Development of Stiffness Monitoring Sensor for Pavement Subgrade

Abstract: Despite the importance of the stiffness of pavement layers, direct and continuous measurement of the stiffness characteristics of pavement sublayers has been a challenging task. Conventional non-destructive field pavement evaluation methods commonly have multiple limitations, including limitations in measurement depth, indirect testing modes, and the need to interrupt traffic for the measurements. A recently developed bender element field sensor, which utilizes shear wave transducers, has demonstrated its capability of monitoring the stiffness of the pavements’ course-grained unbound aggregate layers. Meanwhile, a subgrade of the pavement structure is typically a fine-grained soil, has relatively low stiffness, and functions as a foundation of the pavement structure. This paper presents the result of a recent research effort at the University of St. Thomas aimed to develop an elastic wave-based stiffness sensor for the pavement subgrade layer, capable of continuously measuring the stiffness and Poisson’s ratio of subgrade. Two pairs of piezoelectric transducers, namely bender elements (BE) and piezo disk elements (PDE), were utilized to measure shear wave and compressional wave velocity, respectively. A large-scale testbed was constructed and filled with sandy alluvial soil, which has typical material properties of pavement subgrade materials, followed by multiple layers of compaction for the uniform density of sand. The dimensions of the sensor frame and the size of elastic wave transducers were optimized based on the elastic wave signal quality, through the demonstration test in the large-scale testbed. Finally, various static loadings were applied to the testbed to confirm the stiffness change of the subgrade material. The optimized sensor for subgrade demonstrated its capability to capture stiffness and Poisson’s ratio changes from confinement changes. Through the new sensor technology developed, continuous monitoring of the stiffness properties and Poisson’s ratio properties of the pavement subgrade will enable the analysis of the complex material properties of the pavement subgrade throughout the lifespan of the pavements. Further, the monitoring capability of the sensor can be applied to the analysis of the influence of climate effects on the pavement subgrade, such as flooding or the freeze-thaw cycle.