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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon 4.2 Excavation Materials at the site include 6 inch topsoil underlain by colluvium and fill to a depth of 4 feet underlain by generally fine-grained (silt and clay) soil with variable sand and gravel content that is interpreted to be decomposed volcanic tuff. Excavations in these upper soils may be readily accomplished with conventional earthwork equipment. Trench cuts should stand vertical to a depth of approximately 4 feet, provided no groundwater seepage is present in the trench walls. Open excavation may be used to excavate trenches with depths between 4 and 8 feet, with the walls of the excavation cut at a slope of 1H:1V, provided groundwater seepage is not present and with the understanding that some sloughing may occur. The trenches should be flattened to 1.5H:1V if excessive sloughing occurs or seepage is present. Groundwater was not observed during our field investigation. However, if shallow groundwater is observed during construction, use of a trench shield or other approved temporary shoring is recommended for cuts that extend below groundwater seepage or if vertical walls are desired for cuts deeper than 4 feet. If shoring or dewatering is used, we recommend that the type and design of the shoring and dewatering systems be the responsibility of the contractor, who is in the best position to choose systems that fit the overall plan of operation. These excavations should be made in accordance with applicable Occupational Safety and Health Administration (OSHA) and State regulations. 5.0 CONSTRUCTION OBSERVATIONS Satisfactory earthwork performance depends on the quality of construction. Sufficient observation of the contractor's activities is a key part of determining that the work is completed in accordance with the construction drawings and specifications. We recommend that a geotechnical engineer from PBS be retained to observe general excavation, stripping, fill placement, footing subgrades, and subgrades and base rock for floor slabs and pavements. Subsurface conditions observed during construction should be compared with those encountered during the subsurface explorations. Recognition of changed conditions requires experience; therefore, qualified personnel should visit the site with sufficient frequency to detect whether subsurface conditions change significantly from those anticipated. 6.0 LIMITATIONS This report has been prepared for the exclusive use of the addressee, and their architects and engineers for aiding in the design and construction of the proposed development. It is the addressee's responsibility to provide this report to the appropriate design professionals, building officials and contractors to ensure correct implementation of the recommendations. The opinions, comments, and conclusions presented in this report are based upon information derived from our literature review, field investigation, and laboratory testing. Conditions between, or beyond, our exploratory borings may vary from those encountered. Unanticipated soil conditions and seasonal soil moisture variations are commonly encountered and cannot be fully determined by merely taking soil samples or soil borings. Such variations may result in changes to our recommendations and may require that additional expenses to attain a properly constructed project. Therefore, we recommend a contingency fund to accommodate such potential extra costs. November 23, 2009 Project No. 72852.000 7