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 recommend using a resistance factor that was calibrated based on an evaluation of national data. However, load tests performed in Arizona (primarily in Phoenix and Tucson) have consistently demonstrated much larger values of resistance compared to those predicted using AASHTO specifications. Most Arizona soils are cohesionless, but have coarser aggregates and various degrees of cementation when compared to cohesionless soils in other states. While the current Arizona Department of Transportation (ADOT) specifications are conservative, the new AASHTO specifications are even more so, and using them for ADOT foundations would increase costs by drilling longer-than-necessary shafts. AASHTO encourages owner agencies to develop local resistance factors that are calibrated to local geologic formations based on specific probabilistic, risk-based computational procedures. Using resistance factors based on Arizona-specific geologic formations will shorten length requirements for drilled shafts and reduce the related construction costs. The research would have three main objectives: (1) Supplement current axial load test data with data solicited from other transportation agencies in Arizona and in states (e.g. Nevada, New Mexico, and others) where soil conditions are similar to Arizona’s. The data will be used to develop a database that can be further populated as more data becomes available. (2) Using the available axial load test data, calibrate and verify resistance factors for side resistance of drilled shafts in cohesionless soils. (3) Determine the cost savings of using the newly developed resistance factors.