Estimation of Vehicle Speed, Axle Spacing, and Lateral Position from Earth Pressure Cells for Pavement Field Instrumentation Projects

Abstract: Accelerated pavement testing (APT) is an important tool to inform pavement design and evaluation. Various types of instrumentation are frequently used to measure structural response of pavements, both for APT experiments and for performance monitoring of in-service pavements. Vehicle loading induces stress, strain, and deflection responses in the pavement structure; however, the wheel position, both horizontally and vertically, relative to the instrumentation location results in different magnitudes and types of structural response. To correctly interpret the measured instrumentation response, it is critical to accurately know wheel position relative to the instrumentation. For APT conducted with a simulated loading machine, e.g., a Heavy Vehicle Simulator (HVS), wheel position is tightly controlled and known with high accuracy. However, for live vehicle traffic such as occurs on test tracks and in-service pavement monitoring, wheel position is much less controlled and varies with the wander position. An additional consideration for in-service pavement monitoring is determining axle spacing since truck configuration is not known a-priori, unlike a controlled traffic test track. For flexible pavements, the load rate dependency of asphalt also necessitates accurate knowledge of the wheel or vehicle speed in addition to position. This is particularly important for instrumented monitoring of in-service pavements where vehicle speed is a function of traffic conditions and driver behavior.
The most direct way to measure vehicle speed, wheel position, and axle spacing is with a surface-embedded weigh in motion (WIM) device which uses piezo-electric cables installed perpendicular to traffic and at a diagonal to traffic direction; inductive loop systems and other techniques can also be used. However, installation of WIM systems is invasive and not always desirable, for instance on airfield runways due to concerns over induced cracking or pavement deterioration due to the sawcuts necessary for installation. An alternative solution is to use the structural response instrumentation to determine the desired information. Instrumentation is typically installed in arrays; knowing the array geometry and timing of the measured peak responses allows for determination of wheel speed, vehicle axle spacing, and lateral distribution within the traffic lane. Earth Pressure Cells (EPCs) are robust instruments commonly used for pavement monitoring and their responses are particularly well suited to this purpose; other types of instruments can be used as well.
This paper provides specific examples of the necessary calculations using data from a variety of APT and in-service pavement monitoring projects. Data from both truck and aircraft loading is presented encompassing vehicle speeds from less than 10 mph to nearly 175 mph. Vehicle speed, axle spacing, and lateral position can be computed with high accuracy, with less than 5% error from known true values. Adequate instrumentation arrays and knowledge of precise lateral wheel position from multiple passes of a test vehicle allows for the interpretation of entire stress and strain distributions within a pavement structure. Discussion of important factors to consider when designing an instrumentation array is included as well as ancillary factors that can influence the calculations and should be accounted for.