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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon maximum dry density, as determined by ASTM D 1557. Backfill placed within 3 feet of the wall should be compacted in lifts less than 6-inches thick using hand-operated tamping equipment (e.g., jumping jack or vibratory plate compactors). If flat work (e.g., sidewalks or pavements) will be placed atop the wall backfill, we recommend that the upper 2 feet of material be compacted to 95 percent of the maximum dry density, as determined by ASTM D 1557. We recommend that wall drains will be installed to prevent buildup of hydrostatic pressures behind the walls. A minimum 12 inch wide zone of drain rock, extending from the base of the wall to within 6-inches of finished grade, should be placed against the back of all retaining walls. Perforated collector pipes should be embedded at the base of the drain rock. The drain rock should meet the requirements provided in Section D1.7 of this report. The perforated collector pipes should discharge at an appropriate location away from the base of the wall. The discharge pipe(s) should not be tied directly into storm water drain systems, unless measures are taken to prevent backflow into the wall’s drainage system. Settlements of up to 1-percent of the wall height commonly occur immediately adjacent to the wall as the wall rotates and develops active lateral earth pressures. Consequently, we recommend that construction of flat work adjacent to retaining walls be postponed at least four weeks after backfilling of the wall, unless survey data indicates that settlement is complete prior to that time. 3.5 Seismic Design Criteria PBS completed a site-specific seismic hazard study for this project. This study is included in Appendix C. Results of this analysis indicate that the OSSC 2007 (IBC 2006) Spectra is suitable for the seismic design of the water reservoir. The seismic design criteria, in accordance with the OSSC 2007 (2006 IBC), are summarized in Table 2 below. Table 2: OSSC 2007 (IBC 2006) Seismic Design Parameters Short Period1 Second Maximum Credible Earthquake Spectral AccelerationSs = 0.538 gS1 = 0.217 g Site ClassD Site CoefficientFa = 1.37Fv = 1.966 Adjusted Spectral AccelerationSMS = 0.737 gSM1 = 0.437 g Design Spectral Response Acceleration ParametersSDS = 0.491 gSD1 = 0.285 g Design Spectral Peak Ground Acceleration0.196 g The spectra for 2 and ½ percent damping can be developed by multiplying the 5-percent- damped spectra by 1.2 and 1.5, respectively. The vertical ground acceleration can be taken as 2/3 of the horizontal ground acceleration. 3.6 Pavement Design PBS understands that a gravel maintenance road will be constructed along the side of the reservoir. Assuming that a limited number of maintenance vehicles and fire trucks will be used, we recommend that a 12-inch layer of gravel be used to construct this access road. A November 23, 2009 Project No. 72852.000 5