Liquefaction Hazard Screening of Caltrans Bridge Sites - 3I01

Project goals and objectives

There are two principal goals of this project:

1. Develop a methodology for screening and prioritizing highway bridge sites for retrofit and/or site–specific field investigations based on exposure to liquefaction-induced ground failure hazard, and
2. Apply the methodology(ies) to identify those Caltrans bridge sites at highest risk from liquefaction-related damage.

Role of this project in supporting PEER's mission (vision)

Development and demonstration of a successful cost-effective strategy to prioritize exposure of transportation structures to ground failure hazards can have nationwide applicability. Combining estimates of hazard exposure (this project) with information on structural vulnerability and importance (potential parallel project) can provide an estimate of risk that is a logical basis for prioritizing the expenditure of mitigation funds for earthquake and liquefaction retrofitting. Such an approach may be applicable to a variety of lifeline systems/organizations, not exclusively transportation systems. The results of this study will allow Caltrans to focus its resources on facilities that have been prioritized for future site-specific work using a sound approach that ranks bridge sites from throughout the entire state.

Methodology employed

Development of a screening/prioritizing procedure for liquefaction hazard and application of this screening procedure to Caltrans bridge sites are the goals of this project. Phase 1 screening of the approximately 12,000 Caltrans bridge sites is being performed using available geologic, hazard and seismologic data, much of which is existing data held by various programs within CGS. An integral aspect of Phase 1 is development of a screening methodology in coordination with Caltrans engineers and geologists. Maps of liquefaction hazard, liquefaction zones of required investigation, geology, and strong-ground motion are being evaluated and placed in an evaluation scheme during Phase I. Phase 2 screening will consist of refinement of the Phase 1 results by making use of geotechnical boring data that are available in our GIS database. The screening methodology in Phase 2 will be developed in collaboration with Caltrans and PEER colleagues. Our preliminary analyses tell us that our CGS database contains at least one boring for about 800 bridge sites in Southern California and at least one boring for about 200 bridge sites in Northern California. Part of Phase 2 will include an analysis of the “value-added” by incorporation of one or two borings for a bridge site into the screening analysis. This analysis will include consideration of peak ground motions (10% in 50 years exceedence), and estimation of permanent, liquefaction-induced vertical and horizontal displacements using newly developed approaches (making use of the results of PEER projects 3G01 and 3G02).

Brief Description of previous year's achievements, with emphasis on accomplishments during last year (Year 8)

We have: (1) converted the database of bridge locations provided by Caltrans to a GIS database, (2) begun to query against regional datasets, including a) liquefaction zones of required investigation, b) water bodies and streams, c) liquefaction susceptibility maps, d) probabilistic shaking maps of the state, e) maps of active faults, (3) begun compiling case histories of bridges that have been exposed to liquefaction, and (4) begun laying out draft screening procedures based, in part, on an unpublished “Susceptibility Rating Factor” developed by Professor Steve Kramer at University of Washington.

Other similar work being conducted within and outside PEER and how this project differs

We are aware of work being conducted by Professor Kramer at University of Washington who is working with the Washington Dept. of Transportation, and Professor Dickenson at Oregon State University who recently completed a project for the Oregon Dept. of Transportation. Finally, we are aware of a proposed project by Professor Scott Brandenberg of UCLA who may be working on a project for Caltrans that focuses on ranking bridge foundations as to their susceptibility to liquefaction hazards. Our project differs from the ones in Washington and Oregon in that Caltrans has an inventory of ~13,000 bridges, many more than in the other states, and we are developing a procedure and implementing the screening procedure for all of Caltrans bridges in a GIS. One of the aspects of this study that is different is that we will have widely varying amounts (and scales) of information available for the set of bridges. For some bridge sites we will make use of geotechnical borings in our database, while for other sites the bet available information may be small-scale geologic mapping.

Describe any instances where you are aware that your results have been used in industry

We are in the beginning of the project and thus no results have been applied in industry.

Expected milestones & deliverables

We expect to complete our project and hand over a liquefaction hazard screening tool to Caltrans by December of 2006.

Member company benefits

Development and demonstration of a successful cost-effective strategy to prioritize exposure of transportation structures to ground failure hazards can have nationwide applicability.