The importance of lower-bound capacities in geotechnical reliability assessments

The ability to manage risk in geotechnical engineering relies on a realistic assessment of the probability of failure for designs. Most reliability analyses focus on the mean and variance and an assumed, mathematically convenient distribution to model the left-hand tail of the distribution for capacity. However, the reliability of a geotechnical engineering system is governed by a physical constraint on the smallest available capacity. This lower-bound capacity is usually neglected in conventional reliability analyses.; In this study, databases of load tests conducted on offshore and onshore deep foundations are analyzed to provide evidence for the existence of a lower-bound capacity that can be calculated using site-specific soil properties and information about the geometry of the foundation. Next, realistic probability distributions that can accommodate a lower-bound capacity are proposed and used to relate reliability to the lower-bound capacity. Multiple Load and Resistance Factor Design (LRFD) design-checking formats that include information on the lower-bound capacity in addition to the conventional design information are then introduced. Finally, practical approaches are presented for updating information about lower-bound capacities using installation data, proof-load data, and historical performance of foundations under load.; Databases with deep foundations show clear evidence of the existence of a lower-bound capacity that typically ranges from 0.4 to 0.8 of the predicted capacity in both cohesive and cohesionless soils. Results from reliability analyses indicate that the presence of a lower-bound capacity can have a significant effect on increasing the reliability of a deep foundation. The effect of the lower-bound capacity increases as the coefficient of variation for the capacity increases and as the target reliability index increases. This result indicates that reliability-based design codes need to incorporate information about lower-bound capacities. Incorporation of a lower-bound capacity into design is expected to provide a more realistic quantification of reliability for decision-making purposes and therefore a more rational basis for design.