The Corps of Engineers assisted with the design of a new maintenance hangar for the Air Force Reserve at the Oregon Air National Guard Portland Base. Cornforth Consultants was retained by the Corps to provide geotechnical investigation assistance and recommendations for foundation design and site development.
Two deep exploratory borings were drilled to evaluate subsurface conditions and obtain samples for materials testing. A suite of laboratory soil tests was performed to classify engineering parameters for design. The site is underlain by interbedded alluvial deposits of highly compressible Columbia River Alluvium and loose sands to a depth of approximately 40 feet, and medium dense to very dense sands below this depth. Settlement evaluations precluded the use of shallow footings to support the structure.
Consequently, recommendations were provided for deep pile foundations, including estimates of potential downdrag and parameters for estimating lateral capacity. A seismic evaluation was performed to evaluate the liquefaction susceptibility of the loose sand layers. The analyses indicated that the loose sands are potentially liquefiable under the design earthquake motions; however, due to the relatively level topography of the site, potential liquefaction-induced lateral spread was estimated to be very small. The pile recommendations accounted for temporary strength loss during the postulated earthquake loading. Site grading recommendations were also provided to minimize the potential for damaging settlement adjacent to the pile-supported structure. Pavement design parameters and construction recommendations were developed for design of the rigid base slab.