11831 AIRPORT RD WHOLE SITE FILE Geotech Report 2022-10-25

Earthwork Earthwork Considerations Fill, recessional sand and gravel and glacially consolidated soils were observed in the explorations. We anticipate that these soils can be excavated with conventional excavation equipment, such as trackhoes or dozers. Cobbles and boulders are frequently encountered in glacially consolidated soils. Larger horsepower excavators will be more efficient for excavating the very dense, native soils, or if large cobbles and boulders are encountered in the deeper excavations. The contractor should be prepared to deal with debris in the fill and cobbles and boulders in the native soils. Clearing and Grubbing The existing ground surface along the project corridor is typically vegetated or paved as discussed in the "Surface Conditions"section of this report. Embankment areas covered with vegetation should be cleared and grubbed in accordance with Section 2-01 of the WSDOT Standard Specifications. Subgrade Preparation Prior to placing new fill,subbase or base course materials, larger subgrade areas should be proof-rolled to locate areas of loose, soft or pumping soils. Smaller subgrade areas should be evaluated by probing. Proof-rolling can be completed using a piece of heavy tire-mounted equipment or a loaded dump truck. If soft or pumping soils are observed, such unsuitable subgrade soils should be recompacted or overexcavated and replaced.The depth of overexcavation should be determined by GeoEngineers. Where existing fill is present in proposed foundation areas,we recommend that the upper 18 inches of the fill be re-compacted to a minimum of 95 percent of the MDD estimated in general accordance with ASTM D 1557. As discussed previously, we recommend the foundation area for the station wall and signal pole to be located at Dumas Road and 132nd Street SE be potholed to confirm the depth to native soils or the landfill liner. If native soil conditions are encountered and the area is outside the liner, the wall for this station should be founded on the recompacted native soil or on structural fill overlying native subgrade. If the liner is encountered,we should develop appropriate foundation support using geogrid and lightweight fill based on the depth to the liner,thickness of new fill, and other factors. We recommend we be retained to review these conditions during construction and provide appropriate fill placement and foundation support alternatives. If deep pockets of soft or pumping soils are encountered across some of the project areas, it may be possible to limit the depth of overexcavation by placing a geotextile for separation or soil stabilization (WSDOT Standard Specification 9-33.2) on the overexcavated subgrade and covering the geotextile with structural fill. We recommend using the specified woven fabric for soil stabilization. The geotextile will provide additional support by bridging over the soft material,and will help reduce fines contamination into the structural fill. The need for geotextile fabric and overexcavation should be evaluated by GeoEngineers during construction. GeoEngineers should monitor the subgrade preparation operations to help determine the depth of removal of soft or pumping soils, and to evaluate whether subgrade disturbance or progressive deterioration is occurring. Subgrade disturbance or deterioration could occur if the subgrade is wet and cannot be dried. GEOENGINEERS April 14,2017 Page 21 File No.2095-032-00