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June 13, 2023 HWA Project No. 2022-188-21 4.11.7 Differential Settlement of Roads Roadways supported on geosynthetic stabilized fine-grained native subgrade will settle overtime and dips can develop at the approach area where roads encounter slabs or other pavement structures supported on full -depth structural fills. In order to minimize the amount of differential settlement between these features we recommend that the excavation conducted for the removal of compressible soils beneath the scale facility and transfer station access ramp extend from the base of the cut to subgrade elevation at an inclination of 3H:1 V instead of 1 H:1 V in the direction of travel. 4.12 GENERAL EARTHWORK CONSIDERATIONS 4.12.1 Subgrade Preparation for Foundations In the proposed footing areas, subgrade preparation should begin with the removal of all topsoil, deleterious material, and vegetation. Using a smooth bucket, the soils should be excavated to the proposed subgrade elevation. The exposed subgrade should be inspected by the Geotechnical Engineer, or their representative, and any loose or unsuitable soils should be over -excavated to expose the coarse -grained alluvial soils. Where excavation below the elevations of the foundation or floor slabs is needed, the width of the excavation should extend beyond the edge of the footing a distance equal to the depth of the over -excavation required to reach the bearing soils. After exposing coarse -grained native soils, and prior to placing any fill, exposed subgrade soils should be static rolled with a smooth drum roller under the observation of the geotechnical engineer prior to placement of structural fill. In order to avoid point -load contacts between concrete footings and the supporting coarse - grained soils, it is recommended that the foundations be constructed on a minimum 8-inch thick layer of compacted Crushed Surfacing Base Course (CSBC) meeting the requirements of Section 9-03.9(3) of the WSDOT Standard Specifications (WSDOT, 2023). 4.12.2 Structural Fill and Compaction For the purposes of this report, all fill placed under structures, floor slabs, utilities, sidewalks, roads and other paved areas should be considered structural fill. Structural fill should be a relatively well -graded soil, which is free of organic material, contains non -plastic fines, and should have a maximum particle size no greater than 1/3 the thickness of the lifts being placed. Its moisture content should be within 2% of optimum for compaction purposes. The native sandy silt or clay is not suitable for use as structural fill as it is too wet for proper compaction. We understand that Select Borrow fill material meeting the specifications of WSDOT Section 9-03.14(2) is available locally for use as structural fill. However, in wet weather or wet conditions, structural fill should contain no more than 5% non -plastic fines (material passing the No. 200 US Standard sieve) based on the minus 3/4-inch (19 mm) soil fraction. 2022-188-21 KCTS — Final Geotechnical Report 22 HWA GEOSCIENCES INC.