Generation of low-frequency Rayleigh waves for the spectral analysis of surface waves method.

Abstract

The Spectral-Analysis-of-Surface-Waves (SASW) method is used for in situ evaluation of elastic shear modulus in layered soils and pavements. The advantages are: it is performed from the surface (not requiring boreholes), it is nondestructive, it utilizes a simple procedure and test setup, and is usually very accurate. The major deficiencies of the method involve inversion of the dispersion curve, and limitations in depth of evaluation. The inversion process is inadequate because it is time consuming and it may be ambiguous at sites with irregular soil profiles, i.e. profiles in which shear wave velocity does not necessarily increase with depth. Limitations in evaluation of properties of deep soil deposits result from inability to generate sufficiently strong, low frequency Rayleigh waves. The purpose of this research was to explore elements for design of an impact device for generation of low frequency Rayleigh waves, and evaluation of Rayleigh wave dispersion in irregular soil profiles. The research has three major parts: evaluation of Rayleigh wave dispersion in irregular soil profiles; the solution of vertical oscillations of a circular flexible plate on an elastic layered medium; and the analysis of the dynamics of a falling weight, impact device. The dispersion in irregular soil profiles was evaluated by numerical simulation of the SASW test. The evaluation of vertical oscillations of a flexible plate utilized the ring method. For the overall evaluation of the stiffness matrix of the flexible plate-soil system, the stiffness matrix approach was used for the stiffness matrix of the soil system, while the Finite Difference Energy Method was used for the plate. Solutions of the first two parts were applied to the analysis of the impact device. The major conclusions were: (1) the inversion process should be guided by multiple Rayleigh modes whenever possible, (2) transition of dominant influence from one Rayleigh mode to another is characterized by a localized approach of dispersion curves of two adjacent modes, (3) the response of flexible plates differs from the response of rigid plates in three ways (a) soil reaction distribution, (b) displacement distribution, and (c) variations in impedance functions.