Detection of Composite Underground Obstacles by the SASW Test: Fid-Bau Portal

Detection of Composite Underground Obstacles by the SASW Test

Krstic, Vedrana / Gucunski, Nenad

The Spectral Analysis of Surface Waves (SASW) method is a nondestructive in situ seismic testing method used in evaluation of shear wave velocity profiles of layered systems. The method has been successfully used for twenty years in evaluation of layered media. The assumption of the SASW test is that the system tested consists of a set of homogeneous layers of constant thickness. However, results of theoretical and experimental studies point to significant effects of underground obstacles on wave propagation patterns in soils in general, and surface waves in particular. Specifically, obstacles cause strong fluctuations in the dispersion curve. Increase in the apparent phase velocity was observed with an increase in the obstacle rigidity. This paper focuses on the influence of composite underground obstacles on propagation patterns of surface waves. Composite obstacles are described as objects that combine rigid and soft materials. In order to investigate effects of composite obstacles on apparent phase velocity values and dispersion curve patterns, a field experimental study, complemented by numerical simulations, was conducted. The results pointed out that composite obstacles cause, contrary to the expected, a decrease in the apparent phase velocity. This conclusion is of particular interest in detection of underground utilities buried within loose-backfilled trenches. Compacting backfill to the level that will provide the shear wave velocity equal to the one of the surrounding virgin soil is essential for the successful detection of underground utilities by means of SASW.