The Characteristics of Velocity Slopes of the Pavement Surface from Traffic Speed Deflectometer device

Abstract

With the development of Traffic Speed Deflection (TSD) device, the laser-based deflectograph is increasing used to perform continuous bearing capacity measurement at a range of driving speeds at network level. The two main advantages of the TSD technology are; its ability to carry out continuous deflection measurements rather than at discrete points and the tests can be carried out at traffic speed without the need for lane closure in the case a stationary device such as the FWD. In Australia, the TSD utilises a set of Doppler lasers to measure the vertical velocity of deflected pavement at six locations along the midline of the rear left dual tires and in front of the axle load of 100 kN (10 tonnes) at 100, 200, 300, 600 and 900 mm. The seventh Doppler laser, known as the reference laser, is positioned 3500 mm from the load. The reference laser is presumed to remain relatively unaffected by the load applied by the axle and it is expected to measure very little vertical pavement deflection velocity. The vertical pavement velocities (VV) divided by the horizontal velocity (Vh) of the TSD vehicle results in a term known as the velocity slope (SLP = VV/Vh). There are two commonly used deflection algorithms for computing pavement vertical surface deflection from the TSD deflection slopes. The methods are Euler-Bernoulli beam model (Rasmussen et al, 2008) and ARRB “Area Under the Curve” (AUTC) method (Roberts & Byrne, 2008) and (Muller & Roberts, 2013).

The objective of this paper is to provide an overview of the TSD device and to discuss the characteristics of the velocity slopes of the deflected pavements generated from the TSD device. The approach to determine the non-linearity of the subgrade of the TSD velocity slopes using Simplified Deflection Model (SDM) will be introduced. The study shows that the velocity slopes exhibit distinct characteristics for pavements with linear and non-linear subgrade behaviour. The research concluded that subgrade non-linearity behaviour is observed for slope ratio (SLP600 /SLP900) greater than 3.35 and deflection ratio (D600 /D900) that is greater than 2.00. This non-linearity behaviour can present problems in achieving convergence of the measured and calculated deflection particularly in the laser locations greater that 450 mm from the rear axle. This can affect the back-calculation of resilient moduli of pavement layers and the subgrade non-linearity should be considered in the back-analysis. Finally, a demonstration of the technique for converting the TSD deflections to FWD data using the SDM method will be presented.