UCLA

Dynamic Stresses in Foundation Soils from Soil-Structure Interaction

by

Bahareh Heidarzadeh

Department of Civil and Environmental Engineering

University of California, Los Angeles, Fall 2015

Professor Jonathan P. Stewart, Chair

Professor George Mylonakis, Co-Chair

This research concerns the impacts of Soil-Structure Interaction (SSI) on the seismic stress demands in soil materials beneath the foundation, referred to as ‘foundation soils’. Engineering procedures for evaluation of these stress demands are needed for a variety of applications including ground failure evaluation in foundation soils and possible impacts of SSI on buried structures such as pipelines. Conventional engineering practice typically ignores SSI during evaluation of the seismic stress demands in foundation soils based on the perception that this demand is dominated by wave propagation from site response. The goals of this study are to show how the presence of a structure affects wave propagation in the vicinity of the foundation due to SSI and to propose rigorous procedures by which to assess these demands due to vertical or horizontal point loads, line loads, or arbitrary combinations of these acting within a foundation area.

Following an exhaustive literature review, I find that while integral transform methods exist to evaluate the response of foundation soils to surface loads, such approaches have two major limitations: (1) the resulting equations are so complex that closed form solutions for stresses produced by harmonic surface loads are absent in the literature, despite over a century of research in this subject; and (2) the resulting equations can be solved numerically, but only one such study has been conducted, and that work has limited practical applicability.

Accordingly, I solve the governing equations numerically for harmonically applied surface loads (horizontal and vertical, point and line) acting on a visco-elastic halfspace using the Boundary Element Method via a well-verified software platform (ISoBEM). The results are interpreted within a framework derived from dimensional analysis considerations, specifically applying the Buckingham π theorem to determine the dimensionless fundamental parameters applicable to these problems. I demonstrate that induced stresses normalize using appropriate dimensionless variables, and that these normalized stresses have clear dependencies on dimensionless frequency, location within the soil, and soil Poisson’s ratio. Moreover, I demonstrate that phase angle is associated with wave travel times from source to the point of interest in the foundation soil, and as such varies with aperture angle due to variations in body and surface wave radiation patterns from horizontal and vertical surface loads.

Results for the amplitude and phase of all relevant stress components are presented in dimensionless graphical forms and the effects of fundamental parameters for each case are discussed. For the case of loads applied to rigid surface foundations, I evaluate the distribution of normal and shear stresses at the base of the foundations. Finally, for the flexible foundation case I have developed and verified a numerical code that applies superposition principles to combine soil stress demands associated with uniformly distributed surface loads.