Effect of Constant Energy Source on Coherence Function in Spectral Analysis of Surface Waves (SASW) Testing

Abstract

The quality and acceptability of a waveform data collected during spectral analysis of surface waves (SASW) testing is judged based on the coherence function over the measured range of frequencies. However, many trials and repetitions are required during SASW testing to obtain data with acceptable coherence value (>0.95). This makes the test time-consuming, and in most cases, only small portion of the collected data that satisfies the acceptable coherence criteria can be used for analysis. In this research study, an attempt was made to study the effect of using an impact source of constant energy on the coherence function as compared to the use of traditional handheld hammers. Laboratory experimental studies were performed on sandy clay soil bed filled in a metal box of dimensions 1.5 m × 0.61 m × 0.45 m. Also, a series of field tests were performed to validate the applicability of the laboratory findings. In both laboratory and field testing, a 2.5 kg hammer was used with height of fall as variable parameter. Two sets of tests were performed. One with random height of fall that is similar to current practice and the other with fixed height of fall of 0.13 m to simulate impacts with same energy conditions. Test results depicted that unlike the conventional procedure of testing, the use of constant energy of impact leads to coherence close to 1 over a significantly large frequency bandwidth. This research highlights the effect of using constant and varying impact energy on coherence value over a wide range of frequencies obtained during SASW testing.