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bridgesmall.gif (4301 bytes)MCEER/NCEER Bulletin Articles: Research

Screening Guide for Liquefaction Hazard at Highway Bridge Sites

This article summarizes procedures to conduct a preliminary assessment of the vulnerability of existing highway structures to damage as a consequence of liquefaction induced ground failure. The full procedure is detailed in Screening Guide for Rapid Assessment of Liquefaction Hazard at Highway Bridge Sites, MCEER-98-0005. Comments and questions should be directed to Professor T. L. Youd, Brigham Young University, Department of Civil and Environmental Engineering, 368 Clyde Building, Provo, UT 84602-4081; phone: (801) 378-6327; email: tyoud@byu.edu.

Liquefaction does not occur randomly in natural deposits but is limited to a rather narrow range of seismic, geologic, hydrologic, and soil environments. Taking advantage of relationships between these environments and liquefaction susceptibility, this article introduces a screening guide that highway geotechnical engineers can use to perform rapid assessments of liquefaction hazard. The guide presents a systematic application of standard criteria for assessing liquefaction susceptibility, evaluating ground displacement potential, and assessing the vulnerability of bridges to liquefaction-induced damage. The screening proceeds from least complex, time-consuming, and data-intensive evaluations to the more complex, time-consuming, and rigorous analyses. Thus, many bridge sites can be evaluated and classified as low hazard with very little time and effort. Only bridges with significant hazard need to be evaluated with the more sophisticated and time-consuming procedures. At each level of screening, a conservative assessment of hazard is made. If there is clear evidence that liquefaction or damaging ground displacements are very unlikely, the site is classed as "low liquefaction hazard and low priority for further investigation," and the evaluation is complete for that bridge. If the available information indicates a likely hazard, or if the data are inadequate or incomplete, the site is classed as having possible liquefaction hazard, and the screening proceeds to the next step. If the available site information is insufficient to complete a liquefaction hazard analysis, then simplified seismic, topographic, geologic, and hydrologic criteria are used to prioritize the site for further investigation.

The screening guide is conservative; that is, at each juncture in the screening process, uncertainty is weighed on the side that liquefaction and ground failure could occur. Thus a conclusion that liquefaction and detrimental ground displacement are very unlikely is a much more certain conclusion than the converse outcome–that liquefaction and detrimental ground displacements are possible. This conservatism leads to the corollary conclusion that additional investigation is more likely to reduce the estimated liquefaction hazard than increase it.

The principal steps and logic path for the screening procedure are listed in figure 1. In assessing liquefaction hazard, the recommended procedure is to start at the top of the logic path, perform the required analyses for each step, and proceed downward until the bridge is classified into one of four categories: (1) confirmed high liquefaction and ground failure hazard–very high priority for further investigation and possible mitigation; (2) confirmed liquefaction susceptibility but unknown ground failure hazard–high priority for further investigation; (3) insufficient information to assess liquefaction susceptibility–prioritized for further investigation; or (4) low liquefaction hazard–low priority for further investigation. Based on these outcomes, the following procedures are recommended for setting priorities for further investigation or mitigation of liquefaction hazard.

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Confirmed High Liquefaction Hazard
Sites with confirmed high liquefaction or sensitive soil hazard should be given very high priority for additional investigation and development of possible mitigative measures. Prioritization at this level should consider the following factors: A primary criterion should be the importance of the bridge. Essential bridges, as defined in Standard Specifications for Highway Bridges (AASHTO, 1992), should be given priority for further investigation and mitigation over other bridges. Bridges with higher traffic volumes generally would be given priority over bridges with lower traffic volumes. Older bridges or bridges with weaker or more brittle foundations and structural components should be given priority over stronger and more ductile structures. Bridges scheduled for major renovation or replacement might also be given high priority. These considerations are provided as general guidance; highway agencies should weigh these criteria along with local needs to set priorities for further investigations and hazard mitigation.

Confirmed Liquefaction Susceptibility, but Unknown Hazard
Sites with confirmed subsurface liquefiable, but with unknown ground failure hazard owing to insufficient site information, should be given high priority for further investigation. The further site investigation would usually include CPT and SPT soundings and laboratory testing to provide sufficient site information to conduct an analysis of the ground displacement hazard. Prioritization of sites for further investigation should proceed using the same general guidelines as suggested for the above category, with the following additional guideline. Most past bridge damage caused by liquefaction has occurred at river or other water-channel crossings. Thus bridge sites involving water crossings should be given priority for further investigation over non-water crossings, such as viaducts and overpasses.

Insufficient Information to Assess Liquefaction Resistance or Strength-Loss Potential
Where insufficient information is available, additional site investigations will be required to fully evaluate liquefaction and ground failure hazards. These investigations usually include additional drilling, SPT or CPT, and laboratory testing to identify and delineate liquefiable layers of liquefiable or sensitive soils, and analyses to define ground failure and bridge damage potential. Sites associated with water crossings should be given priority over sites at nonwater crossings. Sites with geologic conditions indicative of high liquefaction susceptibility should be given priority over sites assessed as having moderate or lesser susceptibility. The guidelines listed in the first category should also be considered in setting priorities for further hazard investigation.

Low Hazard and Low Priority for Further Investigation
Sites categorized as low hazard and low priority for further investigation need not be further analyzed or prioritized for further study, except for very critical structures where a high level of performance is mandated. Nevertheless, engineers should apply appropriate screening criteria for liquefaction hazard when new data is developed, such as for a new bridge or highway segment. Liquefiable sediment may exist beneath a small percentage of sites classed as low hazard by the criteria herein for reasons such as unusual local geologic conditions or inaccurately reported site information. Thus evaluations might be made as new information becomes available, but further specific investigations for liquefaction hazard are not required.

References

AASHTO, 1992, Standard Specifications for Highway Bridges, 15th ed., American Association of State Highway and Transportation Officials, Washington, DC.

 


MCEER Bulletin, Summer 1998, Vol. 12, No. 2

 

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