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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon there is a minimum of 10 feet of horizontal distance between the face of the footings and any adjacent, parallel slope. 3.3 Floor Slabs Satisfactory subgrade support for the reservoir slab can be obtained from the native subgrade prepared in accordance with our recommendations discussed above. As discussed before, a minimum 4 feet of soils below the slab should be over-excavated and replaced with compacted granular fill. The top 12 inches should consist of imported floor slab base aggregate (Section D1.8). The imported granular material should be compacted to at least 95 percent of the maximum dry density as determined by ASTM D 1557. A subgrade modulus of 125 pounds per cubic inch (pci) may be used to design the floor slab. 3.4 Reservoir/Retaining Wall Design Parameters Based on our discussion with you, we do not anticipate that the reservoir walls will be embedded below the ground surface. However, if they are, we have provided the following recommendations for the design of the reservoir walls. We recommend that the permanent walls for underground structures be designed as rigid retaining walls. Therefore, the walls should be designed to resist at-rest lateral earth pressure, static water pressure, and seismic pressures. We have provided two sets of wall design recommendations: drained walls and un-drained walls. These wall design parameters are summarized in Table 1 below. Table 1: Retaining Wall Design Parameter Equivalent Fluid Unit Weight (pcf) (Backfill is sloped at 2H:1V) Drainage Passive Friction ConditionsPressures (pcf)Coefficient Un-Hydrostatic Restrained RestrainedPressure Drained Walls405502500.35 Un-drained Walls182662.42000.35 For embedded building walls, a superimposed seismic lateral force should be calculated 2 based on a dynamic force of 4 H pounds per lineal foot of wall, where H is the height of the wall in feet, and applied at 0.6H from the base of the wall. A traffic surcharge equivalent to 2 feet of soil should be added in the top 10 feet of wall height if adjacent streets are present. If other surcharges are located within a horizontal distance from the back of a wall equal to twice the height of the wall, then additional pressures will need to be accounted for in the wall design. Our office should be contacted for appropriate wall surcharges based upon the actual magnitude and configuration of the applied loads. The wall footing should be designed in accordance with recommendations provided in Section 3.2 above. The backfill material placed behind the walls and extending a horizontal distance equal to at least half of the height of the retaining wall should consist of granular retaining wall backfill (Section D 1.6). The wall backfill should be compacted to a minimum of 92 percent of the November 23, 2009 Project No. 72852.000 4