Critical aspects of using vibrating wire piezometers in geotechnical monitoring of dams: how to meet basic operating principles of the devices and avoid common errors that compromise monitoring


. This procedure is particularly relevant for avoiding misinterpretations of pressure variations caused by phenomena such as the desaturation of the porous stone. During the zero-reading process, it is crucial that the piezometer is properly prepared. The porous stone


depends directly on a correct initial reading, as errors in this phase can introduce systematic deviations, compromising the monitoring analysis and leading to incorrect decisions in geotechnical engineering projects[13]


filter must be saturated with deaerated water to eliminate air bubbles that can interfere with the accurate transmission of water pressure to the sensor[15]


. Inadequate saturation can result in the phenomenon


known as “zero drift “, where the sensor provides inaccurate readings due to the presence of air in the internal chamber. This phenomenon can be observed when the readings in do not correspond to the actual applied pressure, as seen in cases where the porous stone loses saturation after installation, generating negative or inconsistent readings.


Another critical aspect is the thermal equilibrium of the sensor before the reading. Manufacturer manuals, such as those from GEOKON and DGSI, recommend a stabilization period of 15 to 30 minutes, especially in environments with significant temperature variations[4,16]


. During this period, the piezometer


should be placed in an area where only atmospheric pressure acts on the sensor, free from external loads or water column pressure. This ensures that the initial reading exclusively reflects zero pressure and not external factors. The zero reading is recorded in terms of digits (Hz2


/1000) and temperature (°C). Comparing this reading


to the calibration values of the sensor is essential to validate the accuracy of the equipment. For example, the RST Instruments manual recommends that the difference between the zero reading and the reference value in the calibration report should not exceed 0.5% of the sensor’s full scale. If this difference is greater, the procedure should be repeated or re-evaluated to verify potential issues, such as filter desaturation, thermal changes, or sensor defects[21]


. The validation process may also include performing consecutive readings, as suggested by the


ENCARDIO-RITE and Soil Instruments manuals. These additional readings help verify consistency and discard isolated anomalies[5,8]


. Additionally, the readouts or dataloggers used must be configured to


operate within the sensor’s specific frequency range, ensuring an adequate signal-to-noise ratio and avoiding interpretation errors. Figure 5 illustrates the zero-reading procedure, detailing the steps that involve removing the filter, filling the chamber with deaerated water, reinstalling the filter, and taking the reading with the readout equipment. This process ensures that the sensor is free from external influences and helps detect potential problems, such as filter desaturation after installation. Proper execution of the initial zero reading is a key component in ensuring the accuracy of measurements throughout geotechnical monitoring. When performed according to the best practices recommended by manufacturers, the risk of errors is reduced, and subsequent readings are more representative of actual field conditions. This strengthens the reliability of safety and stability analyses for dams, preventing incorrect diagnoses that could compromise decision-making in critical situations[13]


.


Vol XXXIII Issue 3 | Dam Engineering | 111


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