What is Piezometer? Definition, Working, Types, Calculation

Piezometers are geotechnical sensors that measure the pressure in the ground based on the flow of pore water. In soils, rock fills, concrete structures, and foundations, the instrument measures changes in pore water pressure. Piezometers measure pore pressure along with its distribution and variation over time, providing quantitative information about the magnitude of the pore pressure.

Understand where potential piping can be installed, how effective controls are, and what causes seepage. This is one of the most important geotechnical monitoring instruments, so studying it is of interest.

What is Piezometer? Definition, Function

Piezometer Definition

Piezometers measure the height at which a column of liquid rises against gravity in a system. Alternatively, it can measure the height to which a column of water rises against gravity at a particular location to determine the pressure of a large amount of groundwater.

Function of Piezometer

• Unlike pitot tubes, piezometers measure static pressure without aiming directly at the flow of fluid, making them more accurate than pitot tubes.
• To measure groundwater pressure, piezometers can be buried or pushed into the ground in durable casings, where they will measure the water pressure at the point of installation.
• By converting pressure into a signal generated by the transducer, pressure gauges or transducers convert pressure into electrical signals.
• Data can be logged using cabled piezometers. In addition, portable readout units enable more frequent or higher rate measurements than can be achieved with standpipe piezometers.

How Does Piezometer Work?

Let’s see the working process of a piezometer:

• The piezometer is simply a tube made of steel and fitted with a fixed cap on one end and a flexible diaphragm on the other. A steel tensioning rod is inserted between the fixed cap and the diaphragm. You change the tension on the wire when you push the diaphragm and change the pressure. Wire tension increases when you pull the diaphragm.
• Through the diaphragm, tension changes as pore pressure changes. With a coil magnet, it plucks a wire and measures the frequency of its vibration – think of it as the vibration of a wire. Under higher water pressure, a wire becomes a slacker, resulting in a lower reading frequency as it becomes a slacker.
• Piezometer readings can be taken manually or automatically. The borehole can be drilled manually by mounting a terminal unit at the top, or by extending a cable and running it in a trench at its bottom engineer has plugged in.  
• When the engineer plugs in the device, the results can be read manually. Piezometers are often connected to a wireless node that leads to a gateway and data logger 

Characteristics of Piezometer

• Geotechnical engineers first used piezometers to measure aquifer levels as early as the 18th century.
• The casing of Casagrande piezometers usually extends down to the depth of interest. To measure the pressure of water in the target zone, there is usually a slotted or screened casing inside the solid casing.
• There are several types of electrical pressure transducers available on the market that can be programmed, which makes it easier for the data collector to collect data than if observation wells were used.
• A piezometer well can have a diameter as small as 5 centimeters, and a standpipe with the same diameter can be used.
• A piezometer may be installed in confined aquifers under artesian conditions, where the piezometer level does not necessarily reflect the aquifer level.
• To prevent surface water from entering the drilling hole and contaminating the groundwater supply near the casing, the casing of the drilling hole is sealed with clay, bentonite, or concrete during Piezometer installation.
• Piezometers and water tables may not match exactly, especially when the flow velocity has a substantially vertical component.

Understanding the Calculations by Using the Formula

The diaphragm in a piezometer deflects proportionally when its pressure changes, which then affects the tension applied to the stretched wire. The tension in the wire is directly affected by any change in pressure within the pores.  

 Wires are pulled by coil magnets using electric coils. The resonance of the wire has a frequency ‘f’ that depends on the tension in the wire, which can be calculated as follows:

f = {[σg/ρ]^1/2}/ 2l Hz

The sign denotes:

• σ: Wire tension is indicated by this symbol.
• g: Gravitational constant is indicated by this symbol.
• ρ: Wire Density is indicated by this symbol.
• l:  Wire Length is indicated by this symbol.

Inducing the wire to vibrate is the alternating current produced by the resonant frequency of the coil magnet. Since pore pressure is proportional to frequency squared, the piezometer readout logger allows you to display it directly on the screen. Alternative methods, such as manual methods, may be chosen when convenience is more important than cost.  Readings can also be taken less frequently with this method. The automated system allows for more readings to be taken and more data to be generated. It can also improve safety and health for employees and reduce the carbon footprint of work sites by reducing lone worker visits. Rather than just seeing snapshots or revealing daily or seasonal changes, clients will get a fuller picture of what’s going on.

Various Types of Piezometers

Pore-water pressure can be measured using several types of piezometers. It is possible to install them in a variety of boreholes as well as in fills. As the cornerstone of soil-water interaction, effective stress is a fundamental component of soil mechanics. Therefore, it is crucial to understand the pressure within pores.

Following are the benefits and drawbacks of different types of piezometers used in geotechnical surveys:-

Open Standpipe type Piezometer

One of the simplest forms of piezometer, the Casagrande piezometer is often referred to as the easiest piezometer. There is usually an open pipe between the filter and the tube in an open standpipe piezometer. 

• Boreholes typically contain a porous filter surrounded by sand that serves as a shield against the earth. To maintain the borehole’s integrity, the top and bottom of the borehole are sealed with bentonite, while the rest is filled with cement or bentonite grout.
• At the center of the porous filter, which is also a measuring point, there must be an equivalent level of ground pore pressure. The water level inside the standpipe must match the pressure within the porous filter above the measurement point.
• In order to determine the level of water inside a standpipe, a measurement device uses an electric diameter. Standpipes emit an audible sound when their diameters come into contact with water. Whenever the tube touches water, light is produced. The top of the standpipe is measured with a tape measure.

It is necessary to take into account a part of the standpipe that sticks above ground level when calculating the pore pressure. The method of open-ended piezometers can be used to determine the permeability in situ. This is done to find out where the top of the standpipe meets the water level inside.

Benefits of using the Open Standpipe Piezometer

• Since they are simple, cheap, and long-lasting if installed correctly, they are generally reliable.
• It is considered self-desiring if the standpipe is larger than 12mm.
• This type of Piezometer can be used to estimate fluid permeability.
• Open Standpipe piezometers can be used to test water samples taken by them.

Drawbacks of using the Open Standpipe Piezometer

• It can take a long time for an open standpipe to respond, depending on its type.
• Ice on the surface of the ground prevents water levels near the surface from being read if they freeze.
• It is possible for fines to penetrate through these types of pipes, resulting in clogging and even failure of the standpipe.
• If the pore-water pressure is negative, it cannot be measured. Water in the ground cannot be detected by a dry standpipe, which cannot detect its location.

Diaphragm type Piezometer

Piezometers with a diaphragm have a pressure sensor and a Low Air Entry filter tip to detect low air entering the chamber, in addition to a small reservoir of water. The diaphragm attached to the pressure sensor separates water in the reservoir from the measuring device, while a filter tip attached to the pressure sensor separates water in the ground from the reservoir.

To measure the signal, a diaphragm deflection sensor transmits the signal via a cable to an access point. Among the types of diaphragm, piezometers are vibrating wire piezometers, strain gauge piezometers, and fiber optic piezometers.

Diaphragm piezometers are used to detect pressure changes in boreholes or fills. To provide extra durability, a sand pack can be applied around them instead of grouting. 

Benefits of using the Diaphragm Piezometer

• There is a fast response time with this type of Piezometer.
• Data logging and remote data reading are possible.
• Suction pressures or negative pressures within pores can be measured by the device.

Drawbacks of using the Diaphragm Piezometer

• The air cannot be removed from this type of piezometer unless suction lines are present.
• Calibration cannot be checked after installation.
• In order for this piezometer to work properly, it requires lightning protection.
• Your system’s performance can be affected by changes in temperature or barometric pressure.

Pneumatic type Piezometer

An air diaphragm is applied pressure to produce a piezoelectric effect in a pneumatic piezometer. On one side of the diaphragm, water pressure acts, while on the other side, gas pressure acts.

Diaphragm separation occurs when gas pressure exceeds water pressure. Gas would be discharged from the vent tube if the gas pressure exceeded the water pressure. A lake’s surface is covered with a layer of gas flowing backward. A surface flow will shut off gas if it detects it flowing backward.

Under water pressure, allowing the diaphragm to return to its original position would decrease the pressure of gas passing through the piezometer, preventing gas from escaping through the vent tube. When the water pressure and gas pressure are equal, pneumatic indicators’ pressure gauges will display the same reading as gas regulators.

Benefits of using the Pneumatic Piezometer

• It does not include any electronics. Using a portable gauge, measurements can be taken at the road’s surface.
• The durability of these types of piezometers is excellent.
• There is no need to calibrate this Piezometer.
• There is no effect of freezing ground temperatures on this type of Piezometer.

Drawbacks of using the Pneumatic Piezometer

• It can take a long time and be complicated to interpret measurements when they are necessary.
• In order to prevent dirt from getting into the system and causing problems, it is imperative to keep it clean.
• In order to keep the valve working properly, it must be used regularly.
• This type of piezometer cannot measure negative pore water pressures.
• Logging of data is not possible. 

Vibrating Wire Peizometer

Piezometers with vibrating wires are designed to measure water pressure and are widely used in civil engineering projects because of their reliability and suitability for demanding environments. Piezometers with vibrating wires have demonstrated long-term stability, with no drift from zero over decades.

Signals are also capable of being transmitted over long distances without degradation. They are also somewhat resistant to interference from external noise and damp wiring conditions.

Lightening protection is provided by a 90V gas plasma arrester on all Geosense piezometers. Temperature is measured by internal thermistors in all Geosense piezometers.

Model EPP – 30V Vibrating Wire Piezometer

Stainless steel body with an electron beam welded to the diaphragm surrounds the vibrating wire and coil magnet assembly of the Model EPP-30V vibrating wire piezometer. In order to protect the sensor from water and other corrosive materials inside, the welding creates a vacuum of about 1/1000 Torr.

Chemical corrosion is not a problem for the piezometer because of its stainless steel body. Ceramic flat filters with grain sizes of 40-60 microns and a low air entry value are used. In mass concrete structures, brick foundations, dam foundations, foundation soils of structures, reclaimed land soils, etc., water penetrates the internal pores or seams. Consequently, the ceramic filter is put under pressure.

Model EPP – 40V Slim Size Piezometer

In small diameter boreholes, slim size piezometers are used to measure pore pressure. An electron beam welding process is used to weld the sensor, and a vacuum of around 1/1000 Torr is created inside it, similar to the EPP-3-V. It contains a ceramic flat filter with a grain size of 40-60 microns with a low air entry value. A cable is connected through the glass to metal seal exactly as with the Model EPP-30V, but if desired, the piezometers may be supplied with the requisite length of cable.

Model EPP – 60V Low-pressure Piezometer

Low-pressure piezometers, such as the Model EPP-60V, are designed to measure low water pressures. It is convenient to use the basic pressure sensor for measuring water pressure and level in boreholes. Groundwater level, groundwater pressure, and their variations over time are provided quantitatively.

Water level evaluation helps monitor the behavior of the structure and its foundation after construction, indicating any potentially hazardous conditions that could affect their stability. Moreover, it provides basic information to promote safer and more economical design and construction.

Hydraulic type Piezometer

Hydraulic piezometers measure the forces exerted by fill materials, such as embankments, but they can also be used in boreholes. A water reservoir is connected by water-filled tubes to a pressure measurement device in this type of filter. The pumps circulate and remove air from reservoirs by pushing them long distances. It consists of a water reservoir and a high-entry filter tip.

Piezometers may contain air when pore pressures are negative in the ground, as they are in compacted clays. Due to the negative pore pressure in the ground, this occurs. A water connection must be maintained between the piezometer and reservoir in order to function accurately and to measure pressure accurately. For the air connection to be maintained, the air must be removed from the reservoir.

Benefits of using the Pneumatic Piezometer

• It is not difficult to use this type of Piezometer.
• The lack of inaccessible moving parts allows for long-term monitoring.
• Recalibration of the system is easy due to easy access to the sensors.
• It can measure small negative pore water suction pressures due to its ability to be removed from air.

Drawbacks of using the Pneumatic Piezometer

• In order to ensure a successful installation, de-aired water and a high air entry filter must be used.
• As this pressure measuring device has a limited range, it is best to place it at the same elevation as the filter tip.
• In the event of a large settlement, hydraulic tubing can be damaged, posing a very dangerous situation.

Flushable Diaphragm type Piezometer

Piezometers are available in various types, including flushable piezometers, which are special cases of hydraulic piezometers. Piezometers generally consist of several parts, including a pressure sensor, a valve, a porous filter tip, and a water reservoir. A pressure sensor detects pressure by being located near valves, filters, and reservoirs.

It is necessary to isolate the pressure sensor in the reservoir from the hydraulic tubes that flush water through the reservoir when there is a negative pore water suction pressure present. As a result of the negative suction pressure, it removes any air that accumulates there.

Benefits of using the Flushable Diaphragm Piezometer

• The response time is comparably faster.
• The height from which it can operate is not limited.
• Calibration can also be checked on site, which has its benefits.
• You can replace it if it becomes damaged or faulty.     
• This device measures the suction pressure of small negative pore water.
• The valve can also be deflated if necessary.

Drawbacks of using the Flushable Diaphragm Piezometer

• The piezometer can be used with a high-quality air entry filter and desired water.
• You can maintain it on a regular basis which is very important.

Component involved in Piezometer Functioning

There are many types of piezometers, all of which contain porous stones or tubes with perforations attached to them and supported by a plastic standpipe. Piezometers are mainly used in the field in two types: open standpipes and Casagrandes.

Porous sections of pipe would be inserted into boreholes and positioned in a way that could measure porosity using the porous section.

Porous filters are placed in the middle of bare holes with annuli filled with sand between them with the sand sealing both sides of the annulus. In response to the groundwater pressure, the water is forced into the standpipe until it reaches the piezometric level above the floor of the porous filter at the elevation where the standpipe water meets the groundwater pressure.

Installing of Piezometer

Vibrating wire piezometers and pneumatic piezometers have been in use for many years, but the most effective method of installing them has generated some debate over the years.

Traditional methodology

In the traditional method of installing an air conditioning system, a hole is drilled into the ground up until the target strata, which is the geologic level where the particular system should be installed.

Piezometers are placed in boreholes using a sand or gravel layer that surrounds the tip of the piezometer 0.6 meters to 1 meter deep in sand or gravel. A bentonite pellet seal is applied on top of the borehole creating an impermeable seal, followed by 1 meter of bentonite pellets. Sealing the installation properly is one of the most important parts of the installation.

When monitoring pore water pressures at different strata, this method can be used to place several piezometers in the same borehole. In the gap between the piezometers, bentonite pellets are used to seal the gap between the piezometer tips. Backfilling the borehole with cementitious bentonite grout is typically done at the top.

Recent methodology

In recent years, however, there has been an increasing number of piezometers being installed using a method in which the grout has been fully embedded.

Following are the steps in the procedure:-

• hang the piezometer in a borehole that has been previously drilled,
• Then, fill the borehole completely with grout.

According to this theory, water pressures are transmitted through the ground horizontally in a horizontal direction.

This method was initially viewed as a potential source of concern as there was a concern that the water pressure would be transmitted up and down the grout column instead of being represented locally by the piezometer tip. While this has not proven to be the case, this alternative method has now been accepted widely as a viable alternative method that is more commonly seen in specifications as it has been found to be effective. 

What are the applications of the Piezometer?

Piezometers measure groundwater level and porewater pressure. Engineers can also benefit from their help with the following things:-

• Using this Piezometer, you can identify the initial site conditions and formulate a plan based on those conditions.
• The rate at which fill can be placed safely must be determined.
• This sensor is capable of predicting slope stability, which is an important skill.
• Using these sensors, structures can be designed and built to withstand horizontal and upward ground pressures.
• Using these sensors, evaluate drainage schemes to determine their effectiveness.
• The integrity of containment systems must be ensured.

Various applications of Piezometer for Earthfill Dams construction

• It is necessary to establish a construction rate and implementation method that is safe.
• It is also possible to use these sensors to determine whether the clay core is performing as expected.
• Seepages can also be monitored with it.
• This piezometer can also be used to monitor cutoff walls.
• It can also be used to calculate uplift pressures in some cases, in addition to determining them. 

Various use of Piezometer for Embankments

• This type of piezometer sensor can be used to determine construction rates and implement safe methods of construction.
• It is possible to monitor drainage flows and outlooks using these piezometer sensors.
• It is also possible to monitor consolidations using these piezometer sensors.

Various applications of Piezometer for landslides

• By measuring slope resistance to gravity, a piezometer can also be used to predict slope stability.
• Piezometer sensors that measure soil vibrations can also be used to determine soil mass.

 Various applications of Piezometer for retaining walls

• Additionally, a piezometer can be used to measure how much load is applied to a wall.
• The piezometer can also be used to determine drainage system effectiveness.

 Various applications of Piezometer for Diaphram Wall

• The piezometer can also be used to determine the initial conditions of a site.
• The load applied to a wall can be calculated using this method.
• Piezometers can also be used to determine the level of the water as well as monitor draw-down due to dewatering.
• As differential pressures are measured by piezometers, they are also effective for monitoring uplift pressures on excavation floors. 

 Various applications of Piezometer in relation to Dewatering

• In addition to evaluating the pumping scheme of a water heater, a piezometer can also be used.
• By placing a piezometer in a suitable place, a piezometer can also provide early flood warnings. 

 Various applications of Piezometer for Dynamic Compactions

• Consolidation can also be evaluated using a piezometer.
• It is also possible to evaluate drainage systems using a piezometer.

Uses of Piezometer

Piezometers can be used for a variety of purposes, including:

• When water is found inside the pores of soils and rocks, it reduces the load-bearing capacity.
• Eventually, the soil’s load-bearing capacity could be completely compromised as the pore water pressure increases, which could be catastrophic in some cases.
• For purposes of determining their source, it is necessary to determine groundwater levels and flow patterns.
• Water flow patterns in concrete dam foundations and earth fill will be determined.
• A phreatic line is drawn to delineate this area. 

Pore Pressure & Piezometer

By allowing water to enter, soil and rock pores lose their ability to hold structures. Soil or rock that has more water in its pores will be less likely to be able to bear weight. Structures built over such soil fail and are destroyed.

Piezometers are used to measure the level and flow of groundwater, one of the most fundamental resources. The phreatic line can be determined by measuring water flow patterns in earth, rockfill, & concrete dams.

Pressure & Temperature Fluctuation while Using Piezometer

It is important to record both pressure and temperature data when a piezometer is installed in a zone whose temperature is likely to fluctuate significantly. This will allow you to evaluate the effects of temperature on pressure readings.

• Temperature effects on piezometers sealed in boreholes or buried in fill are typically small, so it is not necessary to correct for them. In shallow standpipes or wells, however, low range piezometers may be affected by day-to-day temperature changes and seasonal variations.

• The importance of temperature corrections could increase in this situation. There are many factors that influence Piezometer readings, including thermal influences.

• Temperature correction requires first establishing the effects of temperature changes on the piezometer and the medium in which it is installed. Temperature affects the density of water as well. Using an alternative method, it is necessary to verify the head acting on a piezometer accurately to determine the true effects of temperature changes.

• In addition to recording piezometer pressure and temperature, it is important to determine the true depth of water over the piezometer diaphragm. These data can be used to compute both seasonal and daily thermal effects over a full annual cycle. Similar to barometric pressure corrections, corrections would be made based on a similar principle.

How to Install Piezometer?

The most effective method to install vibrating wire piezometers and pneumatic piezometers has been debated for many years.

Traditional methodology

• A hole is drilled into the ground to reach the target strata, which is the geologic level in which the particular air conditioning system should be installed.
• 0.6 to 1 meter of sand or gravel surrounds the tip of piezometers in boreholes when they are placed in boreholes. The borehole is sealed with bentonite pellets, creating an impermeable seal, followed by 1 meter of bentonite pellets. Sealing the installation properly is one of the most important steps.
• It is possible to place several piezometers in the same borehole when monitoring pore water pressures at different strata. A gap between the piezometer tips is sealed with bentonite pellets. The top of the borehole is typically backfilled with cementitious bentonite grout.

Recent methodology

Piezometers have increasingly been installed by embedding the grout fully in the grout in recent years.

This procedure consists of the following steps:-

• Drill a borehole and hang the piezometer in it,
• After that, complete the grouting of the borehole. 

The transmission of water pressure is based on this theory. As a result of this method, water pressure could be transmitted up and down the grout column instead of being represented locally by the piezometer tip. Despite the fact that this alternative method has not proven to be effective, it is now accepted as a viable alternative and is more commonly seen in specifications. The ground should be horizontally oriented.

Benefits of using Piezometer

• There are several advantages to piezometers, including ease of use and high reliability.
• A static pressure gauge measures the static pressure of a fluid in a container.
• Piezometers are commonly used for measuring pore water pressure and groundwater levels, which is one of their most common uses.
• Soil and rock performance can be determined based on these calculations.

Drawbacks of using Piezometer

• Because gas has no free surface, a piezometer cannot measure its static pressure.
• Under vacuum conditions, air will be suctioned into a container when measuring vacuum pressure with a piezometer. The static pressure of the liquid must be greater than the atmospheric pressure for this type of filtration to be effective.
• Since piezometers have high heads, they are not the best instruments for measuring the pressure of lighter liquids. Thus, long glass tubes would be required, which is very challenging.

Our Apps:

Check out our ‘MechStudies – The Learning App’ in iOS & Android

Book

Nice & cheap Book on Pump Question & Answers

You can get Centrifugal Pump Reference Book

Our Videos

Our YouTube Animated Videos

Check out our most interesting articles,

Pump basics

Centrifugal Pump

Screw Water Pumps

Gear Pumps

Steam Turbines

Globe valve

Venturi meter

Siphon

Reference Articles