2400 33RD ST GEOTECHNICAL INVESTIGATION 2018-01-02 MF Import

sc 130 Milbor-Pitaa'vR°C Association of State Highway Transportation Officials (AASHTO) design methodology, we determined that Soil Profile Type II is appropriate for use in design. The corresponding Site Coefficient is 1.2. Design ground acceleration for the project alignment was determined using results from the seismic maps presented in the WSDOT Bridge Design Manual, based on the Nalional Seismic Hazards Mapping Project completed by USGS in 1996. WSDOT's maps present horizontal bedrock accelerations associated with a 10 percent probability of exceedence in a 50-year period. Interpolation between iso-seismic contours on the WSDOT seismic map indicates peak ground acceleration (PGA) at the project site of approximately 0.28 gravities (g). For comparison, using AASHTO design seismic maps indicate a PGA of approximately 0.23g for Everett. It is important to note, however, that the AASHTO maps have not been revised to incorporate the results of the recent USGS mapping project, and are outdated based on that data. Initial results of the Seismic Hazards inventory of Puget Sound (SHIPS) project indicate that the accelerations could be significantly higher due to the geometry of the fault zones surrounding Everett. Some Seismologists have suggested that a PGA of 0.32 would be more accurate. 5.2.2 Regional Seismicity The seismicity of the Puget Sound area is not well understood. This is due to several factors such as the lack of historical records. Another difficulty is the deep deposits of glacial materials that obscure the surface expression of bedrock faults. Recent analyses of the seismic epicenters do not show a specific pattern indicating a fault zone. The SHIPS project uses air gun blasts (in water) and underground explosions (in borings) to create shock waves that are mapped with seismographs. This provides a detailed cross section of the rock and glacial materials. Initial results show a series of faults where the Southern side has been thrust north and over the adjoining block. Everett and Seattle appear to be bracketed by these fault zones. More information will become available after the East-West profile has been completed in September 1999. 5.2.3 Soil Liquefaction Liquefaction occurs when loose, saturated and relatively cohesionless soil deposits temporarily lose strength as a result of vibration or earthquake shaking. Primary factors controlling the development of liquefaction include intensity and duration of strong ground motion, characteristics of subsurface soil, in -situ stress conditions and the depth to groundwater. Potential effects of soil liquefaction include temporary loss of bearing capacity and lateral soil resistance. Excessive ground movements (horizontal or vertical) may also occur as a result of liquefaction. The results of our subsurface investigation indicate that the soils underlying the site consist of cohesive clays and glacially overconsolidated sands and silty sands. In addition, the near -surface soils are not saturated, as the regional groundwater level at the site is typically 35-39 feet below the ground surface. For these reasons, neither the clays nor the glacial sands are susceptible to earthquake -induced liquefaction, and therefore the potential for significant liquefaction -induced settlement and lateral spreading of soils beneath bridge piers is low, in our opinion. 6tviLchnical, Tunnel and Enviromnenhd ERineers 3080 125° Ave NE Phone (425) 969-5778 lkllcwc, WA 98005 P38e9 Fu (425)861-0677