PROCEDURE FOR EVALUATING EARTHQUAKE HAZARDS ALONG EMERGENCY TRANSPORTATION ROUTES

The Missouri Department of Transportation commissioned a study of the earthquake vulnerability of a section of U.S. Highway 60 in southeast Missouri. This section of highway was designated as an emergency vehicle priority access route in the event of an earthquake. It was the first comprehensive study of emergency vehicle priority access routes in the Midwestern United States, and from this study a procedure for evaluating earthquake hazards along emergency transportation routes was developed.

First, a simple ranking system was used to identify the most vulnerable bridges or stretches of highway. The ranking system relied on geologic hazards (natural and fill slope stability, and liquefaction and flooding potential), structural hazards (bridge age, methods of pinning decks to abutments, and pier-soil coupling), and layout hazards (bridge length, potential alternate crossings).

Field investigation of bridges or roadway sections focused on four potential methods of failure: liquefaction of roadway sections or bridge approaches, slope failure of bridge abutments, structural failure of bridges, or flooding of long expanses of the roadway. These analyses required accurate information on sand density, soil strength, ground shaking levels, topography, and bridge geometry.

This data is ideally generated from a limited number of subsurface borings with frequent SPT values, correlated with more frequent CPT soundings. Accurate assessment of water table elevations, as well as anticipated water levels during wet and dry periods is crucial. Laboratory testing should provide fines content for non-cohesive soils, and density and cohesion information for cohesive soils. Rock-base ground-motion levels may be generated from synthetic seismograms or from published values, and propagated upwards to the ground surface using commercial software. SPT data from old boreholes along the roadway are used to interpolate conditions between sites undergoing more detailed study.