Bearing capacity and immediate settlement of shallow foundations on clay

Abstract

Shallow foundations are extensively used to support structures of all sizes and derive their support from near surface soils. Thus, they are typically embedded up to a few meters into the soil profile. Designers of shallow foundations are required to meet two limit states: overall failure of the soil beneath the foundation (bearing capacity) and excessive settlement. Existing bearing capacity design methods use an assumed shearing plane within the soil and perfectly plastic soil behavior to estimate the ultimate resistance available. The immediate settlement of a shallow foundation is typically approximated using an elasticity-based method that does not account for actual, nonlinear soil behavior. A load test database was developed from footing load tests reported in the literature to assess the accuracy and uncertainty in existing design methodologies for calculating bearing capacity and immediate settlement. The assessment of uncertainty in bearing capacity and immediate settlement was accomplished through the application of a hyperbolic bearing pressure-displacement model, and the adaptation of the Duncan-Chang soil constitutive model to footing displacements. The prediction of bearing capacity using the general bearing capacity formula was compared to the bearing capacity extrapolated from the load test database using a hyperbolic bearing pressure-displacement model. On average the general bearing capacity formula under-predicts the bearing capacity and exhibits a significant amount of variability. The comparison was used to develop resistance statistics that were implemented to produce resistance factors for an LRFD based design approach using AASHTO load statistics. The Duncan-Chang model was adapted to predict bearing pressure displacement curves for footings in the load test database and used to estimate governing soil parameters. Bearing pressure-displacement curves fitted to the observed curves were used to back calculate soil stiffness. The soil stiffness was used with an elasticity-based displacement prediction method to evaluate the accuracy of the method. Finally, the back-calculated modulus from the fitted Duncan-Chang model was used to assess the accuracy and uncertainty associated with the elasticity-based K-factor, a correlation based stiffness parameter. In general the comparisons indicate that the current design procedures over-predict the bearing pressure associated with a given displacement and exhibit a significant amount of uncertainty.