The use of the S.A.S.W. to evaluate the deterioration of pavement properties with increase in wheel loads

Abstract

Paper presented at the 20th Annual South African Transport Conference 16 - 20 July 2001 "Meeting the transport challenges in Southern Africa", CSIR International Convention Centre, Pretoria, South Africa. ABSTRACT: The application of a non-destructive test method for the determination of elastic properties of pavement layers and soil layers is being discussed. This method makes use of the seismic wave theory through the analysis of surface waves S A S W (Spectral analysis of surface waves. The SASW method is based on generation and detection of elastic stress waves, known as surface waves. Surface waves have a unique characteristic that, if the wave length is varied, the velocity of propagation of the surface waves also varies. This phenomenon is called dispersion. The dispersion characteristic of surface waves can be utilised to determine the layering and modulus profiles of pavement systems quite accurately. The method is non-destructive. Measurements are made at strains below .001% where the elastic properties of the materials are independent of the strain amplitude. The method is such that both the source and the receivers are located on the ground surface. The source is simply a transient vertical impact, which generates a group of surface waves of various frequencies that the medium transmits. A pilot study was conducted on a test site, with different test sections, that was situated on the road D 2388 near Cullinan in Gauteng Province. These test sections were constructed with different base course materials as part of a labour intensive project. The aim was to find out how well the SASW method could be applied to obtain information on the deterioration of the pavement, in terms of the change in stiffness, with the increase in number of wheel load repetitions. The results obtained were such that a relatively complete picture of the status of the structure of the road could be obtained. The deterioration with the increase in load repetitions was well defined. The testing contributed to a better knowledge and understanding of what is happening underneath a loaded pavement structure.