Reliability-based geotechnical foundation design focusses on soil and structure analysis that meets necessary safety, performance and/or serviceability criteria, calibrated based on probabilistic analyses and an accepted level of risk. As the civil engineering community seeks to better harmonize geotechnical and structural design methodologies, reliability-based design is being incorporated more into geotechnical limit states analysis, for example ultimate limit state (ULS; e.g., bearing capacity) and serviceability limit state (SLS; e.g., settlement) foundation design. However, additional work is required to develop robust design procedures that can be easily implemented in practice.
The main objective of this study is to advance the underlying knowledge of reliability-based serviceability limit state (RBSLS) design for shallow foundations. Particularly, this study focusses on foundations supported on plastic, fine-grained soil (e.g., clay) and aggregate pier-improved plastic, fine-grained soil.
As part of this work, two new RBSLS models were developed for footings supported on clay and aggregate pier-reinforced clay. Both SLS models were developed and calibrated using probabilistic analyses and databases compiled from the literature for high-quality footing loading tests with immediate (undrained) settlement. The new models capture the nonlinear bearing pressure-displacement behavior that is typical of footings on plastic, fine-grained soils, even at relatively small (e.g., service-level) loads. A calibrated, lumped load and resistance factor is also introduced for both models that can be easily implemented in conjunction with a pre-selected level of risk and/or reliability index.
The study continues with further discussion of calibration procedures, with focus on the impact of the correlation structure of individual load-displacement parameters and suitable factors to account for the propagation of model error and other sources of uncertainty. This phase of work also focused on re-evaluating the calibrated SLS model for footings supported on aggregate pier-reinforced clay and providing an independent evaluation using additional full-scale loading tests completed at the Oregon State University geotechnical engineering field research site (OSU GEFRS).
Finally, the calibrated load-displacement model for a footing supported on fine-grained soil was used to develop a non-linear soil-spring model within the computer program OpenSees. The foundation spring model was used in combination with a previously developed 3-story steel moment frame building model to complete a series of Monte Carlo simulations with varying levels of soil variability to investigate the role of both inherent soil variability and soil-structure interaction on foundation and structural performance.