Elastic wave propagation in highway pavements

Abstract

The objective of this study was to investigate the problems associated with determining the velocity of propagation of the direct compressional and shear wave in thin layers of highway building materials using a portable seismograph. Velocity determinations, of materials serving as structural components in a specially designed highway test facility, are reported. Measurements were made with the seismograph on each of the materials as they were placed sequentially during construction of this facility. The test facility was constructed at the Texas A&M Research Annex as part of the National Cooperative Highway Research Program Project 1-6 and 2-8-62-32. Velocities determined with the seismograph were established from the first arrivals (first energy recorded) at six detectors placed at distances of 5, 10, 20, 25 and 30 feet from the seismic source (a hammer blow). These velocities were initially assumed to be those of the direct compressional wave, as the compressional wave travels with the greatest velocity of any phase according to the theory of elasticity. However, a comparison of the velocity determined in this manner for each of the materials with the compressional velocity determined for the same material using an ultrasonic technique showed a lack of agreement too great to be attributed to experimental error in either method. An analysis of data from both methods indicated that the velocity determined from the first arrivals recorded on the portable seismograph was not the velocity of the compressional wave. A discussion is presented to show this was the result of the direct compressional wave not being recorded because of one or a combination of the following: (1) the recording system of the portable seismograph discriminated against the high frequencies of the direct arrivals; (2) the high frequencies were attenuated severely over even short distances; (3) the seismic source was possibly biased toward low frequencies. It was concluded that a portable seismograph was not a suitable instrument with which to measure the velocities of direct elastic waves (compressional or shear) in thin layers of road building materials.