A piezometer is a geotechnical sensor used to measure pore water pressure (piezometric level) in the ground. Designed to measure pore water pressure in soil, earth/rock, foundations and concrete structures.
Piezoelectric or pore pressure gauges provide important quantitative data on the magnitude and distribution of pore pressure and its evolution over time. It also helps to assess seepage patterns, potential areas of piping and the effectiveness of seepage control measures that have been implemented.
It is considered one of the most important geotechnical monitoring tools, it is also interesting to study. Let’s take a closer look at piezometers, their types, working principles, applications and functions.
What is a Piezometer?
A piezometer is a device used to measure fluid pressure in a system by measuring the height to which a column of fluid rises against gravity, or groundwater pressure (more precisely, a piezometric head) at a specific point. A piezometer is designed to measure static pressure, so unlike a pitot tube, it is not directed toward the fluid flow.
Observation wells provide information about the water level in the formation, but must be read manually. It can automatically read multiple types of electrical pressure transducers, making data collection more convenient.
The first piezometer in geotechnical engineering was an open well or standpipe (sometimes called a Casagrande piezometer) placed in an aquifer. Casagrande piezometers usually have a solid chamber to the desired depth and a slotted or screened chamber in the area where the water pressure is measured.
Casings are sealed over drilled holes with clay, bentonite, or concrete to prevent surface water from contaminating groundwater sources. In confined aquifers, the piezometer water level does not exactly correspond to the water surface, especially if the vertical component of the flow velocity is large.
In an artesian aquifer, the piezoelectric water level represents the aquifer pressure, but not necessarily the water level. Piezometer wells can be much smaller in diameter than production wells, and 5 cm diameter standpipes are common.
Piezometers in durable housings can be buried or pushed into the ground to measure groundwater pressure at the installation site. A pressure gauge (transducer) is a vibrating wire, air pressure, or manometer that converts pressure into an electrical signal.
These piezometers can be read faster or longer than basic readout piezometers because they are attached to a surface that can be read by a data logger or portable reading unit.
Why is a piezometer used?
The use of piezometers has the following purposes:
- Studying the effect of water in soil or rock is to reduce the bearing capacity of soil or rock. This effect becomes more obvious at higher pore water pressures and eventually leads to the complete loss of soil bearing capacity in some cases.
- Determination of groundwater level and flow patterns
- Determines water flow patterns in soil/rock embankments, concrete dams and their foundations.
- To delineate the phreatic line.
How does a piezometer work?
Piezometer is used to measure underground water pressure. It converts the water pressure into a frequency signal through a diaphragm and a tensioned steel wire. As the diaphragm pressure changes, the wire tension changes.
A vibrating wire piezometer basically consists of a high-strength magnetic tension wire, one end of which is fixed and the other end of which is attached to a fixed diaphragm that deflects in proportion to the increase in applied pressure.
As the pressure changes, the diaphragm deflects proportionally, which affects the tension of the stretched wire. Therefore, changes in pore pressure directly affect wire tension.
The wire is pulled by a coil magnet. Proportional to the tension of the wire, it resonates at the frequency “f”, which can be determined as follows:
f = {[σg/ρ] ^1/2}/ 2l Hz
Where:
- σ = tension of the wire
- g = gravitational constant
- ρ = density of wire
- l = length of wire
The resonant frequency at which the wire vibrates induces an alternating current in the magnet of the coil.
Pore pressure is proportional to the square of frequency and the Encardio-rite Model EDI-51V Readout Logger (Data Logger) can display this directly in engineering units.
Types of Piezometers
1. Standpipe Piezometers
The stand pipe piezometer is the most basic type of piezometer. It consists of a filter tip attached to a riser tube that extends to the surface. Water flows through the filter tip into the riser tube. Readings are taken at the water level indicator.
2. Vibrating wire piezometer
Vibrating wire piezometer is the most common type of piezometer. Suitable for most applications, it can be installed in a borehole, embedded infill, or suspended on a standpipe. Readings are taken with a portable readout or data logger.
3. Pneumatic Piezometers
A pneumatic piezometer is activated by gas pressure. It can be installed in a borehole, embedded with a filler or hung on a large diameter standpipe. Readings are taken with an pneumatic indicator.
4. Titanium piezometer
The titanium piezoelectric meter is a 4-20 mA device compatible with industrial data loggers. Typically used for drop down testing.
Piezometer Applications
Piezometers measure pore water pressure and groundwater level and provide essential information for:
- Understand basic site conditions.
- Determining the safety factor for filling
- Prediction of slope stability.
- Designed for lateral earth pressure.
- Evaluating the effectiveness of the drainage plan.
- Check the operation of the containment systems.
Advantages of Piezometer
- Simple and reliable
- It is used to measure the static pressure of fluids in vessels.
- One of the main applications of piezoelectric transducers is the measurement of pore water pressure/groundwater level.
- It helps to calculate soil and rock performance.
Disadvantages of Piezometer
- A piezometer cannot be used to measure the static pressure of a gas because gas does not form a free surface.
- Piezometers are not suitable for measuring vacuum pressure (although they can be used). Because vacuum pressure pulls air into the container/vessel. It is only useful when the static pressure of the liquid is greater than atmospheric pressure (gauge pressure).
- Piezometers are not suitable for measuring high pressure in light liquids. Because in this case the piezometric head is very high. It requires a long glass tube and is difficult to handle.
