Calibrating Side-Resistance Factors of Drilled Shafts in Arizona Cohesionless Soils

The 7th Edition of American Association of State and Highway Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications (2014) contains some significant changes in the analysis for drilled shafts with respect to estimating side resistance. The AASHTO specifications are based on an evaluation of national data. AASHTO encourages owner agencies to develop local resistance factors, calibrated to local geologic conditions based on specific probabilistic, risk-based computational procedures. The current Arizona Department of Transportation (ADOT) specifications are conservative, but the new AASHTO specifications are even more so; using them for ADOT foundations could increase costs by requiring the drilling of longer-than-necessary shafts. Using resistance factors based on Arizona-specific geologic formations will shorten length requirements for drilled shafts and control the related construction costs. The study is using available load testing from Arizona sites and supplementing with axial load test data from other states (such as Nevada) with similar geological conditions to Arizona to develop those local resistance factors for drilled shaft design. To determine potential savings from using those newly developed resistance factors, the study is also gathering drilled shaft cost information from ADOT and other state transportation agencies.