Piezometer classification and model selection is one of the most difficult issues in the field of geotechnical monitoring. Choosing the right solution to measure groundwater pressure or water level in soil, rock or at the interface between a structure and the ground must take into account the permeability of the soil and the objective of the measurement.
The two main piezometer classifications are ‘open piezometers’ and ‘closed piezometers’.
Open piezometers (also called standpipe piezometers) are pipes consisting mainly of a lower part with a filter (Casagrande porous filter or slotted pipe) and a single or double blind pipe connecting the filter to the surface. The water flows through the filter and after a certain time (a function of soil permeability) reaches a balanced water level. If the filter is not sealed within a given water table or is buried in the phreatic layer, the open piezometer will read the water table level. If, on the other hand, the filter unit is sealed within a certain layer and only collects water from that specific layer, the open piezometer measures the pore water pressure of this layer. In this case, we speak of Casagrande piezometers.
Casagrande piezometers are often used to monitor pore water pressure in medium permeability soils. Open piezometers are read with a water level indicator (WLI), also called a Dip Meter, or with a relative pressure transducer (model Sisgeo P252R). An absolute pressure transducer can also be used if the barometric pressure is compensated using local barometer data, for accurate results.
Closed piezometers are buried directly in the ground, rock or rock-soil interface and in the structure at the level where the pore water pressure is to be monitored. The absolute pressure transducer (usually simply called a piezometer) is available with different filter porosities, depending on the permeability of the soil, and with different sensor technologies. Resistive piezometers (model P235) are typically used for short-term monitoring and high-frequency measurements, such as in pumping tests; vibrating wire piezometers (model PK45 or PK20) are used when high reliability and long-term monitoring is required, such as in dam projects. If it is necessary to monitor water pressure on several levels, the most suitable solution is the installation of a chain of multi-point vibrating wire piezometers (model PK45M). The suggested installation technique is the ‘fully grouted’ method, which allows rapid installation and response to pressure variations. For some requirements, the installation of piezometers in sandbags, sealed together with a cement or bentonite plug, may be advisable but difficult to implement.
V-notch weirs are instruments for monitoring flow rates in open channels: the operating principle is that the water flow rate is proportional to the water level above the weir. The water flows into the stilling basin upstream and is measured with a pressure gauge or, for automatic measurement, with a pressure transducer installed in the stilling basin. The flow rate in the open channel is calculated with an equation. One of the most important applications is the monitoring of water loss from a dam: the amount of water loss is a parameter for assessing the performance of a dam and is an important part of the long-term safety monitoring project.
A lot of information and suggestions regarding piezometer applications can be found in the international standard ISO 18674-4 “Measurement of pore water pressure: Piezometers”.