Piezometers: Types, Functions, & How it Works?

Piezometers are the geotechnical sensors that are used to measure pore water pressure (piezometric level) in the ground. It is designed to measure pore water pressure in the soil, earth/rock fills, foundations, and concrete structures.

The piezometer or pore pressure meter provides significant quantitative data on the magnitude and distribution of pore pressure and its variations with time. It also helps in evaluating the pattern of seepage, zones of potential piping, and the effectiveness of seepage control measures undertaken.

Considered as one of the most important geotechnical monitoring instruments, it’s interesting to study as well. Let’s discuss more piezometer, their types, working principle, applications, and function in detail.

What is a Piezometer?

A piezometer is either a device used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity or a device that measures the pressure (more precisely, the piezometric head) of groundwater at a specific point. A piezometer is designed to measure static pressures and thus differs from a pitot tube by not being pointed into the fluid flow.

Observation wells give some information on the water level in a formation, but must be read manually. Electrical pressure transducers of several types can be read automatically, making data acquisition more convenient.

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The first piezometers in geotechnical engineering were open wells or standpipes (sometimes called Casagrande piezometers) installed into an aquifer. A Casagrande piezometer will typically have a solid casing down to the depth of interest, and a slotted or screened casing within the zone where water pressure is being measured.

The casing is sealed into the drill hole with clay, bentonite, or concrete to prevent surface water from contaminating the groundwater supply. In an unconfined aquifer, the water level in the piezometer would not be exactly coincident with the water table, especially when the vertical component of flow velocity is significant.

In a confined aquifer under artesian conditions, the water level in the piezometer indicates the pressure in the aquifer, but not necessarily the water table. Piezometer wells can be much smaller in diameter than production wells, and a 5 cm diameter standpipe is common.

Piezometers in durable casings can be buried or pushed into the ground to measure the groundwater pressure at the point of installation. The pressure gauges (transducer) can be vibrating-wire, pneumatic, or strain-gauge in operation, converting pressure into an electrical signal.

These piezometers are cabled to the surface where they can be read by data loggers or portable readout units, allowing faster or more frequent reading than is possible with open standpipe piezometers.

Piezometers

Why is a piezometer used?

The use of a piezometer has the following purposes:

  • To study the effect of water in pores of soil or rock is to reduce load bearing capacity of soil or rock. Effect is more pronounced with higher pore water pressure leading eventually in some cases to total failure of load bearing capacity of the soil.
  • To determine level and flow pattern of ground water
  • To determine the flow pattern of water in earth/rock fill, 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 water pressure to a frequency signal via a diaphragm and a tensioned steel wire. A change in pressure on the diaphragm causes a change in the tension of the wire.

The vibrating wire piezometer basically consists of a magnetic, high tensile strength stretched wire, one end of which is anchored and the other end fixed to a diaphragm that deflects in some proportion to applied pressure.

Any change in pressure deflects the diaphragm proportionally and this, in turn, affects the tension in the stretched wire. Thus, any change in pore pressure directly affects the tension in the wire.

The wire is plucked by a coil magnet. Proportionate to the tension in the wire, it resonates at a 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, with which wire vibrates, induces an alternating current in the coil magnet.

The pore pressure is proportional to the square of the frequency and the Encardio-rite model EDI-51V readout logger (data logger) is able to display this directly in engineering units.

Types of Piezometers

1. Standpipe Piezometers

The standpipe piezometer is the most basic type of piezometer. It consists of a filter tip joined to a riser pipe that extends to the surface. Water flows through the filter tip into the riser pipe. Readings are obtained with a water level indicator.

2. Vibrating Wire Piezometers

The vibrating wire piezometer is the most commonly deployed type of piezometer. Suitable for most applications, it can be installed in a borehole, embedded infill, or suspended in a standpipe. Readings are obtained with a portable readout or a data logger.

3. Pneumatic Piezometers

The pneumatic piezometer operates by gas pressure. It can be installed in a borehole, embedded infill, or suspended in a large diameter standpipe. Readings are obtained with a pneumatic indicator.

4. Titanium Piezometer

The titanium piezometer is a 4-20mA device that is compatible with industrial data loggers. It is generally used for drawdown tests.

Piezometer Applications

Piezometers measure pore-water pressure and groundwater levels, providing information essential to:

  • Understanding initial site conditions.
  • Determining safe rates for placement of fill
  • Predicting slope stability.
  • Designing for lateral earth pressures.
  • Evaluating the effectiveness of drainage schemes.
  • Checking the performance of containment systems.

Advantages of Piezometer

  • Simple and reliable
  • Used to measure static pressure fluid in a container.
  • One of the main applications of piezometer is that it is used to measure pore water pressure/ ground water level.
  • They are helpful in calculations to find performance of soil and rock.

Disadvantages of Piezometer

  • Piezometer cannot used to measure the static pressure of gas because gas do not form form a free surface.
  • Piezometer not suitable (but it can be used) to measure the vacuum pressure, because vacuum pressure cause suction of air into the container/vessel. It convenient only when static pressure of liquid is greater than the atmospheric pressure (gauge pressure)
  • Piezometer are not suitable to measure large pressure of lighter liquid, because for this condition piezometric head is very high. It would require long glass tube, which are difficult to handle.