Development and optimization of a subgrade stiffness monitoring sensor using piezoelectric transducers in a laboratory testbed

Pavement foundations are critical for structural longevity, yet current evaluation methods lack the capability for real-time, continuous monitoring of resilient modulus, particularly in subgrade layers where distress frequently originates. While Bender Element field sensors have been successfully deployed in unbound base layers, assessment tools for low-stiffness subgrades remain underexplored. This study presents the development and optimization of a Subgrade Stiffness Monitoring sensor designed specifically for weak granular subgrades. The sensor integrates Bender Elements and Piezo Disc Elements to measure shear and compressional wave velocities, enabling the direct calculation of Poisson’s ratio and small-strain modulus. Large-scale laboratory tests were conducted in a testbed filled with sandy subgrade material to optimize transducer geometry. Experimental results indicated that a 30-cm sensor spacing yields an optimal Signal-to-Noise Ratio (SNR) for both wave types. Concurrently, repeated load triaxial tests established a robust correlation between the sensor-measured small-strain modulus and the design-relevant resilient modulus, capturing the stress-hardening behavior of the material. The study demonstrates that the optimized subgrade stiffness monitoring sensor can effectively evaluate subgrade stiffness variations under diverse static loading conditions. These findings highlight the sensor’s laboratory-demonstrated potential for the continuous monitoring of transportation infrastructure foundations.