Method Uncertainty for Five Axial Pile Capacity Design Methods

The paper describes two approaches for deriving the mean, standard deviation and probability density function of the method uncertainty for five axial capacity pile design methods, namely the API, Fugro, ICP, NGI and UWA methods. A new unified database of pile load tests recently developed in a joint industry research project (Lehane et al., 2017) is used for the quantification of method uncertainty. The focus of this paper is on the statistical description of the method uncertainty parameters for each of the pile design methods for predicting the axial capacity of piles in sand and in clay. Probabilistic calculations of the axial pile capacity for typical offshore piles using the above five design methods (API, Fugro, ICP, NGI and UWA methods) showed that method uncertainty is a major contributor to the uncertainty in pile foundation capacity. The method uncertainty has therefore a strong influence on the calculated annual probability of failure, and thus on the associated safety level. Establishing the statistics of the error in a capacity prediction model from the measured values (Qm) in pile load tests and the calculated values (Qc) of pile capacity requires careful consideration of several factors. Issues of importance to the derivation of method uncertainty statistics include the effect of different sized databases for the different pile design methods, the effect of case histories with particularly low Qm/Qc values and the possible dependence of method uncertainty on pile length and/or pile diameter. The paper presents two different interpretations for the characterization of method uncertainty and demonstrates their application through a case study for an offshore piled jacket. The effect of method uncertainty on the calculated annual probability of failure is illustrated.