Correlations between Vs and SPT-N by different borehole measurement methods: effect on seismic site classification

The average shear wave velocity of the top 30 m of the subsurface profile (Vs30) is a critical parameter to characterize the seismic site class. Since Vs30 is a quantitative index and measurable by geophysical techniques, the use of Vs30 becomes popular and is widely used in practice. However, the Vs30 of a site may vary due to the different Vs measurement methods used. This could result in a different seismic site class and design force. To quantify the effect of measurement methods on Vs and seismic site classification. This study collected high quality geotechnical investigation reports with standard penetration test (SPT) N-values, and shear wave velocities measured by suspension logging, SCPT, down-hole test and cross-hole logging. The correlations between Vs and SPT-N by different methods were then built and compared. The results show that Vs by suspension logging is relatively high in comparison with those by SCPT and cross-hole logging. The effect on the site classification resulting from the different Vs measurement methods is then demonstrated with several examples.