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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon geotextile fabric, as recommended in Appendix D, should be used below the gravel to provide separation from the underlying soils. 4.0 PERTINENT CONSTRUCTION RECOMMENDATIONS 4.1 Site Preparation The water reservoir tank will be placed close to the elevation of the existing ground. Minimal excavation and cuts will therefore be required. The upper topsoil and unsuitable fill should be over-excavated and removed from the tank area. The trees and shrubs would also be removed from the improvement area during this operation. Materials generated during excavation and demolition should be transported off-site or stockpiled in areas designated by the owner. Asphalt, concrete, gravel fill, and base rock materials may be crushed and recycled for use as general fill. Such recycled materials should meet the criteria described in the “Structural Fill” section in Appendix D of this report. 4.1.1 Proofrolling Following stripping and prior to placing fill, at the water reservoir base slab, foundations, or pavement, the exposed subgrade should be evaluated by proofrolling. The subgrade should be proofrolled with a fully loaded dump truck or similar heavy rubber-tire construction equipment to identify soft, loose, or unsuitable areas. A member of PBS’ geotechnical staff should observe the proofrolling. Soft or loose zones identified during the field evaluation should be compacted to an unyielding condition or be excavated and replaced with structural fill, as discussed in the “Structural Fill” section of this report (Appendix D). 4.1.2 Wet Weather/Wet Soil Conditions Trafficability on the near-surface soils may be difficult during or after extended wet periods or when the moisture content of the surface soil is more than a few percentage points above optimum. Soils that have been disturbed during site preparation activities, or soft or loose zones identified during probing or proofrolling, should be removed and replaced with compacted structural fill. Track-mounted excavating equipment may be required during wet weather. The thickness of the granular material for haul roads and staging areas will depend on the amount and type of construction traffic. A 12- to 18-inch-thick mat of imported granular material is sufficient for light staging areas. The granular mat for haul roads and areas with repeated heavy construction traffic typically needs to be increased to between 18 to 24 inches. The actual thickness of haul roads and staging areas should be based on the contractor’s approach to site development and the amount and type of construction traffic. The imported granular material should be placed in one lift over the prepared, undisturbed subgrade and compacted using a smooth- drum, non-vibratory roller. Additionally, a geotextile fabric should be placed as a barrier between the subgrade and imported granular material in areas of repeated construction traffic. As an alternative to placing thick rock sections to support construction traffic, the subgrade can be stabilized using cement amendment. The depth of treatment and percentage of cement required depends on the site conditions at the time of construction. Additional recommendations will be provided during construction, if this approach is used. November 23, 2009 Project No. 72852.000 6