3800 RIVERFRONT BLVD Geotech Report 2023-05-04

May 13, 2019 HWA Project No. 2015-061-21 Lateral Spread Modeling HWA has conducted a lateral spread evaluation using the methodology outlined in the 2012 CALTRANS Guidelines on Foundation Loading and Deformation Due to Liquefaction Induced Lateral Spreading. The computer program Slide 5.0 was used for these evaluations. This methodology uses a soil-structure interaction displacement-based approach to calculate the anticipated lateral load and displacement during a liquefaction event. The goal of this analysis is to develop a more realistic assessment of the extent of lateral spreading that will occur. This model can incorporate pile foundations or ground improvement measures. However, these types of conditions were not utilized for this evaluation. The CALTRANS guidelines first require a slope stability analysis be performed to estimate the depth of the critical failure mass, including the maximum anticipated extent of the failure. As part of this modeling the residual strengths for the anticipate liquefiable layers are used. Once the maximum extent of lateral spreading is determined, the geotechnical engineer repeats slope stability analyses at a particular point, within the predicted area of lateral spreading, to determine anticipated displacements. This is done by reducing the estimate ground acceleration until the location is found meet the stability requirement. Then the determined stable ground acceleration for that point is used to calculate the amount of lateral displacement for the design event. The following assumptions we used to determine anticipated lateral spreading magnitudes; 1. Liquefied soil residual shear strengths were calculated using the equation provided by Kramer and Wang(Kramer, 2015). 2. The initiation of liquefaction was determined based on the 2,475-year return period earthquake as outlined in the IBC. 3. One half of the site modified peak ground acceleration (0.2735g) for the slope stability model is appropriate. 4. The anticipated displacements were evaluated using equations outlined in Bray and Travasarou (2007). 5. Lateral spreading magnitudes were estimated ignoring the effects of proposed pile foundations. Lateral Spread Analysis Results For our analysis, cross sections B-B' and C-C' were evaluated as they were determined to be the most critical cross sections relating to the site's potential for lateral spreading. Cross sections B-B' and C-C', showing the anticipated areas of potential liquefaction, are shown in Figure 2 and 3 attached to the addendum. Based on this information HWA utilized Slide 5.0 to perform our stability analysis in accordance with the CALTRANS Mythology described above. Results of the slide analyses are shown in Figures 4 and 5 attached to this addendum report. Based on our analysis HWA has determined that no lateral spreading is predicted to occur along the proposed Riverfront Boulevard or within the footprints of the proposed Phase I or Phase II Everett Riverfront Development Addendum 4 HWA GeoSciences Inc.