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20 certerra.com 4660 Main Street, Suite 100, Springfield, Oregon 97478 T: 541.393.6340 After stripping and excavating to the proposed subgrade level, as required, the building area should be inspected and proofrolled with a fully loaded, tandem axle dump truck. The proofroll should be observed by a representative of the Geotechnical Engineer. If the subgrade cannot be accessed with a dump truck, then the subgrade will need to be visually evaluated by a representative of the Geotechnical Engineer by probing. If fill is required, the structural fill should be placed on the prepared subgrade after it has been proofrolled or probed. Soils that are observed to be soft or are otherwise judged to be unsuitable should be undercut and replaced with properly compacted structural fill. Given the current site grades and proposed construction, we envision that fill benches will be necessary on this site. Fills constructed on slopes steeper than 5H:1V (11 degrees), should be benched prior to fill placement. Level benches should be a minimum of 4 feet wide laterally, and should be cut into the slope for no greater than every 5 feet of vertical rise. The placement of structural fill should begin at the base of the slope. All benches should be inspected by a representative of the Geotechnical Engineer and approved prior to placement of structural fill. If evidence of seepage is observed in the bench excavations, a supplemental drainage system may need to be designed and installed to prevent hydrostatic pressure buildup behind the fill. Permanent slopes should be graded no steeper than 2H:1V and should remain permanently vegetated. Temporary slopes may be excavated steeper than 2H:1V in accordance with OSHA regulations. During construction, slopes should be protected from erosion during the wet season with grass seeding, plastic sheeting or the installation of jutte matting. We recommend that any tree stumps on sloped ground either remain in place, or if removed, should be backfilled with properly compacted structural fill as outlined in Section 3.3 below. When fine-grained soils on this site are exposed, construction equipment may severely disturb the upper one to two feet of the subgrade during initial phases of site clearing and grubbing, especially if site preparation work is performed during wet weather. This may result in the need for undercutting and replacement of the disturbed soils if care is not taken by the contractor to protect the moisture sensitive soils. The contractor should construct temporary construction roads to protect the subgrade soils from becoming disturbed. If repeated construction traffic is anticipated, we recommend covering these areas with 18 inches of coarse gravel underlain by a geotextile fabric to prevent soil contamination of the rock and to protect the underlying subgrade. 3.3 Structural Fill We recommend structural fill consist of imported crushed rock gravel. If crushed rock gravel is imported to the site, it should be relatively well-graded and have a maximum particle size of 1-1/2 inches. Structural fill materials should be free of organics or other deleterious materials, contain at least 5 percent soil passing the U.S. #200 sieve, and have a liquid limit less than 45 and plasticity index less than 25. Alternatively, the weathered Eugene Formation siltstone could be used as structural fill provided it is clean and free of deleterious materials such as organics, and free of clay, and has been properly moisture conditioned as outlined below.