Reliability-based serviceability limit state design for immediate settlement of spread footings on clay

While many spread footings constructed on clayey soils are designed using consolidation settlement analyses for the serviceability limit state (SLS), immediate settlement, or undrained displacement, of the footing may also contribute a significant portion of the total and/or differential settlement. Owing to possible magnitudes in immediate settlement, and with regard to stress history, assessment of the contribution of immediate settlement comprises an essential task for the understanding of the performance of a foundation system. This study proposes a simple reliability-based design (RBD) procedure for assessing the allowable immediate displacement of a spread footing supported on clay in consideration of a desired serviceability limit state. A relationship between the traditional spread footing bearing capacity equation and slope tangent capacity is established, then incorporated into a bivariate normalized bearing pressure–displacement model to estimate the mobilized resistance associated with a given displacement. The model was calibrated using a high quality database of full-scale loading tests compiled from various sources. The loading test data was used to characterize the uncertainty associated with the model and incorporated into an appropriate reliability-based performance function. Monte Carlo simulations were then used to calibrate a resistance factor with consideration of the uncertainty in the bearing pressure–displacement model, bearing capacity, applied bearing pressure, allowable displacement, and footing width. An example is provided to illustrate the application of the proposed procedure to estimate the bearing pressure for an allowable immediate displacement of a footing at the targeted probability and serviceability limit state.