Nondestructive testing using stress waves: wave propagation in layered media

Abstract

The use of stress waves in several civil engineering applications such as nondestructive testing of soil deposits or pavement systems has become extremely popular over the last few years. In all cases, a dynamic impulse is applied to the surface of the investigated medium, and the corresponding motions associated with the propagation of stress waves are recorded by receivers located at different points away from the source of loading. In many applications, these are primarily surface (Rayleigh) waves. The properties of the medium and the potential existence of defects can be determined from the appropriate interpretation of the recorded motions. Although current interpretation processes are performed considering accurate solutions to the dynamic problem, a situation of interest arises when a soil stratum is underlain by a much stiffer material. Researchers have established that no surface waves will propagate through a soil medium below a threshold frequency for the case when the soil base is assumed to be infinitely rigid. When dealing with vertical loads, some researchers had originally suggested that the threshold frequency corresponds to the natural frequency of the soil layer in dilatation/compression. This would imply that for a saturated soil having a value of Poisson's Ratio close to 0.5, surface waves would never be generated - a conjecture which is clearly incorrect. The goals of the research project were to determine the threshold frequency as a function of Poisson's Ratio of the soil layer and to investigate the surface-wave propagation phenomena for values close to the mentioned frequency. Two approaches were utilized to achieve the research objectives. The first approach consisted in developing analytical expressions for the relations between displacements and stresses due to plane waves propagating in a soil layer. The second approach considered the study of the phase shift in the motions at different points under steady state conditions. Both approaches used wave propagation results provided by computer simulation programs. The results of this study are figures showing the variation of the apparent wave propagation velocity corresponding to the threshold frequency divided by the shear wave velocity of the medium as a function of Poisson's ratio and a comparison with the P-wave velocity.