Comparison of cross-hole, seismic cone penetrometer, spectral wave (SASW) to characterize bridge sites in the New Madrid Seismic Zone

摘要

The New Madrid Seismic Zone (NMSZ) is the location of three of the most powerful earthquakes in United States history. Scientists estimate that there is a 9-in-10 chance of a magnitude 6 to 7 temblor occurring in the NMSZ within the next 50 years. Because of differences in the geology east and west of the Rocky Mountains, the effects of a magnitude 7 quake in the mid-continent United States could be far worse than those of the 1989 Loma Prieta, California, earthquake. Additionally, the Mississippi Embayment is one of the largest soil deposits in the North American continent that are susceptible to liquefaction and extends to depths never explored by geotechnical engineers (> 500 m). A major task is to evaluate the low strain dynamic soil properties at two bridge locations using field and laboratory tests. Three different field tests have been used to study the maximum shear modulus: seismic cone penetration, cross-hole velocity, and spectral analysis of surface wave (SASW). Being able to characterize the entire soil profile above bedrock remains a challenge, given that soils within this geologic region extend to depths of over one thousand meters. This paper will report on a study comparing the results of seismic cone penetrometer (down-hole), cross-hole velocity and SASW tests. Having the opportunity of using these three geophysical methods performed at the same site allowed developing confidence for the use of surface geophysical methods, such as the SASW to investigate greater depths with this non-invasive methods, which was not practically possible with the seismic pie-zocone and cross-hole. These results will be compared to various published correlations for the evaluation of shear modulus, G.