Calibration of partial safety factor for CPT-based axial pile capacity design methods

The API RP 2GEO (2011) and ISO 19902 (2007) guidelines include, in addition to the API-method, four CPT-based methods for calculating the axial capacity of piles in sand, namely the Fugro, ICP, NGI and UWA methods. However, the values of partial safety or resistance factors to be used with the CPT-based methods are not given. The designer is required to select an appropriate safety factor when using the CPT-based methods. The difficulty lies in deciding which safety factor or combination of partial safety factors will ensure a consistent safety level for different soil conditions and pile dimensions. In a recent Joint-Industry Project, a reliability-based approach was adopted to calibrate the resistance factor of the four CPT-based methods with 13 case studies. The goal of this case-by-case calibration was to ensure that the annual failure probability of the designed pile foundation was less than a target annual probability, chosen for this study as 10 −3 , 10 −4 or 10 −5 , and to compare the results with the safety level implied by the current API-pile design method. To achieve consistent results, the code calibration was based not only on the computed annual failure probabilities for each pile design method, but also on the comparison of the predicted axial capacities with each method for the individual case studies relative to each other. The paper explains the working hypotheses and describes the approach adopted for the calibration and optimization of the partial safety factor. The paper makes recommendations for the resistance factors to be used with the four CPT-based axial pile capacity design methods in the API and ISO guidelines.