2202 100TH ST SW Geotech Report 2022-10-25

Subsurface Exploration, Geologic Hazard,and Stephens Everett Property Preliminary Geotechnical Engineering Report Everett, Washington Preliminary Design Recommendations Pavement areas should be prepared in accordance with the "Site Preparation" section of this report. If the stripped native soil or existing fill pavement subgrade can be compacted to 95 percent of ASTM D-1557 and is firm and unyielding, no additional overexcavation is required. Soft or yielding areas should be overexcavated to provide a suitable subgrade and backfilled with structural fill. The upper 2 feet of pavement subgrade should be recompacted to 95 percent of ASTM D-1557. If required, structural fill may then be placed to achieve desired subbase grades. Because the site soils were substantially above optimum moisture content and they generally contain large amounts of soil particles smaller than the U.S. No. 200 sieve, some remedial subgrade preparation may be required below the paving. Remedial preparation measures could include removal of some of the existing native soils below the planned pavement section and restoring the planned subgrade elevation with select imported structural fill, treating the native soil subgrade with powdered cement to stabilize the wet soils, or aeration and drying of existing soils prior to compaction of the road subgrades. We recommend that the final determination of how to prepare the road subgrades be made at the time of construction when weather and field conditions are known. We anticipate the interior parking lot and aisles will be subject to light traffic loads from passenger vehicles driving and parking. In these light traffic load areas, we recommend a pavement section consisting of 3 inches of hot-mix asphalt (HMA) underlain by 4 inches of crushed surfacing base course is the recommended minimum in areas of planned passenger car lanes and parking. Additionally, we anticipate that select areas of pavement onsite will encounter heavier loading from semi-trucks, fire trucks, and garbage trucks, along with passenger vehicles. In these heavy traffic areas, a minimum pavement section consisting of 4 inches of HMA underlain by 2 inches of crushed surfacing top course and 4 inches of crushed surfacing base course is recommended. The crushed rock will provide improved and consistent drainage, which will extend the service life of the parking lot.The crushed rock courses must be compacted to 95 percent of the maximum density, as determined by ASTM D-1557. All paving materials should meet gradation criteria contained in the current Washington State Department of Transportation (WSDOT) Standard Specifications. Depending on construction staging and desired performance, the crushed base course material may be substituted with asphalt treated base (ATB) beneath the final asphalt surfacing. The substitution of ATB should be as follows: 4 inches of crushed rock can be substituted with 3 inches of ATB, and 6 inches of crushed rock may be substituted with 4 inches of ATB. ATB should be placed over a native or structural fill subgrade compacted to a minimum of 95 percent relative density, and a 11/2- to 2-inch thickness of crushed rock to act as a working surface. If ATB is used for construction access and staging areas, some rutting and disturbance of the ATB surface should be expected. The general contractor should remove affected areas and replace them with properly compacted ATB prior to final surfacing. March 29,2019 ASSOCIATED EARTH SCIENCES,INC. TG/ms-180567E001-2-Projects120180567WE\WP Page 18