Seismic Vulnerability of the Highway System

Task C2-2: Pile Group Effects
Subject Area: Long Span Bridges — Ground Motion and Geotechnical Studies

Principal Investigator(s) and Institution(s)
Po Lam, Earth Mechanics, Inc.
Geoffrey R. Martin, University of Southern California

Evaluations of the earthquake response of bridges normally divide the bridge system into two equivalent substructure types, a bridge substructure and a foundation substructure, usually separated at a pile cap or at mudline pile nodes. The foundation substructure requires the development of an appropriate foundation stiffness matrix and foundation forces (reflecting excitation due to ground motion) for use in the bridge substructure dynamic analysis. As described in a prior FHWA/MCEER task (Task 106-E-4.10 of Project DTFH61-92-C-00106), substructuring methods may be deployed in the time domain or the frequency domain, or possibly via a hybrid frequency-time domain. Each of these methods have unique merits and degrees of difficulty.

Whereas engineers have a good understanding of the pros and cons of these methods and the associated questions of the relative importance of inertial and kinematic interaction in the case of smaller (say 3x3) pile groups, the appropriate application of these methods in the case of very large groups involving hundreds of piles normally associated with long span bridges, remains to be resolved. For this case, ignoring group effects can lead to serious errors, and the use of an elasticity approach is known to be deficient.

By incorporating advances in constitutive modeling and the application of 3-D finite element analysis, this task will establish a practical approach to determine stiffness matrices appropriate for very large pile groups, and examples where the use of the developed approach will be illustrated by application to pile groups representative of long span bridges in both the Western and the Eastern U. S.

The following subtasks will be undertaken during the first year of this two-year study:

Subtask 1 - Large Pile Group Data Base. To provide a baseline for analytical studies, a data base will be compiled describing pile configurations, pile types and related site soil conditions for long span bridges representative of various regions of the country.

Subtask 2 - Development of Analysis Methodology. Initially, nonlinear 3-D finite element solutions for smaller pile groups will be back fitted to smaller (say 3x3) pile group data. The associated calibration of the constitutive model will then be used to predict group effects of much larger pile groups by using periodic boundary conditions. These solutions will then be used to evaluate equivalent linear soil properties to match static group effects, and the results will be compared with the more conventional elasticity approach to establish group effects. The results of this analysis will then be used to establish design guidelines for the determination of a static stiffness matrix for large pile groups as a function of pile configuration and soil type.

In Year 2, the research will be extended by applying the equivalent linear soil properties available in elasto-dynamic programs (such as SASSI) to extend static group effects to the evaluation of the importance of dynamic group effects, including radiation damping and kinematic interaction

In addition, a preliminary evaluation of the research framework described above will be extended during Year 1 to bridge caisson systems, as an initial first step in refining the scope of work for future Task C2-5 on "Soil-Structure Interaction for Caissons." Included in the evaluation will be the establishment of a
bridge caisson data base.

Anticipated Start Date and Duration
January 1, 1999 - 24 months