Resistance Factors for Side Friction and End Bearing of Drilled Shafts in Shales

Side resistance factors and tip resistance factors were calibrated for use in service limit state (SLS) designs of side friction shafts and end bearing shafts in shales. The SLS design procedure utilizes the t-z method and adopts the factored strength approach to account for variability and uncertainty in the design and associated input parameters. The variability and uncertainty considered for calibration of the resistance factors were derived from full-scale load tests performed on drilled shafts embedded in shales in the central United States. Monte-Carlo simulations were used to calibrate the resistance factors. The range for side resistance factors is 0.05-0.2, and the range for tip resistance factors is 0.1-0.5, depending upon shale uniaxial compressive strength coefficient of variation, normalized loads, and probability of failure. Under the same conditions, side resistance factors are smaller than tip resistance factors. The obtained resistance factors are substantially low but reasonably reflect the collective variability and uncertainty presented in the design of drilled shafts in shales.