Piezometer

Piezometers are widely used in civil engineering in order to monitor pore water pressure or the level of the water table.

The most common applications are: Earth fill dams and embankments, Up-lift pressure in dam foundations, ground water level, pore water pressure, seepage monitoring, water pressure behind tunnel linings, potential landslides and instable slopes, dewatering and pump tests, foundations and excavations with retaining walls and slopes.
Piezometer classification and model selection are one of the most difficult theme in the field of geotechnical monitoring. Finding the right solution for measuring groundwater pressure or water level in the soil, rock or the interface between a structure with the ground has to take into account ground permeability and the measuring target.
The two main piezometer families are "open piezometers" and "closed piezometers".
Open piezometers (also named standpipe piezometers) are tubes primarily composed by a bottom part with a water filter (Casagrande porous filter or slotted tube) and a blind single or twin tube that connects the filter to the surface. Water flows through the filter and after a certain time (function of the soil permeability) reaches a balanced water level. If the filter is not sealed within a certain strata or is buried in the phreatic layer, the open piezometer will read the level of the groundwater table. If, on the other hand, the filter unit is sealed within a certain level and collects water only from that specific level, the open piezometer measure the pore water pressure of this layer. In this case we speak about Casagrande piezometers. They are often used to monitor pore water pressure in medium permeable soil. Open piezometers are read using a water level indicator (WLI), also named dipmeter, or with a relative pressure transducer (Sisgeo P252R model). Also an absolute pressure transducers can be used if barometric pressure is compensated to achieve precise results. The WLL is an automatic operated and simple system to monitor the water table level and water temperature of standpipe piezometers, wells, basins, reservoirs and other technical facilities. It is composed of a small diameter cylindrical body that includes a pressure transducer, a temperature sensor and a data logger. The WLL shall be is programmed with a PC/notebook to activate automatic measurements and then fixed to a steel wire lowered into the borehole. The software to setup the WLL enables also data download and producing graphs and numerical data tables.
Closed piezometers are directly buried within the soil, rock or interface between rock and ground and structure at the level where the pore water pressure is to be monitored. The absolute pressure transducer (usually simply named "piezometer") is available with different filter porosities, function of the soil permeability, and with different transducer technologies. Resistive type piezometers are typically used for short term monitoring and high-frequency measurements like in pumping test; vibrating wire piezometers are used where high reliability and long term monitoring is required, like in dam projects. If a multi-layer water pressure monitoring is required, the most suitable solution is the installation of a chain of vibrating wire multipoint piezometers (PK45M model). The suggested installation technique is the "fully grouted" method, that allow fast installation and rapid response to pressure changes. For certain requirements an installation of the piezometers in sand pockets, that are sealed between each other with a cement or bentonite plug, can be advisable, but really difficult to be realized.
V-notch weirs are instruments to monitor flows in open channels: the working principles is that the water flow is proportional to the water level over the weir. The water flows into the upstream stilling basin and is measured with a staff gauge or, for automatic measurement with a pressure transducer installed within the stilling basin. The flow rate in the open channel is calculated with an equation. One of the most important applications is to monitor the water leakage of a dam body: water leakage quantity is a parameter to evaluate the performance of a dam and is an important part of long term safety monitoring project.