Reliability of Shallow Foundations Designed Against Bearing Failure Using LRFD

There is a growing interest world-wide in the development of a rational reliability-based geotechnical design code. The reason for this interest is a least two-fold; first of all, geotechnical engineers face uncertainties which are commonly significantly larger than those faced in other fields of engineering and so there is a need to properly characterize and deal with this uncertainty. Secondly, structural engineers have been using a reliabilitybased design code for decades now, and there is a need to develop the same for geotechnical engineers so that the two groups can ‘speak the same language’. This paper develops a theoretical model to predict the probability that a shallow foundation will exceed its supporting soil’s bearing capacity. The footing is designed using characteristic soil properties (cohesion and friction angle) derived from a single sample, or ‘core’, taken in the vicinity of the footing and used in a load and resistance factor design approach. The theory predicting failure probability is validated using a two-dimensional random finite element method analysis of a strip footing. Agreement between theory and simulation is found to be very good, so that the theory can be used with confidence to perform risk assessments of foundation designs and develop resistance factors for use in code provisions.