Piezometers
Applications
- Monitoring pore water pressures to determine safe rates of fill or excavation.
- Monitoring pore water pressures to evaluate slope stability.
- Monitoring dewatering systems used for excavations.
- Monitoring ground improvement systems, such as vertical drains and sand drains.
- Monitoring pore pressures to check the performance of earthfill dams and embankments.
- Monitoring pore pressures to check containment systems at landfills and tailings dams.
Types of Piezometers
Standpipe Piezometers
The standpipe piezometer is the most basic of piezometer types. It consists of a filter tip joined to a riser pipe. Water flow from the surrounding soil into the standpipe. Readings are obtained with a water level indicator.
- Advantages: Simple, reliable, not electrical, no calibrated components.
- Limitations: Accuracy depends on skill of operator; reading requires a man on site; remote reading not possible; slower to show changes in pore-water pressure.
Vibrating Wire Piezometers
The vibrating wire piezometer is the most commonly deployed piezometer and is suitable for almost all applications. It consists of a vibrating wire pressure transducer and signal cable. It can be installed in a borehole, embedded in fill, or suspended in a standpipe. Readings are obtained with a portable readout or a data logger.
- Advantages: Easy to read, very accurate; good response time in all soils; easy to automate; reliable remote readings.
- Limitations: Must be protected from electrical transients.
Multi-Level Vibrating Wire Piezometer
The multi-level VW piezometer system is used to monitor pore-water pressure at multiple zones in a borehole. It consists of a number of VW piezometers in special housings, signal cable, a grout fitting, and some user supplied components (mainly PVC pipe). The system is grouted into a borehole. Readings are obtained with a portable readout or a data logger.
- Advantages: Solves or avoids most of the problems with traditional multi-level piezometer installations.
- Limitations: Same as VW piezometers.
Vented Vibrating Wire Pressure Transducers
The vibrating wire piezometer consists of a vibrating wire pressure transducer, a vented signal cable, and a desiccant chamber. It is designed for monitoring water levels in wells, stilling basins, and wiers. Readings are obtained with a portable readout or a data logger.
- Advantages: Easy to read, accurate, and can be connected to data loggers. Requires no barometric pressure compensation.
- Limitations: Electrical noise from a pump in the same well can interfere with operation.
Titanium Pressure Transducer
The titanium pressure transducer is a 4-20mA device that is compatible with industrial data loggers. It is use for monitoring water levels in pumping wells and for monitoring pore-water pressure in environments that would corrode stainless steel.
- Advantages: Suitable for dynamic measurements, easy to read, highly resistant to electrical noise.
- Limitations: 4-20mA circuits require more power so are less suitable for battery-operation. Long term stability may not be good enough for some applications.
Pneumatic Piezometers
The pneumatic piezometer operates by gas pressure. It consists of a pneumatic pressure transducer and pneumatic tubing. It can be installed in a borehole, embedded in fill, or suspended in a standpipe. Readings are obtained with a pneumatic indicator.
- Advantages: Reliable, remote reading possible, not electrical, indicator can be calibrated at any time.
- Limitations: Accuracy depends on skill of operator; difficult and expensive to automate, so reading requires man on site; reading time increases with length of tubing; pneumatic tubing can be blocked by condensation if not frequently charged with dry nitrogen gas.
Comparison of Piezometers
| Standpipe | Pneumatic | Vibrating Wire | |
|---|---|---|---|
| Response | Slow | Fast | Fast |
| Accuracy | High | Medium | High |
| Repeatable Readings | Need Technique | Need Patience | Easy |
| Obtain Readings Remotely | No | Yes | Yes |
| Connect to Data Logger | No | No | Yes |
| Potential for Lighting Damage | No | No | Yes |
| Main Expense | Drilling Borehole | Drilling Borehole | Drilling Borehole |