Incorporating Lower-Bound Capacities into LRFD Codes for Pile 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 (1) investigate the possibility of a lower -bound capacity using pile load test databases; (2) study how a lower -bound capacity could affect the reliability for a pile foundation; and (3) propose a Load and Resistance Factor Design (LRFD) design-checking format that includes information on the lower -bound capacity in addition to the conventional design information. Databases with driven pipe piles show clear evidence for the existence of a lower -bound capacity in both clays and sands. This lower-bound capacity is a physical variable that can be calculated based on mechanics with site-specific soil properties. For driven pipe piles, the calculated lower-bound capacity typically ranges from 0.5 to 0.9 times the calculated predicted capacity. Re liability analyses illustrate that the presence of a lower-bound capacity can have a significant effect on increasing the reliability of a deep foundation. Its effect increases as the coefficient of variation for the capacity increases and as the target re liability index increases. Finally, it is shown that information about the lower-bound capacity can be incorporated into LRFD codes using two alternative formats that would not require substantive changes to existing codes. 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.