Variation of Shear Wave Velocity with SPT-N Values in the City of Sharjah
Bismillah, Niamatullah Haji
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Modern building codes require the evaluation of a shear wave velocity profile of the foundation soils for a dynamic analysis of structures and sites to occur. Wave velocities are typically evaluated by conducting laboratory and field tests. Due to the rapid pace of development in the UAE,consultants often rely on shear wave velocity (VS) obtained from correlations of shear wave velocity (VS) with N values from Standard Penetration Tests (SPT). Significant variationsin previous studies that predict wave velocities justifythis new study in order to develop correlations specific to the region and applicable to areas of similar geology. These variations can be attributed to factors such as biases in conducting field tests and shortcomings in seismic techniques. This study presents the correlations developed between wave velocity and N values for the city of Sharjah by performing seismic tests in various locations. The velocity profiles are evaluated by using vertical geophones that are traditionally used, as well as, horizontal geophones to evaluate their ability in calculating similar profiles. The analysis of surface waves for velocity evaluation is preferred due to its ability in delineating hidden layers of weaker velocities and for the better characterization of shallow depths from the generation of higher frequency phases. The benefits of using a seismic technique based on surface waves are presented and the results of the regression of VS-N pairs are compared with the findings of previous studies discussed here. The effect of using horizontal and vertical geophones on the results is evaluated in order to validate the theoretical basis of particle motion during the propagation of surface waves. The results indicate many previous models underestimate the wave velocity as function of N valuesas compared to the proposed model, especially at N values larger than 15. Most recent studies, however, tend to agree with the prediction model of this study;only one model significantly overestimates the shear wave velocity. The velocity profiles computed from horizontal and vertical geophones agree, indicating that the theoretical basis of elliptical particle motion with both horizontal and vertical components in the propagation of surface waves is valid.