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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon 2.3 Subsurface Conditions Subsurface conditions are based on our field explorations and a review of general site geology. The approximate location of the test pits are shown on Figure 2 (Appendix A). Both test pits were excavated to a depth of 17.0 feet below ground surface (bgs). Topsoil was observed in the upper 9-inches of both test pits. Colluvium was observed below the topsoil in Test Pit TP-1 to a depth of 2.8 feet bgs. Fill from the cut portion of the reservoir pad was encountered below the topsoil in Test Pit TP-2 to a depth of 3.8 feet bgs. Soil interpreted as decomposed tuff was observed below the colluvium in Test Pit TP-1 and the fill in Test Pit TP-2. TOPSOIL: Soft, brown, organic sandy silt with trace amounts of clay. COLLUVIUM: Medium-stiff, brown, sandy silt with trace clay. Only observed in Test Pit TP-1. FILL:Medium-stiff, brown, sandy silt with trace clay. Only observed in Test Pit TP-2. DECOMPOSED Medium-stiff to very stiff, dark gray to brown, sandy silty clay TUFF:to sandy with some cobbles. A layer of very dense sandy gravel and cobbles with a matrix of sandy silt was observed from 9.8 to 12.5 feet bgs in Test Pit TP-1 that is underlain by medium-dense, gray, silty sand to 14 feet bgs. Silty sand with some gravel was also observed from 7.5 to 9.0 feet bgs in Test Pit TP-2. 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. Samples from the upper portion of the decomposed tuff were analyzed for Atterberg Limits. The results are summarized on the test pit logs and graphs of liquid limit versus plasticity index are included in Appendix B. The liquid limit and plasticity index of the samples from Test Pit TP-1 (at 4.5 feet bgs) were 66 and 29 percent, respectively; whereas in Test Pit TP- 2, they were 63 and 25 percent, respectively. Thus, these results plot below the A-line in the MH or OH field. 3.0 SITE CONDITIONS 3.1 Discussion The results of our field explorations show that the site is underlain by topsoil, colluvium and fill to a maximum depth of 4 feet bgs. Below these near-surface soils to the depth explored of 17 feet bgs is generally fine-grained (silt and clay) soil with variable sand and gravel content that is interpreted to be decomposed volcanic tuff. The tuff is stiff to very stiff below 4.5 feet bgs. The tuff unit also contains interbeds of very dense gravel and cobbles with a matrix of silty sand and medium-dense silty sand with trace to some gravel up to an observed thickness of approximately 2.6 fee bgs. Groundwater was not observed to the November 23, 2009 Project No. 72852.000 2