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Geotechnical investigation s Proposed Courthouse Square - Salem, Oregon ,, ; s ~ Ground water was encountered in all borings. Depth to ground water, measured at the time of r r drilling, ranged from 11 feet to 15 feet below ground surface. 6.0 CONCLUSIONS Based on the results of our investigation, it is our opinion that the site soil conditions are adequate for construction of the proposed structure and appurtenant features as long as the design incorporates ~~ the recommendations found in Section 7.0 of this report. Field exploration and laboratory testing indicate that the project site is underlain by generally dense to very dense naturally occurring alluvial deposits of silty gravels overlying sandy gravels. A spread footing foundation system, consisting of individual column load pads, and continuous spread footings if required, is suitable for this site. Estimated total settlement of foundation materials supporting the proposed structure should be less than 1 inch and differential settlement should not exceed 'h inch in a 20-foot span. Proposed below ground parking will not extend all the way to the western edge of the building. Along this western side, footings will bear on native gravelly soils at shallow depth. An existing partial basement under the Senator Hotel in the southwest portion of the site may impact bottom of footing elevations. In addition, demolition may result in limited areas of excavation below the bottom of the parking garage. To utilize the maximum allowable bearing value in these areas, all footings must extend through.any fill or backfill to bear on dense native gravelly soils. Preliminary design indicates that the lowest parking gazage level will be approximately 20 feet below existing grade. Observations during this study and records from on site monitoring wells indicate that seasonal ground water levels vary from approximately 11 feet below ground surface to a summer time low of 17 feet below ground surface. Dewatering of the lower levels of the garage will be required during construction. To the extent possible, dewatering should be scheduled for the lowest mid to late summer ground water conditions. Liquefaction is a phenomenon in which saturated cohesionless soils are subject to a temporary but essentially total loss of shear strength because of pore pressure build-up under the reversing cyclic shear stresses associated with earthquakes. Where this occurs at shallow depths, damage to surface structures can occur. Liquefaction risk assessment considers the presence of potentially liquefiable ~~~ centurywest EN6INEERING CORPORATION 4' March 7, 1997 40808.001.08 ~ I~