Evaluating the Performance of Flexible Piezoresistive Sensors for Measuring Static Contact Stress

In geotechnical engineering applications, flexible piezoresistive sensors (also known as tactile pressure sensors) have been commonly used to measure soil contact stress at solid boundaries. However, studies have rarely discussed the performance of this kind of sensor against various calibration methods in granular soils. This study reviewed and evaluated various calibration methods about the application of puck and methods of sensor conditioning. The performance of a point-type piezoresistive sensor installed in an oedometer cell to measure static stresses was assessed through several sensor performance measures, including repeatability, short- and long-term drifts, hysteresis, and accuracy. The effects of soil particle size, soil density, and puck material (hence stiffness) on these measures were also investigated. Results show that the puck application improved the sensor’s short- and long-term drifts and eliminated the stress dependency of the drift, whereas conditioning before every use improved the sensor’s repeatability. Moreover, the data analysis showed that it is important to use the same material that will be used in the testing phase to perform calibration to improve the measurement accuracy. A new term, puck-soil stiffness ratio, is proposed to explain the effects of puck thickness and stiffness on the sensor response because of arching and stress concentrations developed in soil. Reducing soil particle size (D₅₀), with respect to the sensor diameter (D_sens), significantly improved repeatability and drift errors. At a high D_sens/D₅₀ ratio, the sensor performance was independent of soil density.