Task E1-2: Foundation Design for
Liquefaction-Induced Lateral Displacements
|Principal Investigator(s) and
Geoffrey R. Martin, University of Southern California
The vulnerability of bridges to earthquake-induced liquefaction lateral ground deformation (or spread) has been clearly demonstrated in past earthquakes. Liquefaction mitigation options through soil remediation have been addressed in several tasks previously conducted under the FHWA/MCEER Highway Project DTFH61-92-C-00106. However, further research is needed on mitigation options related to foundation design or retrofit as current methods of soil improvement are very costly and time-consuming to implement. A number of case histories and a limited number of analyses have indicated that, with appropriate design, foundations can accommodate relatively large ground deformation demands from lateral spreads.
The results from this study will establish: (1) practical analysis approaches to evaluate the ability of foundation systems normally associated with short to medium span bridges (pile footings and drilled shafts) to accommodate displacement demands arising from lateral spreads; (2) a case study data base (including verification studies using the above analysis approach) which will document field observations in past earthquakes and results of centrifuge modeling simulations; and (3) general design guidelines on conditions suitable for structural retrofit or a structural mitigation option versus ground remediation solutions.
The first year of this two-year task will entail the following subtasks:
Subtask 1 - Case History Data Base. Observations from recent earthquakes in New Zealand and Japan have identified several examples of bridges where foundation systems have accommodated lateral spreads without failure. These case histories and related structural and foundation information will be documented. A number of centrifuge model tests have also recently been performed (U.C. Davis, Rensselaer Polytechnic Institute) where pile foundations have been subjected to lateral spread deformations and measurements taken of pile response. This data will be compiled in the database. In addition, Caltrans has exercised the ground mitigation option for a number of bridge retrofit projects involving lateral spreads over the past several years. These case histories and related design approaches will also be documented.
Subtask 2 - Development of Analysis Methodology. A relatively simple model will be developed and tested to simulate the distribution of ground deformations occurring in a lateral spread soil zone during earthquake ground shaking. The model will be based on the one-dimensional DESRA-MUSC site response computer code developed during the previous FHWA/MCEER project. The constitutive model is presently being modified to allow one-directional permanent displacements by including a lateral static stress bias.
In addition, models will be developed to simulate the interaction between soil in a lateral spread zone and pile or pier foundation elements and pile caps. Existing models developed by researchers at Earth Mechanics, Cornell University, U.C. Davis and U.C. Berkeley (primarily based on p-y interface elements for piles) will be evaluated for adoption. Conventional beam-column elements will be adopted for piles in numerical analyses. Specific attention will be paid to failure criteria and displacement capacity simulation for foundation elements and connections to pile footings. The resulting model will then be integrated with the ground deformation model described above.
In Year 2 of this task, combined model capabilities will be tested against case histories, to validate overall behavior. Test simulations will then be performed for a variety of representative foundation and lateral spread soil profile configurations to provide basic information and to formulate design guidelines.
Anticipated Start Date and Duration
January 1, 1999 - 24 months