3400 RIVERFRONT BLVD Geotech Report 2022-10-25

• • • • evaluation estimates the amount of settlement that can occur in sand due to the vibrations from • an earthquake. Based on the results of our analysis, we estimate that the non-liquefaction settlement will be less than % inch. • • 5.3.4 Lateral Spreading Lateral spreading is a liquefaction-related seismic hazard and occurs on gently sloping or flat sites underlain by liquefiable sediments that are adjacent to an open face (such as riverbanks). • Liquefied soil adjacent to open faces can flow, resulting in surface cracking and lateral displacement down the slope or towards the open face. The magnitude of lateral spread • typically decreases with distance from the slope or open face. • We evaluated the potential for lateral spreading at the site from a design-level earthquake using the USGS computer program SLAMMER (Jibson et al., 2013). We input the site yield acceleration, • PGA, and magnitude into SLAMMER and then used the program to perform a simplified rigid • sliding block analysis. Yield accelerations were obtained by using the computer program SLOPE/W version 8.13.1.9253 to model the riverbank slope. Soil properties were selected based on published correlations with blow count data collected at the site. Based on the average • results of five different analysis methods used in SLAMMER, the amount of lateral spreading at • the site will typically be up to 12 inches along the riverbank at the eastern edge of the site. The • structural engineer should determine if the magnitude of lateral spreading is excessive. If so, • then lateral spreading is a hazard that will need to be mitigated during design for all structures located within approximately 150 feet of the top of the riverbank slope along the east side of the • site. • • 5.3.5 Surface Fault Rupture The USGS fault and fold database was reviewed to search for known earthquake faults present in • the site vicinity. Quaternary faults (defined as rupturing within the last 1.6 million years) are not • mapped at the site. The closest mapped fault to the site is the Southern Whidbey Island fault • zone that is approximately 6 miles southwest of the site (USGS, 2015). Since active faults are not • mapped at the site, surface fault rupture is not considered a hazard. • 5.4 RIVERBANK SLOPE STABILITY • We evaluated the stability of the riverbank along the east side of the site using the computer • program SLOPE/W version 8.13.1.9253. Soil properties were selected based on published • correlations with blow count data collected at the site. According to the results of our analysis, the riverbank has a static safety factor of approximately 1.5 and will be stable under static • loading conditions. The riverbank may not be stable under seismic loading conditions, which • could result in slump failures and lateral spreading. Lateral spreading is discussed in further • detail in the "Seismic Hazards" section of this report. • We note that the project plans call for a block wall supporting a paved path to be installed along • the top of the riverbank. Based on the results of our analysis, the block wall and paved path • appear to be stable under static loading conditions and at risk of failure during seismic loading • conditions, similar to the situation of the overall riverbank. We note that we have only evaluated the global stability of the riverbank and have not designed the block wall. • G EODESIGINN 11 Polygon-128-01:091615