Model Errors in Bearing Capacity of Vertically Loaded Foundations

This paper scrutinizes the model error in the ultimate bearing capacity of grouped-pile foundations and shallow foundations. First, the model uncertainty of (Observed value)/(Calculated value) is evaluated for grouped-pile foundations using the statistics of the load test data sets of the several single piles at the same locations. The system reliability of a pile group is estimated considering the fact that the variability in the average bearing capacity of piles decreases as the number of the piles increases. Second, the model uncertainty is evaluated for shallow foundations, considering the scale effect on the bearing capacity. The statistics shows that the model error is not dependent on the effective footing width when the calculation considers the scale effect relevantly. The bearing capacity on soft rocks tends to be overestimated and an additional modifier should be considered regarding the resistance factor or in-situ plate loading tests should be conducted in addition to typical laboratory tests for estimating characteristic soil parameter values.