Improvement of the Geotechnical Axial Design Methodology for Colorado’s Drilled Shafts Socketed in Weak Rocks (05-0894)**
Naser Mahmood Abu-Hejleh, Federal Highway Administration
Michael Wayne O'Neill (Deceased), No Organization
Dennis Hanneman, Geocal, Inc.
William J. Attwooll, Terracon Consultants

Drilled shaft foundations embedded in weak rock formations support a large percentage of bridges in Colorado. Since the 1960s, empirical methods have been used for the axial geotechnical design drilled shafts in Colorado that entirely deviate from the AASHTO design methods. The margin of safety and expected shaft settlement are unknown in these empirical methods, however, both are needed for the implementation of the new and more accurate AASHTO Load and Resistance Factor Design (LRFD) method in CDOT (Colorado Dept. of Transportation) design guidelines. Load tests on drilled shafts provide the most accurate design information and research data for improvement of the design methods. In 2002, four Osterberg axial load tests were performed on drilled shafts embedded in the typical range of weak rocks encountered in Denver: soil-like claystone to very hard sandy claystone to even much harder clayey sandstone. To maximize the benefits of this work, the O-Cell load test results and information on the construction and materials of the test shafts were documented, and an extensive program of simple geotechnical tests was performed at the load test sites. This included standard penetration tests (SPT), unconfined compressive strength tests (UCT), and pressuremeter tests (PMT). Analysis of all test data, information, and experience gained in this study were employed to provide: 1) best-fit equations to predict the unconfined strength of weak rocks from SPT, and PMT data, 2) assessment of the CDOT and AASHTO/FHWA design methods; and 3) recommended design equations to predict the shaft ultimate unit base resistance (qmax), side resistance (fmax), and an approximate load-settlement curve as a function of the results of simple geotechnical tests for the weak rock investigated in this study. Other products are developed to help CDOT with implementation of accurate and feasible LRFD methods for the design of drilled shafts.