Piezometers: what they are, the main types and how to automate readings
Casagrande, pneumatic and vibrating wire: what each piezometer measures, how a vibrating-wire reading becomes pressure, and what changes when the instrument reads itself.
A piezometer is the instrument that measures pore water pressure in soil, rock or tailings, known simply as pore pressure, at a defined point in the embankment or foundation of a dam. Converted to a height of water column, that pressure gives the piezometric level, the reading engineers track to understand how water is seeping through the structure.
Why pore pressure matters
The strength of a soil depends on effective stress: the share of the load actually carried by the grains. When the water in the pores gains pressure, it relieves the contact between grains and strength drops. This is Terzaghi's principle of effective stress, and it is why an unexplained rise in piezometric level is one of the first signals a dam engineer wants to see.
The three most common types
Casagrande (open standpipe). A pipe with a porous tip installed in a borehole, sealed above and below the zone of interest. The reading is the water level inside the pipe, taken with an electric dip meter that beeps when it touches the water. Simple and robust, but slow to respond in low-permeability soil, and always manual unless fitted with a transducer.
Pneumatic. A diaphragm separates the pore water from a gas circuit. The readout unit injects gas until it balances the water pressure and reads the value. There are no buried electronics, but reading is manual and slow.
Vibrating wire. A tensioned steel wire is attached to a diaphragm. Water pressure deflects the diaphragm, changing the wire tension and, with it, the frequency at which the wire vibrates when plucked by a coil. The instrument also includes a thermistor. It is the most widely used type in mining dams today, and the easiest to automate, because the signal is a frequency, which is barely affected by cable length.
How a vibrating-wire reading becomes pressure
The readout measures the resonant frequency and usually expresses it in "digits": the frequency squared divided by one thousand. Conversion to pressure uses the constants from each instrument's calibration certificate. The usual form used by manufacturers is:
P = G × (R0 − R1) + K × (T1 − T0)
where G is the calibration factor, R0 and R1 are the readings in digits at installation and now, K is the temperature correction factor, and T0 and T1 are the corresponding temperatures. Each model has its own exact form and units: the correct conversion is the one on the certificate for that serial number, which is why entering the constants comes before any chart.
What changes when readings are automated
With manual reading, a technician walks the instruments with a portable readout and writes the values on a spreadsheet, usually once a week or every two weeks. Automation connects a datalogger with a multiplexer to the existing cables and reads every instrument at the programmed interval. The gains:
- frequency: from one reading a week to several a day, or more during rain;
- consistency: same time, same readout, no transcription errors;
- response time: an out-of-range reading shows up on the dashboard within minutes, not at the next field campaign;
- record: every reading stored with the time it was taken, available to the engineer, the auditor and the regulator.
Field precautions
- Use what is already there. The installed piezometer stays; what changes is who reads it.
- Protect against surges. A long cable at the top of an embankment is a lightning antenna: the logger needs protection on its inputs.
- Self-contained power. A battery with a solar panel, and low consumption between readings.
- Commission side by side. For the first few weeks, run automated and manual readings together until the numbers match, instrument by instrument.
- Two paths for the data. Radio to a gateway and, from there, cellular and satellite: if the tower goes down in a storm, the readings keep coming in.
Where IoTData fits in
The network (LoRa radio, cellular and the DG satellite network, which delivers in minutes), the weather station and the platform already exist. The vibrating-wire reader with multiplexer is under development and will be piloted with the first customer, reading piezometers that are already installed. See dam monitoring.
Sources: principle of effective stress (K. Terzaghi); technical documentation from vibrating-wire piezometer manufacturers (digits and the conversion formula with the calibration certificate constants).