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Geotechnical Investigation ^ Proposed Courthouse Square - Salem, Oregon extend into the western edge of the building. The proposed location of the building is shown on Figure 2. Spread footings are the preferred foundation type. 2.0 GEOLOGIC SETTING The Oligocene sea, which covered the Salem area and most of northwestern Oregon, receded about 25 million years ago (mya). During the Miocene (5-15 mya), Columbia River Basalts from fissures ~ and vents in northeastern Oregon, poured through the Columbia gorge covering the coastal plain from what is now Portland to just south of Salem. In the Pliocene ~mya), the broad coastal plain ~~~(~ began to change with the uplift and tilting of the Coast Range and the Willamette Valley began to `"' form. With the formation of the valley, the rugged erosional surface of the Columbia River Basalts began to be covered with river alluvium. The course of the ancient river is marked by sand and gravel deposits, while the valley margins are more chazacteristically underlain by fine grained overbank deposits. These alluvial sediments are on the order of 300 feet in thickness in the Salem .~ area. 3.0 FIELD EXPLORATIONS Subsurface conditions were explored by drilling 5 borings within the accessible areas of the site. Three of these borings were drilled using hollow stem auger and, because of difficult drilling conditions, 2 borings were d~i~led using mud rotary. The borings were sited to provide general profiles of subsurface conditions within the proposed areas of development; however, selection of drilling locations was limited by existing buildings on site which occupy approximately 50 percent of the project azea. Because of these existing buildings, no borings could be drilled in the southwest one-quarter of the site or the northeast one-quarter of the site. Locations of the borings are indicated on the Site Plan, Figure 2. The borings were drilled as part of a field program that included drilling 20 additional borings, to a typical depth of 15 feet, for the purpose of environmental site characterization. All borings were reviewed as part of this study. ~nly the geotechnical borings, which extended below the proposed depth of site excavation, are included in this report. Because the geotechnical boring were part of a larger overall program, the numbering of the borings are not sequential. The borings were advanced using a 4-inch ID continuous flight hollow-stem auger or mud rotary technique, powered by a CME-75 truck mounted drill rig. Samples of subsurface materials were taken at intervals of 5 feet, using a standazd split-spoon sampler which is driven into :~~ ~ centurywest ENGINEERING CORPORATION 2 March 7, 1997 40806.001.08 ~ L__1