Reliability Based Calibration of Resistance Factors for Axial Capacity of Driven Piles

Resistance factors were calibrated using the framework of reliability theory for the Load and Resistance Factor Design (LRFD) of driven pile's axial capacity in North Carolina utilizing pile load test data available from the North Carolina Department of Transportation (NCDOT). A total of 140 Pile Driving Analyzer (PDA) data and 35 static load test data were compiled and grouped into different design categories based on four pile types and two geologic regions. Resistance statistics were evaluated for each design category in terms of bias factors. Reliability analysis of the current NCDOT practice of pile foundation design was performed to evaluate the level of safety and to select the target reliability indices. Resistance factor calibration was performed for the three methods of static pile capacity analysis commonly used in the NCDOT: the Vesic (1977), the Nordlund (1963), and the Meyerhof (1976) methods. Two types of First Order Reliability Methods (Mean Value First Order Second Moment method and Advanced First Order Second Moment method) were used for the reliability analysis and the calibration of the resistance factors. The calibrated resistance factors varied significantly for the different pile types and geologic regions. The advanced first order second moment method resulted in larger resistance factors than the mean value first order second moment method. Recommended resistance factors are presented for the three methods of static pile capacity analysis and for seven different design categories of pile types and geologic regions.