The Importance of Lower-Bound Capacities in Geotechnical Reliability Assessments

Summary

Note: This study is part of a broader project “Suction Caissons: Seafloor Characterization for Deepwater Foundation Systems” (MMS Project 362).

The objective of this research is to investigate the effect of having a lower-bound capacity on the reliability of geotechnical engineering systems and to provide a methodology for incorporating information about lower-bound capacities in design.

These goals will be achieved through the following tasks:

1. Investigate the possibility of a lower-bound capacity using databases of load test for driven piles and suction caissons.
2. Propose realistic distributions for capacity that can accommodate a lower-bound capacity, and use these distributions to provide a realistic model of the uncertainty in the capacity of driven piles and suction caissons under axial loading.
3. Investigate the effectiveness of numerical and semi-analytical methods in assessing the probability of failure for problems where the distribution for capacity has a lower bound.
4. Study the effect of having a lower-bound capacity on the reliability of geotechnical engineering systems, and investigate the effect of uncertainty in the lower-bound capacity on the magnitude of increase in the reliability.
5. Propose an LRFD design-checking format that includes information on the lowerbound capacity in design.
6. Propose practical approaches for updating information about lower-bound capacities using installation data, proof-load data, and historical performance of foundations under load.

Related Publications: Gilbert, R. B., Choi, Y. J., Dangyach, S. and Najjar, S. S. (2005), “Reliability-Based Design Considerations for Deepwater Mooring System Foundations,” Proceedings, ISFOG 2005, Frontiers in Offshore Geotechnics, Perth, Western Australia, 317-324.

Gilbert, R. B., Najjar, S. S. and Choi, Y. J. (2005), “Incorporating Lower-Bound Capacities into LRFD Codes for Pile Foundations,” Proceedings, Geo-Frontiers 2005, Geotechnical Special Publication No. 132 (CD-ROM), ASCE, Reston, Virginia.

Gilbert, R. B., Najjar, S. S. and Shields, M. K. (2005), “Importance of Residual Strengths in Factors of Safety and Reliability,” Proceedings, Geo-Frontiers 2005, 18th GRI Conference on Geosynthetics Research and Development in Progress (CD-ROM), ASCE, Reston, Virginia.

Najjar, S. S. and Gilbert, R. B. (2005), “Lower-Bound Estimates of Capacity in Geotechnical Reliability Assessments,” Proceedings,ICOSSAR 2005, Intl. Conf. on Structural Safety and Reliability, Rome, Italy, 955-962.