Piezometers: A guide

What is a piezometer?

The word piezometer is formed from two Greek-derived elements: the prefix piezo-, meaning “pressure,” and the root meter, meaning “to measure.” And a piezometer does exactly that: it measures pressure. More specifically, a piezometer measures static water pressure, usually in one of two contexts: measuring the pressure of groundwater contained in the surrounding rock and soil—the pore water pressure—or the pressure of a body of water against the sides of its container.

What are piezometers used for in geotechnical monitoring?

In geotechnical monitoring contexts, determining the pore water pressure is essential to understanding how the ground behaves. Drained and undrained soils of the same type have different properties, especially when placed under loads. The measurements provided by piezometers allow engineers, among other things, to determine groundwater levels, track water flow through the earth, and monitor water levels in wells and standpipes.

The primary geotechnical applications for piezometers’ ability to measure the pressure exerted by an open body of water occur in dams. Here, piezometers’ measurements of the lateral pressure exerted on retaining walls and dams is important for monitoring their stability and preventing problems before they occur.

What are the different kinds of piezometers?

The term piezometer refers to the kind of measurement (water pressure) that an instrument makes, and there are several types of instruments that fill this role. Most piezometers make use of the same basic mechanism: groundwater comes into contact with an impermeable diaphragm, which deforms under pressure. The main difference between piezometers is the instrument used to measure the diaphragm’s deformation.

Vibrating wire piezometer. In a vibrating wire piezometer, or VWP, a wire is attached to the diaphragm on one end and anchored to the instrument on the other. This wire is electrically “plucked” with a magnet, and the effects of the diaphragm’s deformation due to water pressure on the vibrations are translated into a reading.

Strain gauge piezometer. Like a VWP, strain gauge piezometers use a deformable diaphragm to read water pressure. However, they use an electrical device mounted on a flexible backing to measure the pressure behind the diaphragm.

Pneumatic piezometer. A pneumatic piezometer uses changes in a compressed gas to measure the water pressure on the instrument’s diaphragm.

Casagrande standpipe piezometers. These are the simplest kind of piezometer, and the only commonly used type that does not employ a flexible diaphragm to measure water pressure. A Casagrande standpipe piezometer contains no moving parts, taking advantage of the groundwater pressure’s ability to push the groundwater up the borehole where the piezometer is installed. Groundwater enters the instrument through a porous section in its bottom and is pushed up by the pore water pressure into a pipe extending the length of the borehole. The height the water reaches corresponds to the pore water pressure at the depth of the piezometer’s porous section.

RST Instruments’ piezometers

Vibrating Wire Piezometers. RST instruments makes several kinds of vibrating wire piezometers to suit the wide range of situations where water pressure monitoring is needed:

• The VW2100 Standard Vibrating Wire Piezometer is designed for the widest range of applications.
• The VW2100-HD Heavy Duty Vibrating Wire Piezometer is used in more physically demanding conditions, such as direct burial in fills and dam embankments, as well as in high-pressure boreholes.
• The VW2100-DPC and VW2100-DPEW are drive point models made to be driven into fine-grain materials like sand, silt, and clay.
• For low-pressure environments, RST produces the VW2100-L, an unvented model, and the VW2100-LV, a vented model.
• When space is at a premium, there are RST’s VW2100-M, at 17.5 mm in diameter, and the VW2100-MM, at only 11.1 mm in diameter.
• For especially difficult environments, RST offers the VW2190, with a special bladder for protection in saltwater, and the VW2191, with a bladder for protection against corrosion in acidic conditions.
• RST also offers preassembles multi-piezometer systems to take readings at multiple depths from the same borehole.

Strain Gauge Piezometer. RST’s strain gauge piezometers feature a diaphragm sensor that is isolated from the surrounding environment by a silicon pressure cell. This makes them excellent for applications where the instrument is exposed to liquids or gasses that would harm the sensor with direct contact.

Casagrande Standpipe Piezometer. The RST Casagrande standpipe piezometer is designed for taking measurements in rock and soil in applications where only occasional, non-time-sensitive readings are needed.