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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon C.1 INTRODUCTION This appendix presents the results of the PBS Engineering + Environmental (PBS) site-specific seismic hazard analysis for the proposed water reservoir facility. The proposed site is located near the north end of Guy Moore Drive in the city of Detroit. The site location relative to surrounding physical features is shown on Figure 1 (Appendix A). This facility qualifies as an “Essential Facility,” in accordance with 2007 Oregon Structural Specialty Code (OSSC), Chapter 18. A site-specific seismic hazard analysis is therefore required and was conducted in general accordance with Section 1802.6. C.2 SITE CONDITIONS C.2.1 Geologic Setting As presented in Section 2.1 of this report, published geologic mapping (Walker & Duncan, 1989) indicates that the site is underlain by volcanic lava flows and tuffs with some interbedded sedimentary rocks (map unit Tu) of Miocene and Oligocene age. This unit is described as a heterogeneous assemblage of basalt and andesite, including flows and breccias as well as ash-flow and air-fall tuffs. The unit also contains lahar deposits, tuffaceous sedimentary rocks, and volcanic conglomerates. C.2.2 Subsurface Conditions PBS explored subsurface conditions by excavating two test pits on November 9, 2009. Test pit locations are shown on Figure 2 – Site Exploration Plan (Appendix A). Details of our field explorations and subsurface conditions are provided in Sections 1.2 and 2.3 of this report. Summary logs of the test pits are contained in Appendix B. The interpreted geologic profile, shown in Table C-1, below, is based on our explorations and review of geologic and subsurface information summarized. Table C-1: Estimated Geologic Profile Profile DepthShear Wave Velocity Geologic Unit (feet)(feet per second) 0 to 4Medium-stiff sandy silt deposits (fill and colluvium)300 Stiff to very stiff, silty clay to clayey silt with some 4 to 17600 – 1,200 sand and gravel (decomposed bedrock volcanic tuff) 17 to 100Lava flows, volcanic tuff, sedimentary rocks600 – 2,200 C.2.3 Groundwater Conditions Groundwater seepage was not observed in the test pits excavated to 17.0 feet bgs. The elevations of the creeks to the west and east of the ridge suggest that permanent groundwater is likely between 20 and 40 feet bgs. Groundwater levels are expected to fluctuate seasonally and will be higher during the rainy season and/or in response to storm events. Seasonal shallow, perched groundwater may also be present within the upper 5 feet during the rainy season. Moisture contents in the decomposed tuff ranged between approximately 11 and 42 percent. November 23, 2009 Project No. 72852.000 C-1