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A physical property (or physical quantity)  whose value determines the characteristics or behaviour of something.

Passive earth pressures

The minimum value of earth pressure which exists when a soil mass is permitted to yield sufficiently to cause its internal shearing resistance along a potential failure surface to be completely mobilised.

PDA Personal Digital Assistant

A personal digital assistant (PDA) is a mobile device that functions as a personal information manager and is used typically as a readout unit for digital instruments including MEMS inclinometers, tilt meters and ThermArrays.


Permeability (commonly symbolised k) is a measure of the rate at which soil or rock allows water to pass through it. Low permeability soils can allow pore water pressures to build up during consolidation which if uncontrolled can lead to failure.             

Physical quantity

(or 'physical magnitude') A physical property of a phenomenon, body, or substance, that can be quantified by measurement.


Piezometers measure pore-water pressure in fully or partially saturated soil and rock and ground water levels. There are several types of piezometer including:

Standpipe Piezometer: The standpipe piezometer (often known as a Casagrande piezometer) is the most basic type of piezometer. It consists of filter tip connected to a riser pipe that extends to the surface. To measure the borehole water level, the filter tip zone is filled with sand and then backfilled above. To isolate pore water pressure at the filter tip, a bentonite seal is required between the sand filter zone and the backfill. Water flows through the filter tip into the riser pipe. Readings are obtained with a water level indicator.

Vibrating Wire Piezometer:The vibrating wire piezometer is the most widely used type of piezometer. Suitable for most applications, it can be installed in a borehole, suspended in a standpipe or embedded in fill. Readings are obtained with a portable readout or a data logger.

Multipoint Piezometer: Multi-point piezometers comprise of a series of piezometers placed in a single borehole and are typically installed using the Fully Grouted Method which is becoming the industry preferred method for installing vibrating wire piezometers. See Fully Grouted Method.

Strain Gauge Piezometer: Strain gauge piezometers are suitable for dynamic monitoring such as pumping tests. Readings are obtained with a portable readout or data logger

Pneumatic Piezometer: The pneumatic piezometer operates by gas pressure. It can be installed in a borehole, embedded in fill, or suspended in a large diameter standpipe. Readings are obtained with a pneumatic readout.

Polynomial accuracy

As the behaviour of all sensors is inherently non-linear it is common practice within the geotechnical instrumentation industry to provide information on their accuracy based on a 2nd order polynomial expression of its characteristics. Whilst this approach is valid on mathematical and practical grounds its main purpose is for manufacturers to state a higher degree of accuracy. In our opinion there is little to be gained using the polynomial factors and recommend using the non-linearity factor as standard. See Non-linearity. 

Pore water pressure

Pore water pressure refers to the pressure of groundwater held within a soil or rock, in gaps between particles and is measured relative to atmospheric pressure. Where the pressure is atmospheric (i.e. zero) this is defined as the water table or phreatic surface.

Both positive and negative pore water pressures can be measured with vibrating wire piezometers. See effective stress.        


The precision of an instrument also called reproducibility or repeatability is the degree to which repeated measurements under unchanged conditions show the same results.  It is normal to express precision as a ± number and the number of significant figures quoted reflects the precision e.g ±1.00 indicates a higher precision than ±1.0.


Not precise, average accurate

Precise but not accurate

Precise and accurate