PIEZOMETERS

Piezometers are used for measuring pore water pressures. They are generally installed in boreholes but can also be placed in fill.

Pore water pressures are important because they are a fundamental component of the effective stress, which is the cornerstone of soil mechanics. The principle of effective stress states that all measurable effects of a change of stress (e.g. compression distortion and a change of shearing resistance) are exclusively due to changes in the effective stress (taking due account of time effects).

Put simply the effective stress is the difference between the total stress acting at a point in the ground and the pore water pressure acting at the same point. Reasonable estimates can be made of the total stresses (although sensors do exist for measuring total stress) but the pore water pressures can only be measured. Therefore piezometers are very important.

OPEN STANDPIPE PIEZOMETERS

The simplest form of piezometer is the open standpipe, sometimes called the Casagrande piezometer. Typically an open standpipe piezometer consists of an open pipe with a cylindrical porous filter at the bottom. The porous filter is placed in the centre of a borehole and typically surrounded by a sand. Above and below the sand are seals (normally formed of bentonite) and the rest of the borehole is filled with a cement / bentonite grout.

The pore pressure in the ground at the centre of the porous filter (the measuring point) is equivalent to the height of the water inside the standpipe above the measuring point. The level of the water inside the standpipe is measured using an electric dipmeter. The dipmeter is lowered inside the standpipe and when it makes contact with the water it emits a audible sound and/or a light. The distance from the top of the standpipe to the water level inside the standpipe is measured using a tape.

In calculating the pore pressure it is important to take account of any part of the standpipe that is sticking up above the ground level.

Open standpipe piezometers can be used to estimate the insitu permeability.

Advantages

• They are simple, cheap and if they are installed well they are generally reliable.

• If the standpipe is larger than 12mm, open standpipes are self de-airing.

• They can be used to estimate permeability.

• They can be used to take samples of water.

Disadvantages

• The response time of open standpipes can be long.

• They cannot be read if the water level is near the ground surface and it freezes.

• They can allow fines to penetrate and maybe even block the standpipe.

• They cannot be used to measure negative pore water pressures. A dry standpipe reveals nothing about where the water level is in the ground.

Diaphragm piezometers

Advantages

• Quick response time.

• Can be datalogged and read remotely.

• Can measure small negative pore pressures (suctions).

Disadvantages

• Cannot remove air if suctions are present.

• No means of checking the calibration once installed.

• Needs lightning protection.

• Can be affected by changes of temperature and barometric pressure.

Pneumatic Piezometers

Advantages

• No built in electronics. Measurements are made on a portable gauge at the surface.

• Excellent longevity.

• No calibration required.

• No problem with freezing ground conditions.

Disadvantages

• Measurements can take a long time and can require a skilled operator to interpret.

• Requires scrupulous cleanliness. Dirt in the system can be a problem.

• Requires regular use to keep the valve operating properly.

• Cannot measure negative pore water pressures.

• Cannot be datalogged.

Hydraulic piezometers

Hydraulic piezometers are typically installed in fill materials (e.g. embankments) but can be installed in boreholes too. They consist of a high air entry (HAE) filter tip and water reservoir that are connected to a pressure measuring device using water filled tubes, which can be run for long distances and are used for circulating water into and removing air from the reservoir. Air can be present inside the piezometer if the pore pressures in the ground are negative (i.e. suction), which is often the case in compacted clays. It is vital to remove the air so that the water connection between the reservoir and the pressure measuring device can be maintained and the piezometer can thereby remain responsive and accurate.

Advantages

• Simple.

• No inaccessible moving parts, so good for long-term monitoring.

• Easy to recalibrate because the sensors are accessible.

• Can measure small negative pore water pressures (suctions) and will remain useable because air can be removed from the piezometer.

Disadvantages

• Requires a high air entry filter and de-aired water.

• Has limited range. Best if the filter tip and the pressure measuring device are at the same elevation.

• Hydraulic tubing can be damaged (needs protecting) if large settlements occur.

Flushable Diaphragm Piezometers

Flushable piezometers are a special case of hydraulic piezometers, for use in boreholes. Each piezometer has a pressure sensing device, a valve, a high air entry (HAE) porous filter tip and a water reservoir. The pressure sensor is located close to the valve, filter and reservoir. The valve is used to isolate the pressure sensor from the hydraulic tubes that are used to flush water through the reservoir and remove any air that forms there when a negative pore water pressure (suction) is present.

Advantages

• Quick response time.

• Range of operation isn’t limited by elevation.

• Calibration can be checked in-situ.

• Can be removed and replaced if damaged or faulty.

• Can measure small negative pore water pressures (suctions).

• Air can be removed.

Disadvantages

• Requires high air entry filter and de-aired water.

• Requires maintenance.