Overburden Effect on the Axial Resistance of Instrumented CFA Piles

Static pile loading tests and high strain dynamic tests were performed to confirm the foundation design for a major expansion to a hospital on the Stanford University campus in Palo Alto, California. The foundations were 610 mm (24 in) diameter continuous flight auger (CFA) piles to be installed at the bottom of a staged excavation. Advantages to the construction schedule made it desirable to execute this test program on piles installed approximately 6.1 m (20 ft) above the final excavation level; however, such an approach was recognized to require accounting for the overburden effect prior to application of the results to design. Therefore, the test piles were instrumented with strain gages and subsequently monitored throughout testing in an effort to allow meaningful interpretation of the overburden effect. Dynamic testing of one of the test piles after excavation to grade suggests that the implemented methodology resulted in a conservative geotechnical design.