4290805 - Laserfiche WebLink

Home Browse Search

Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon 1.0 INTRODUCTION 1.1 General PBS Engineering + Environmental (PBS) has completed a geotechnical investigation for the proposed water reservoir site in Detroit, Oregon. The reservoir site is located near the north end of Guy Moore Drive, at the approximate location shown on Figure 1 (Figure 1 – Vicinity Map, Appendix A). The purpose of this report is to evaluate subsurface conditions at specific locations within the reservoir site and provide geotechnical design criteria and construction recommendations for earthwork and reservoir foundations. We have also completed a site- specific seismic hazard study in accordance with the 2007 Oregon Structural Specialty Code (OSSC) Section 1802.6. 1.2 Proposed Construction The proposed project consists of an approximately 210,000-gallon water reservoir to be constructed on City-owned property adjacent to Guy Moore Drive. We understand that the proposed reservoir will be 35 to 50 feet in diameter. The site is along the east side of a gently sloping ridge. According to the aerial topographic survey map provided to us, the ground elevation at the reservoir site is approximately 1,870 feet above mean sea level (AMSL). The reservoir will be constructed at the same location as the previous water tank. The concrete pad from the previous water tank is still at the site. 1.3 Field Exploration and Testing Subsurface exploration at the site included excavation of two test pits on November 9, 2009. The test pits were excavated by Kevin’s Backhoe Services of Detroit, Oregon. Test pit locations are shown on Figure 2 – Site Exploration Map (Appendix A). The subsurface materials encountered were logged in general accordance with the Manual- Visual Classification Method (American Society for Testing and Materials [ASTM D 2488]). Representative grab samples were collected from the test pits. The test pits were backfilled with the excavated material and lightly packed with the bucket of the backhoe. The soil samples were re-examined in the laboratory to verify field classifications. Interpreted test pit logs are presented in Appendix B. Moisture contents and Atterberg Limits were determined for selected soil samples with the results shown on the test pit logs (Appendix B). 2.0 SITE AND SUBSURFACE CONDITIONS 2.1 Geologic Setting According to published geologic mapping (Walker & Duncan, 1989) the site is underlain by an assemblage of tuffaceous sedimentary rocks, volcanic lava flows and tuffs (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. 2.2 Surface Conditions The proposed reservoir site is located on the eastern side of a south trending ridge. The north edge of the existing concrete pad is approximately 30 feet from the edge of Weber Street. The existing concrete pad is about 7 feet lower than the road grade. The terrain around the reservoir pad slopes at approximately 15 degrees to the southeast. The existing reservoir pad appears to have been constructed as a cut/fill, with the northwest side of the pad being the cut side and the southeast side being the fill area of the level pad. The reservoir pad is accessed from a gravel driveway on the west side of the pad. The area around the reservoir pad is forested with mature conifer trees. November 23, 2009 Project No. 72852.000 1