Detecting the depth and thickness of weak layer in soil media using phase velocity spectrum and theoretical dispersion curve of rayleigh wave

The presence of subsurface anomalies such as cavities, sinkholes, weak subsurface layers, faults, or tunnels that are man-made or natural can pose serious risks to the environment or engineering project. One of the anomalies is weak or loose layer between two hard layers which increases the settlement potential, so detecting the subsurface anomalies such as void and weak layer at different depths and thicknesses plays an important role in engineering designs. In this paper, the multi-channel analysis of surface waves (MASW) is used. It is fast and comfortable among the various methods for identifying subsurface anomalies. In this regards, the effect of weak layer between hard soil layers is evaluated by using the finite element method in Abaqus and MATLAB software. The effect of depth and thickness of weak layer on the phase velocity spectrum and theoretical dispersion curve of Rayleigh (R) wave is compared. Different thicknesses (2, 4 & 8 meters) and depths (2, 4 & 8 meters) for weak layers are considered. In the following, the output results of each simulation are processed in MATLAB software by using the MASW method.The results showed that the presence of jumps in phase velocity spectrum in high and low frequencies are related to the existence of weak layer. The location of the jump is moved to the low frequencies of the phase velocity spectrum and also shifted to higher modes of theoretical dispersion curve of R-wave due to the increasing depth of weak layer. For example, the jumps are accrued at the frequencies of 37, 27 and 17 Hz in the phase velocity spectrum due to the presence of the weak layer at depths of 2, 4 and 8 meters, respectively. This event is caused by the more energy of the higher modes at special frequencies related to the depth of the weak layer. By increasing the thickness of the weak layer, the number of jumps is increased in the phase velocity spectrum and the phase velocity spectrum is shifted from fundamental mode to the first, second and other modes at the location of each jump. The number of jumps for a weak layer with a thickness of 8, 4 and 2 meters is equal to 6, 2 and 1, respectively, which indicates a higher number of jumps for a thickness of 8 meters. Considering these changes in inversion analysis gives rise to more acceptable S and P wave velocity profiles.