On the Cautious Estimation of Characteristic Soil Strength for Axial Pile Capacity

Ronold, Knut O. (DNV GL)


Assuming perfect knowledge of soil strength and prescribing load factors for loads, we calibrate the necessary requirement for the material factor on characteristic soil strength by tuning a reliability analysis to meet a prescribed target failure probability. Keeping this calibrated material factor unchanged, the reliability analysis is repeated with the stochastic model for soil strength altered to include statistical uncertainty owing to limited soil data. A reduced "cautious" value of the characteristic soil strength is determined such that the failure probability resulting from the analysis is maintained equal to the target. Based on this reduced value, the minimum confidence level needed for characteristic value estimation is interpreted. Using soil strength for prediction of axial pile capacity as an example, this paper outlines a procedure for reliability-based calibration of the minimum confidence needed when estimating characteristic soil strength, defined as the mean value, with confidence for use in offshore pile design.