Effects of Gravels, Cobbles, and Boulders on Drilled Shaft Capacity Cypress Avenue Bridge, Redding, California

Drilled shaft capacity computations are routinely performed by geotechnical professionals using the Alpha and Beta methods outlined in the Federal Highway Administration (FHWA) Publication FHWA-IF-99-025. For cohesionless intermediate geomaterials, the analysis relies heavily upon the Standard Penetration Test sampler blowcounts obtained from exploratory borings. In the presence of alluvial gravel, cobble and boulder deposits, such evaluation cannot be reliably performed. This paper presents information obtained from Osterberg Cell load testing of 2.5 meter (98-inch) diameter shafts advanced using the casing oscillator method at the Cypress Avenue Bridge over the Sacramento River in Redding, California. The relationships between side shear transfer, end bearing, and settlement in cobble and boulder rich deposits with various matrix materials and degrees of weathering are demonstrated. The effects of particle dilation in the development of shaft side resistance and zones of higher stress concentration are also discussed. The results of instrumented pile load testing are compared with the results of drilled shaft capacity estimates made using the FHWA methods in an effort to better correlate field performance with analytical methods.