Evaluating Subgrade Properties with a High-Speed Rolling Deflectometer

Forty laser range finders mounted on a truck are used to assess two transverse profiles. One profile constitutes an unloaded case. It is placed 2.5 m behind the front wheel axle, where the road is little influenced by any wheel load. The other profile, 0.50 m behind the rear axle of the vehicle, constitutes the loaded case. By subtracting the front cross profile from the corresponding rear one, the "transverse difference profile" is assessed. The difference is assumed to vary with the stiffness of the road. A high sampling rate is needed for filtering the macro texture of the pavement. Averaging profiles over some section length should also mitigate effects of vehicle and instrument vibration. It is assumed that the slope of the deflection basin near the load is influenced by near surface elastic properties. The total area and the slope of the deflection profile far from the load can be attributed to layers deep in the structure. Thus, using simple area parameters the influence by the subgrade stiffness on the deflection basin caused by the load can be estimated. During the spring and summer of 1998 the first major test program was carried out with the new rolling deflection tester. The present paper compares deflection basins assessed with those calculated with a finite element method. Many different subgrade conditions were tested and the results from these tests are presented. Further, the results are compared to processed data achieved from falling weight deflectometer testing.