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Evaluation of Flexible Pavements and Subgrades Using the Spectral- Analysis-of-Surface-Waves (SASW) Method
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Marwan F. AOUAD
|
Univ. |
Texas / Austin |
Spec. |
Civil Engineering |
Deg./Ph.D. |
Year 1993 |
# Pages/251 |
An experimental investigation was undertaken to determine the material properties of asphalt concrete (AC) pavement systems including the AC surface layer, base and subgrade. The Spectral-Analysis-of-Surface-Waves (SASW) method was the primary field method used in testing. This method is a seismic method, which is based on the measurement of surface waves, which can be generated and sensed with instrumentation placed solely on the pavement surface. The objectives of this study included the investigation of the following aspects in relation to the SASW method. First, a variety of surface sources and receivers was studied in the field. The performance of each component was evaluated under various climatic and pavements conditions. The importance of the sources and receivers in SASW testing is that they should be carefully selected according to their frequency range to adequately characterize the material properties of the system. As an example, the Wilcoxon Research (WR) Model F‑7 shaker and the WR Model 736 accelerometers proved to be effective in the high frequency range (10‑50 kHz) required sampling the top asphalt concrete layer. The frequency generated in the AC is very dependent on the temperature of the surface layer.
Second, an experimental program at the test facility of the Texas Transportation Institute (TTI) in Bryan, TX was employed to study the effect of some parameters on SASW measurements. Parameters such as the temperature and thickness of the surface layer and the frequency content of the surface wave were studied. At high temperatures, results showed that energy could not be generated at high frequencies due to the viscous nature of the AC surface layer. In case of thick pavements (thickness, H ≥5 in. (12.7 cm)), wavelengths shorter than the thickness of the surface layer could be generated even at temperatures as high as 140º F. However, when the AC surface layer was thin (H < 3 in. (7.6 cm)), is encountered, other techniques such as compression wave tests should be used in conjunction with the SASW method to evaluate Young's modulus of the surface layer.
Finally, a series of tests was conducted at a newly constructed site in Austin, TX, and at 24 pavements sites around the state of Texas to understand the effect of the above parameters on the SASW measurements and to correlate data from the SASW tests with other in situ techniques such as the Falling Weight Deflectometer (FWD). Experimental data from the SASW and FWD methods correlate well if adjusted for frequency and strain effects. In the case of shallow bedrock depths, however, the dynamic effect (waves reflected of the bedrock) should be taken into account in the analysis of the FWD data.
