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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon depth explored. Based on the subsurface conditions encountered, we anticipate that groundwater may be present within the depth of exploration (i.e., less than 17 feet bgs). Based on our discussions with you, the reservoir base will be at an approximate elevation of the existing ground surface. Due to likely presence of topsoil, fill layer, and medium-stiff colluvium, PBS recommends that the soils below the base of the reservoir be over- excavated and replaced with imported granular fill material. The depth of over excavation will be on the order of 4 feet. The over-excavated soils below the top 12 inches should be backfilled with compacted 4 to 6 inch stabilization rock (Section D1.4). The 12 inches immediately below the slab should be filled with compacted aggregate base rock (Section D1.8). The tank base slab can then be placed on top of the aggregate base rock. We understand that you may also lower the level of the tank which may remove the unsuitable soils. We recommend that, at a minimum, a 12-inch layer of aggregate base rock (Section D1.8) be installed below the tank base. We do not anticipate any significant cuts and fills on this site which are greater than 5 feet. Due to the gently sloping nature of the site, the temporary slopes can be cut at 1H:1V. If deeper and/or vertical cuts are desired, a shoring system may be required. Recommendations for the design of the shoring system will be provided, if required. A site-specific seismic analysis in accordance with OSSC Section 1802.6 is presented in Appendix C of this report. As discussed in Appendix C, we do not anticipate any significant seismic hazard at this site. 3.2 Foundation Design As discussed earlier, the reservoir will be placed close to the existing ground level at the previous location of the existing tank slab. Based on our field exploration, PBS recommends that the top 4 feet of soil below the new tank slab be over-excavated and replaced with granular fill. We understand that the reservoir will have a ring foundation with base slab on grade. Footings placed on firm native soil or on structural fill should be designed for an allowable bearing capacity of 2,500 pounds per square foot (psf). The recommended allowable bearing pressure can be doubled for short-term loads such as those resulting from wind or seismic forces. We recommend that the footings be at least 24 inches wide. The bottom of exterior footings should be at least 18 inches below the lowest adjacent exterior grade. Based on our analysis, the total post-construction settlement is calculated to be less than one (1) inch with differential settlement of less than a ½ inch over a 50-foot span for proposed reservoir loading. Lateral loads on footings can be resisted by passive earth pressure on the sides of the structures and by friction at the base of the footings. An allowable passive earth pressure of 250 pounds per cubic foot (pcf) may be used for footings confined by native soils and new structural fills. Adjacent floor slabs, pavements, or the upper 12-inch depth of adjacent, unpaved areas should not be considered when calculating passive resistance. For footings in contact with native soils, a coefficient of friction equal to 0.35 may be used when calculating resistance to sliding. Both of these numbers contain a factor of safety of 1.5. The footings should be founded below an imaginary line projecting at a 1H:1V slope from the base of any adjacent, parallel utility trenches. The footings must also be embedded so November 23, 2009 Project No. 72852.000 3