Development of a Liquefaction Hazard Screening Tool for Caltrans Bridge Sites

We have developed a liquefaction hazard screening tool for the California Department of Transportation (Caltrans) that is being applied in evaluating liquefaction hazard to approximately 13,000 bridge sites in California. Because of the large number of bridge sites to be evaluated, we developed a tool that makes use of parameters not typically considered in site-specific investigations. We assessed geologic, topographic, seismologic, and subsurface conditions at about 100 sites of past liquefaction in California. Among the parameters we found common to many of these sites are: (a) low elevations, (b) proximity to a water body, and (c) presence of geologically youthful deposits or artificial fill materials. The nature of the study necessitated the use of readily available data, preferably datasets that are consistent across the state. The screening tool we provided to Caltrans makes use of the following parameters: (1) proximity to a water body, (2) whether the bridge crosses a water body, (3) the age of site geologic materials and the environment in which the materials were deposited, as discerned from available digital geologic maps, (4) probabilistic shaking estimates, (5) the site elevation, (6) information from available liquefaction hazard maps [covering the 9-county San Francisco Bay Area and Ventura County] and CGS Zones of Required Investigation. For bridge sites at which subsurface boring data were available (from CGS’ existing database), we calculated Displacement Potential Index values using a methodology developed by Allison Faris and Jiaer Wu. The screening tool builds on unpublished work by Professor S. Kramer. Caltrans’ staff are using this hazard-screening tool, along with other tools described in this session, to prioritize site-specific investigations. INTRODUCTION This paper describes a project whose goals were: (1) to 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) to apply the methodology to screen those Caltrans bridge sites at highest risk from liquefaction-related damage. Caltrans plans to use the screening tool we have developed, in combination with other tools that address bridge structures, to evaluate the liquefaction hazard to the approximately 13,000 bridges for which it is responsible. Our focus has been on hazards at bridge sites, not on the bridge structures. Development and demonstration of a cost-effective strategy to evaluate exposure of transportation structures to ground failure hazards has applicability elsewhere. Combining estimates of hazard exposure (this project) with information on structural vulnerability and importance (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 and consistent approach that ranks bridge sites from across the entire state. APPROACH The goals of the project were accomplished through a phased approach. All of the phases made intensive use of Geographic Information System (GIS) software and analyses. The first phase of the project involved reviewing about 100 California liquefaction case histories (Table 1) to identify parameters that would be useful in predicting future liquefaction-related ground failures. Because the study region is so expansive (the state of California), it was necessary to identify parameters for which consistent, state-wide, digital (GIS) data layers are available. After the predictive parameters were identified, in the second phase of the project we developed a screening tool that is based on readily available geological, geographical and seismological information. The third phase of the project included intensive GIS analysis in which bridge locations were intersected with a number of regional databases and a master database of parameters for each bridge site was compiled. During this phase, we used geotechnical boring information in the vicinity of bridges that had been previously collected and entered into the CGS GIS database to develop preliminary quantitative estimates of liquefaction-related ground failure. We used the subsurface data, together with recently developed approaches to estimate magnitudes of liquefaction-related ground failure (Faris, 2004 and Wu, 2002), to quantify the liquefaction hazard and further screen the bridge sites for more detailed study or retrofit. In the first phase of the project, we compiled information on about 100 California liquefaction case histories. Table 1 lists the eleven earthquakes and the number of case history sites for each earthquake that have been characterized. In developing and characterizing these sites, we relied on recent compilations of liquefaction sites by others that were developed primarily for refining liquefaction triggering relationships and lateral spread magnitude relationships (e.g. Bartlett and Youd, 1992; Cetin et al., 2004; Faris, 2004; Moss, 2003; Rauch, 1997;Youd et al., 2002). Some of the sites we include liquefied in more than one of the earthquakes listed in Table 1. Because a digital compilation of San Francisco Bay Area liquefaction sites is available (Knudsen et al., 2000), the analysis is heavily weighted toward this area. In evaluating the potential success of different hazard indicators in predicting liquefaction, we tracked separately the relationships for native material and artificial fill sites. During the first phase of the project, we identified several “hazard indicators” that seem to be good predictors of hazard, which are typically not included in site-specific analyses. Among these parameters, are elevation and proximity of the site to a water body. Table 1. Case History Sites and Earthquakes Earthquake Mw Number of sites 1865 Santa Cruz Mountains ~6.5 2 1868 Hayward ~7 3 1906 San Francisco ~7.9 3