Use of Constant Energy Source in SASW Test and Its Influence on Seismic Response Analysis

The quality and acceptability of the results of the spectral analysis of surface waves (SASW) field test are dependent upon the coherence function, in conjunction with the transfer function. The coherence value is a measure of the quality of the test, and the transfer function denotes the phase lag between signals that are received by the geophones. This article presents a methodology for obtaining high-quality field test data, using a source capable of producing constant impact energy for several repeated impact strikes in an SASW test. Both laboratory and field investigations were performed to assess the effectiveness of the constant impact energy source at improving the coherence value. Tests were conducted on the surface of the soil compacted in a metal box and on the crest of an earthen dam using (a) handheld hammers, resulting in variable impact energy and (b) a drop hammer, dropped from a predetermined fixed height, resulting in constant impact energy. The variation in the shear wave velocity (V_s) profile obtained using the two testing methodologies and its impact on the seismic response analysis of an earthen embankment structure were studied. The SASW tests performed in the laboratory using constant impact energy were more efficient and repeatable than those performed using varying impact energy, and the results showed increased coherence values over a wide range of frequencies. A similar improvement in coherence data was observed in the field studies, and the V_s profiles were found to be significantly different for tests conducted using both methodologies. It was observed that the peak and spectral accelerations at the crest of embankments are significantly different when embankments with different V_s profiles are subjected to seismic excitation. This study emphasizes the importance of performing the SASW test, using a constant impact energy source to obtain a reliable estimate of V_s profiles of subsurface layers.