Prior to 1997, WSDOT used the Engineering News Record (ENR) Formula for driving piling to the design capacity. Washington State Department of Transportation (WSDOT) sponsored research published in 1988 had shown that the ENR formula was quite inaccurate, and that moving toward the Gates Formula would be a substantial improvement Fragaszy et al. 1988). Hence, in 1996, an in-house study was initiated to update the driving formula used for pile driving acceptance in the WSDOT Standard Specifications.
Recently compiled databases of pile load test results were used as the basis for developing improvements to the Gates Formula to improve pile bearing resistance prediction accuracy. From this empirical analysis, the WSDOT driving formula was derived.
Once the WSDOT driving formula had been developed, the empirical data used for its development were also used to establish statistical parameters that could be used in reliability analyses to determine resistance factors for load and resistance factor design (LRFD). The Monte Carlo method was used to perform the reliability analyses. Other methods of pile resistance prediction were also analyzed, and resistance factors were developed for those methods as well.
Of the driving formulae evaluated, the WSDOT formula produced the most efficient result, with a resistance factor of 0.55 to 0.60. A resistance factor of 0.55 is recommended. Dynamic measurement during pile driving using the pile driving analyzer (PDA), combined with signal matching analysis (e.g., CAPWAP), produced the most efficient result of all the pile resistance prediction methods, with a resistance factor of 0.70 to 0.80.