SCREENING GUIDE FOR RAPID ASSESSMENT OF LIQUEFACTION HAZARD AT HIGHWAY BRIDGE SITES

Abstract

Liquefaction-induced ground and foundation displacement has been a major cause of earthquake damage to bridges. For example, during the great Alaskan earthquake of March 27, 1964, liquefaction-induced lateral ground displacement inflicted structural distress to 266 railway and highway bridges, collapsing about 20 and damaging many others beyond repair. This destruction, primarily at river crossings, disrupted the surface transportation system in southern Alaska for many months after the earthquake. Numerous other occurrences of bridge damage or collapse have occurred around the world as a consequence of liquefaction. Nearly all of this damage has occurred at river or other water crossings where flat to gently sloping topography was underlain by loose, saturated, granular sediment. These types of deposits and terrain are highly vulnerable to liquefaction and lateral spread. Because of the high potential for bridge damage and consequent disruption to transportation systems, evaluation of liquefaction hazard is a major part of any assessment of seismic hazard to highway systems. As an aid to seismic hazard assessment, this report provides a for systematic evaluation of liquefaction hazard at bridge sites and a guide for prioritizing sites for further investigation or mitigation. This guide is intended for use by highway engineers with expertise and experience in geotechnical engineering practice, but not necessarily specialized knowledge in seismic hazard evaluation. The guide presents a systematic application of standard criteria for assessing liquefaction, ground displacement potential, and vulnerability of bridges to damage. The screening proceeds from least complex, least time-consuming, and least data-intensive evaluations to the more complex, time-consuming, and rigorous analyses. Thus, many bridge sites can be evaluated and classified as hazard with very little time and effort. Only sites 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 priority for further investigation, and the evaluation is complete for that site. If the available information indicates a higher hazard rating, or the data is inadequate, incomplete, or unclear, 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, geologic, and hydrologic criteria are used to prioritize the bridge site for further investigation.