7000 HARDESON RD 2020-06-05 - Laserfiche WebLink

Geotechnical Engineering Report Community Transit Merrill Creek Operating Base 7100 Hardeson Road Everett, Washington February 28, 2018 RN File No. 3224-002A Page 5 Resources Conservation Service (NRCS) to determine the erosion hazard of the on-site soils. The site surface soils were classified using the SCS classification system as 'Pits'. This unit consists of areas that have been excavated for sand or gravel. While a corresponding geologic unit for these soils is not noted, our site explorations encountered advance outwash sand and gravel. We expect the erosion hazard for the soil on the surrounding slopes to be moderate to severe depending on the slope inclinations. We have included the soil survey as Appendix A. Seismic Hazard: It is our opinion based on our subsurface explorations that the Soil Profile in accordance with the 2015. International Building Code (IBC) is Site Class D with Seismic Design Category D. We used the US Geological Survey program "U.S. Seismic Design Maps Web Application." The design maps summary report for the 2012/15 IBC is included in this report in Appendix B. Additional seismic considerations include liquefaction potential and amplification of ground motions by soft soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. The underlying dense outwash deposits are considered to have a very low potential for liquefaction and amplification of ground motion and seismically induced lateral spread. The project is mapped on Faults and Earthquakes in Washington State (Jessica L. Czajkowski and Jeffrey D. Bowman, USGS OFR 2014-05) as located in the Southern Whidbey island fault zone. This is a class A fault and is considered to have a low potential for surface displacement because of the thickness of the glacial overburden. The location of the fault zone is shown on the map in Appendix B. Site Preparation and Grading The first step of site preparation should be to strip the asphalt and loose fill soils to expose medium dense or firmer soils in pavement areas. Because the fill thickness ranges from about 14 to 19 feet in the pavement replacement areas, it is not feasible to completely remove all fill material. We therefore recommend approximately 2 feet of existing fill be removed from the underlying pavement. The excavated material should be removed from the site, or stockpiled for later use as landscaping fill. The resulting subgrade should be compacted to a firm, non- yielding condition. Areas observed to pump or yield should be repaired prior to placing hard surfaces. The on-site fill likely to be exposed during construction is considered moisture sensitive, and the surface will disturb easily when wet. We expect these soils would be difficult, if not impossible, to compact to structural fill specifications in wet weather. We recommend that earthwork be conducted during the drier months. Additional expenses of wet weather or winter construction could include extra excavation and use of imported fill or rock spalls. During wet weather, alternative site preparation methods may be necessary. These methods may include utilizing a smooth-bucket trackhoe to complete site stripping and diverting construction traffic around prepared subgrades. Disturbance to the prepared subgrade may be minimized by placing a blanket of rock spalls or imported sand and gravel in traffic areas. Cutoff drains or Robinson Noble, Inc.