Material Model Parameters for Shallow Foundation Numerical Analysis

In material modeling, a number of physical properties such stiffness, strength and post-yield capacity can be directly determined through element-level tests. However, to actually generate a numerically robust representation of a materials response, additional parameters describing the backbone and/or cyclic response (e.g., the shape of transitions from linear to nonlinear regions and the full shape of the post-yield region) are needed. These backbone parameters are difficult to characterize directly from an element-level experiment, and thus are not well correlated with typical soil parameters. However, they may be more readily obtained from system-level tests. Therefore, this study focuses on the parameters needed to describe the nonlinear response of Winkler springs, used in the context of Winkler-based shallow foundation modeling. The foundation subgrade is assumed as a system of discrete, mechanistic, uncoupled springs having nonlinear uplift, sliding and damping capabilities. For this type of modeling problem, elastic-plastic spring representations are commonly used in practice. However, under cyclic rocking, physical changes to the underlying soil occur (mostly due to the densification and shearing at the edges), therefore an elastic-plastic representation will not provide a realistic description of the soil-footing behavior. In this paper, data sets from centrifuge and large-scale footing experiments, whereby the response is controlled in a single degree of freedom, are used to determine the parameters describing the nonlinear backbones of mechanistic springs. For this purpose, models of each test are constructed and through comparison of model results with the experimental response, and isolating one parameter at a time, best-fit parameters are determined. Finally, a sensitivity study is carried out to observe the effect of parameters that describe the spring backbone response.