a review of the United States Geological Survey Fault Database, the closest fault to the site is the northernmost fault in the Southern Whidbey Island Fault Zone, which is approximately 5 miles southeast of the site. As a result, HWA believes that site has a low potential for ground rupture. Geotechnical Report Revision 3 26 2019.doc 11 HWA GEOSCIENCES INC.