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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon soon as possible after grading. Surface water runoff should be collected and directed away from slopes to prevent water from running down the face of the slope. D.3 DRAINAGE CONSIDERATIONS D.3.1 Surface and Subsurface Drainage Requirements The Contractor shall be made responsible for temporary drainage of surface water and groundwater as necessary to prevent standing water and/or erosion at the working surface. We recommend removing only the foliage necessary for construction to help minimize erosion. The ground surface around the structures should be sloped to create a minimum gradient of 2 percent away from the building foundations for a distance of at least 5 feet. Surface water should be directed away from all buildings into drainage swales or into a storm drainage system. “Trapped” planting areas should not be created next to any building without providing means for drainage. The roof downspouts should discharge onto splash blocks or paving that direct water away from the buildings, or into smooth- walled underground drain lines that carry the water to appropriate discharge locations at least 10 feet away from any buildings. D.3.2 Foundation Drains We recommend foundation drains around the perimeter foundations of all structures. The foundation drains should be at least 12 inches below the base of the slab. The foundation drain should consist of perforated collector pipes embedded in a minimum 2- foot-wide zone of angular drain rock. The drain rock should meet specifications provided in the “Structural Fill” in this appendix. The drain rock should be wrapped in a geotextile fabric. The collector pipes should discharge at an appropriate location away from the base of the footings. Unless measures are taken to prevent backflow into the wall’s drainage system, the discharge pipe should not be tied directly into the stormwater drain system. November 23, 2009 Project No. 72852.000 D-4