What Is A Piezometer? Its Application, Types & How it Works
Geotechnical sensors called piezometers are used to measure the pore water pressure (piezometric level) in the ground. They are made to measure the pore water pressure in soil, earth/rock fills, foundations, and concrete structures.
The piezometer, also known as a pore pressure meter, gives important quantitative information about the amount and distribution of pore pressure and how it changes over time. It also aids in assessing the seepage pattern, possible piping zones, and efficacy.
In this reading, we’ll explore what a piezometer is, its application, diagram, type, & how it works. We’ll also explore the advantages and disadvantages.
Let’s begin!
What Is a Piezometer?
A piezometer is a device that measures the pressure (more specifically, the piezometric head) of groundwater at a particular location or the height to which a column of liquid rises against gravity in order to determine the liquid pressure in a system. A piezometer is not aimed into the fluid flow as a pitot tube is since it is made to detect static pressures.
Although they need to be manually read, observation wells provide some information on the water level in a formation. Data acquisition is made more convenient by the automated reading of several types of electrical pressure transducers.
Open wells or standpipes (also known as Casagrande piezometers) inserted into an aquifer were the first piezometers used in geotechnical engineering. A Casagrande piezometer usually has a slotted or screened casing inside the zone where water pressure is being measured and a solid casing down to the depth of interest.
To stop surface water from polluting the groundwater supply, the casing is sealed into the drill hole using concrete, bentonite, or clay. The water level in the piezometer would not precisely match the water table in an unconfined aquifer, particularly if the vertical component of flow velocity is substantial.
The water level in the piezometer, rather than the water table, is a good indicator of the aquifer’s pressure in a restricted aquifer under artesian circumstances. A 5-cm-diameter standpipe is typical for piezometer wells, which can have a significantly smaller diameter than production wells.
To monitor the groundwater pressure at the installation site, piezometers with sturdy casings can be pushed or buried in the ground. The transducer, which converts pressure into an electrical signal, can be either a strain gauge, vibrating wire, or pneumatic.
Unlike open-standpipe piezometers, these piezometers are cabled to the surface so that data recorders or portable readout devices may read them more quickly or often.
Applications
Here are the following piezometer applications listed here:
Measure pore-water pressure and groundwater levels.
It helps in understanding initial site conditions.
It determines safe fill placement rates.
It predicts slope stability.
It is designed for lateral earth pressures.
It evaluates drainage schemes effectiveness.
Finally, check containment system performance.
Diagram
Types of Piezometer
Here are the main different types of piezometers:
Casagrande or Open Standpipe Piezometers,
Electric Piezometers
Hydraulic Piezometers
Pneumatic Piezometers
Titanium Piezometer
Casagrande or Open Standpipe Piezometers
An Open Standpipe Piezometer, also known as Casagrande, is a plastic pipe with a small diameter (up to 20 mm) and a porous section at the bottom where the piezometric level is to be measured. The pipe is installed inside a borehole, with the porous section at the bottom where the piezometric level is to be measured.
The annulus between the standpipe and the porous filter has sand in it that has been sealed on both sides with bentonite, and the rest of the borehole is filled with cement mortar. The groundwater pressure forces water into the standpipe until the level of water inside the standpipe (h) equals the piezometric level (pore water pressure) in the ground at the elevation of the porous filter. This is how the piezometer water level is used and monitored.
Electric Piezometers
The next kind of piezometers are electric piezometers, which are made up of a porous filter and a deflecting diaphragm separated by a little water reservoir. The diaphragm deflections are calculated using a strain gauge or a vibrating wire. Following accurate calibration, the piezometer-measured values are translated into an equivalent pressure.
The following process is identical to that of the open-standpipe piezometers. After filling the annulus between the porous filter and the borehole with cement/bentonite grout, the electric piezometer is placed into the borehole.
Due to buoyant force, groundwater pushes itself into the reservoir, causing the diaphragm to deflect until the reservoir’s internal pressure equals the ground’s pore water pressure (piezometric level) at the height of the porous filter.
The water in the piezometer will be pulled out of the reservoir if an electric piezometer is placed above the present groundwater table because the pore pressure in the soil is negative. This will cause the piezometer to become filled with air, which will cause it to malfunction.
Hydraulic Piezometers
The third kind of piezometer is the hydraulic one, which is made up of a water reservoir in a porous filter that is isolated from a pressure gauge by means of sturdy, water-filled tubes.
To eliminate air and guarantee that the reservoir stays full of water, these tubes carry water throughout the system. The piezometer calculates pore pressure by measuring the entire hydraulic pressure.
Pneumatic Piezometers
The pneumatic piezometer, which is powered by gas pressure, is the next kind of piezometer. Piezometers can be used in large diameter standpipes, fills, and boreholes. A pneumatic indicator is then used to compute the readings.
Titanium Piezometer
The titanium piezoelectric meter is a 4-20 mA device compatible with industrial data loggers. Typically used for drop-down testing.
How does Piezometer Works?
In essence, a magnetic, high-tensile-strength stretched wire with one end secured to a diaphragm that deflects proportionately to applied pressure makes up the vibrating wire piezometer.
Changes in pressure are necessary for the piezometer to work. The tension in the stretched wire is impacted by the diaphragm’s proportionate deflection in response to any change in pressure. Therefore, the tension in the wire is directly impacted by any change in pore pressure.
The coil magnet pulls the wire. It resonates at a frequency “f,” which is proportional to the wire’s tension and may be found as follows:
[σg/ρ] ^1/2}/ 2l Hz = f
Where σ is the wire’s tension.
gravitational constant, or g.
ρ is the wire’s density.
l = wire length
An alternating current is induced in the coil magnet by the resonance frequency at which the wire vibrates. This clarifies the formula or equation for the piezometer. The Encardio Rite type EDI-51V readout logger (data logger) can immediately show the pore pressure in engineering units, which is proportional to the square of the frequency.
Advantages of Piezometer
Simple, reliable tool for measuring fluid static pressure.
Mainly used for pore water pressure/groundwater level measurement.
Aids in calculating soil and rock performance.
Disadvantages of Piezometer
It cannot measure the static pressure of gas due to a lack of free surface.
Its unsuitable for measuring vacuum pressure as it pulls air into a container or vessel.
It’s not suitable for high pressure in light liquids due to the high piezometric head and difficulty in handling.
FAQs
What is a piezometer used for?
A piezometer is a tube with an open bottom that is used to determine the depth of groundwater by drilling it into the ground at various depths. It helps in measuring the depth of water in the tube to assess the direction of flow vertically and horizontally.
What is a piezometer instrument used to measure?
piezometer (pie-ZOM-uh-ter)
An instrument used to measure the pressure head in a pipe, tank, or soil. For pipes and tanks, it usually consists of a small pipe or tube. One end is connected or tapped through the wall of the pipe or tank being measured.
What is another name for a piezometer?
Piezometers or pore pressure meters are pressure transducers that are installed beneath the ground to measure the sub-surface piezometric level within groundwater level, soil, or rock. Now that you are well-versed with the piezometer definition, let’s move forward to understanding its usage in the geotechnical domain.
What is the difference between a piezometer and a pressure gauge?
A piezometer is an ordinary tube that is vertically connected to a liquid system for which the pressure must be measured. A pressure gauge is used to measure fluid intensity, as well as to set up and tune fluid power machines.
