Laboratory thermal calibration of contact pressure cells installed on integral bridge abutments

Abstract

Hydraulic contact pressure cells were installed on the abutments of an integral abutment bridge to monitor changes in earth pressure over an extended period of time. The accuracy of field data from such instruments is affected by a number of factors. In particular, temperature changes are one of the key factors that can influence earth pressure measurements. Thermal calibration factors supplied by manufacturers of such cells tend to only account for the effect of temperature on the pressure transducer, rather than on the instrument as a whole. Therefore, in an effort to quantify the effect of temperature on data collected from the contact pressure cells installed on the integral abutment bridge, laboratory thermal calibration of these sensors was undertaken. Construction-related time constraints precluded extensive testing of the instruments installed in the field; however, extensive tests were conducted on an identical contact and earth pressure cell. Laboratory thermal calibration tests were conducted on the sensors for an unloaded, unconfined condition and with sensors confined in soil and loaded with uniform pressure. All tests were conducted in a cold room where temperatures could be controlled over a wide range. Results indicate that both temperature and applied pressure affect the performance of hydraulic pressure cells. Thermal correction factors were developed from linear-regression analysis of the unloaded, unconfined test data; however, application of these factors to the loaded, confined test data was found to account for only a portion of the pressure variation, with the remaining variation still being significant. Similar correction factors by linear regression analysis could not be developed from the loaded, confined pressure test data. However, when considering the range of temperatures experienced by the pressure cells installed on the integral abutment bridge, it was concluded in a 2013 study by the authors that the thermal pressure variations present in the field data should not exceed ±10 kPa. The results of this research demonstrate the need for extensive laboratory calibration of these types of pressure cells for proper interpretation of field data.