Importance of Lower-Bound Capacities in the Design of Deep Foundations

There is generally a physical limit to the smallest possible capacity for a deep foundation. However, a lower bound on the capacity has rarely been accounted for in performing reliability analyses and developing reliability-based design codes. The objectives of this paper are to investigate the effect of having a lower-bound capacity on the reliability of a geotechnical engineering system and to propose a load and resistance factor design (LRFD) checking format that includes information on the lower-bound capacity in design. It is concluded that a lower-bound capacity can cause a significant increase in the calculated reliability for a geotechnical design even if it is an uncertain estimate. Two alternative LRFD formats that incorporate lower-bound capacities and that would not require substantive changes to existing codes are proposed. Real-world examples dealing with the design of onshore and offshore foundations indicate that the 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 and efficient basis for design.