Abstract

Liquefaction is an earthquake-induced process in which a loose, sandy deposit below the water table loses much of its strength as load is transferred from grain-to-grain contacts to the water in the pore space. Liquefaction occurs when the pressure in the pore water equals the weight of the sediment column above the sandy layer. The effects of liquefaction have been observed in Utah in historical earthquakes, and evidence of liquefaction in the Wasatch Front region has been preserved in the stratigraphy of Lake Bonneville deposits. Liquefaction in historical earthquakes has caused significant damage to dams, buildings, buried utilities, roads, and bridges.

Mapping of liquefaction hazards in the Wasatch Front began in 1980 with a National Earthquake Hazards Reduction Program Grant to Utah State University for a study of Davis County. Davis County was selected because the urbanized part was relatively small, ground water was known to be shallow, and a substantial amount of subsurface geotechnical information was available. Liquefaction hazards were mapped on the basis of the susceptibility of subsurface sand deposits as indexed by Standard Penetration Test (SPT) blow counts using a conventional geotechnical procedure. The mode of potential liquefaction-induced ground displacement was equated to ground slope. The acceleration required to induce liquefaction was calculated for each site where subsurface data were available, and compared to the results of a probabilistic seismic ground motion evaluation done in 1978. Liquefaction-hazard maps showing high, moderate, low, and very low liquefaction potential were constructed using the computed values of critical acceleration integrated with engineering geology to guide the position and shape of the hazard-zone boundaries. Salt Lake County was mapped next in a similar way, except Cone Penetrometer Tests (CPT) were used to supplement the SPT blow counts. Utah County was the third area to be mapped, followed by Weber, eastern Box Elder, and Cache counties. The final liquefaction hazard evaluation included selected areas in central Utah from Park City to Richfield. In 1994, the Utah Geological Survey digitized and published summary maps of liquefaction hazard for parts of Davis, Salt Lake, Utah, and Weber counties based on this research.

Research following earthquakes in the 1980s has led to an appreciation that the potential for liquefaction-induced ground displacement is not necessarily accurately indexed to liquefaction susceptibility of sand layers. The thickness of and depth to susceptible sand layers was found to be important in determining the likelihood and extent of possible ground displacement. The importance of this factor was demonstrated in Salt Lake County where the liquefaction hazard maps were adopted by the Planning Department, and developments located in areas of mapped high liquefaction potential were required to have specific studies to evaluate the extent of the liquefaction hazard. Only about half of the sites were found to have high liquefaction susceptibilities. The maps of liquefaction potential developed in the 1980s along the Wasatch Front represent a first generation of this type of hazard mapping. Refinement is needed to translate the liquefaction-susceptibility maps into liquefaction-induced ground displacement maps showing not only the potential for such displacement, but the amount of displacement that can be expected for different probabilities of earthquake ground motion.