What Is Piezometer: Features, Working, Types & uses

Piezometers are geotechnical sensor devices that help to measure pore water pressure in the ground. It is made for measuring pore water pressure in soil, foundations, earth fills, and concrete structures. It is an important tool for geotechnical monitoring systems.

In this post, we will cover detailed features for piezometers and related factors. Let’s get started.

 What Is a Piezometer?

• The piezometer is a device used for the purpose of measuring the pressure of groundwater at a certain point or height where a liquid column rises against gravity to find the liquid pressure in the system.

• The piezometer is not physically put into fluid like a pitot tube, but it has features to detect static pressure.
• It needed manual reading and observation to provide water level details in the information.
• Automated reading of different electrical pressure transducers makes data acquisition easy.
• Standpipes put into aquifers are the first example of piezometers used for geotechnical engineering.
• That, also called a Casagrande piezometer, comes with a slotted casing in the zone where water pressure is measured.
• To avoid surface water polluting the groundwater supply, casing is sealed into the drill hole with the use of concrete or clay.
• The water level in the piezometer does not properly match the water table in the unconfined aquifer in case of required vertical component flow of velocity.
• The water level of the piezometer compared to the water table is a good sign of aquifer pressure in a limited aquifer in artesian conditions.
• The five-centimeter-diameter standpipe is used for piezometer wells, which have a smaller diameter as compared to production wells.
• For monitoring groundwater pressure at the installation area, piezometers are pushed into the ground with sturdy casings.
• The transducers that transform pressure into electrical signals can be strain gauges or pneumatic.

What is the need for a piezometer?

The main motive of the piezometer is

• It helps to find the water effect in the pores of soil or rock for minimizing load-bearing features of soil. The effect is highly pronounced through high pore water pressure, causing net damage for the load-bearing capacity of soil
• The piezometer is used for groundwater monitoring for finding the level and flow layout of groundwater.
• It also helps to find the flow pattern of water in rock fill and concrete dams and also delineate the phreatic line.

Parts of a piezometer 

• The piezometer comes with porous stone or perforated pipe and a plastic standpipe. For piezometers, normally open standpipe piezometers are also used.
• The pipe connected in the borehole, and the porous part is designed for measuring porous values.
• The annulus between the porous filter and the bare hole exists, covered from sand and sealed from both sides.
• The groundwater pressure sends water to the standpipe until the standpipe has the same water level as the piezometric level in the ground, compared to the elevation of the porous filter.

Types of Piezometers

 Casagrande or Open Standpipe Piezometers

 

• The basic tool used for measuring piezometeric level in the ground is the open standpipe piezometer. That is a plastic-made pipe that comes with a diameter of 20 mm and a porous part at the lower side.
• The pipe is connected to the inner side of the borehole, and the porous part is placed lower, where the piezometric level is measured.
• The annulus existing between the porous filter and the borehole comes with sand and is sealed with bentonite on both sides, and the remaining borehole is filled with cement mortar.
• Groundwater pressure sends water into the standpipe till the water level in the standpipe is the same as the piezometric level in the ground at the porous filter elevation.
• In this way, the piezometer water level is monitored and used.

Electric Piezometers

• Electric piezometers come with a deflecting diaphragm and a porous filter parted with a small water reservoir. the vibrating wire used for measuring deflection of the diaphragm. The piezometer-calculated values are transformed into equivalent pressure with the use of an accurate calibration process.
• The next process is the same as opening standpipe piezometers. The electric piezometer is put into the borehole, and the annulus between the porous filter and the borehole is filled with bentonite grout.
• Water from the ground pushes due to buoyant force in the reservoir and deflects the diaphragm until the pressure in the reservoir is similar to the pore water pressure value in the ground at the porous filter elevation.
• If we connect an electric piezometer over the current groundwater table, pore pressure in the soil is negative, and water in the piezometer gets from the reservoir. In this condition the piezometer has air and causes dysfunction.

Hydraulic Piezometers

• It comes with a porous filter that has reservoir water, separated from the pressure gauge through resilient and water-filled tubes.
• These tubes cause water to flow in the system to remove air and fill the reservoir with water.
• A piezometer measures total hydraulic pressure which helps to measure pore pressure.

Pneumatic Piezometers

• This type of piezometer operated with gas pressure; it is used for boreholes, fills, or for larger diameter standpipes. Values are measured through a pneumatic indicator.

Vibrating Wire Piezometers

This type of piezometer is used for measuring pore water pressure in completely or partially saturated soil and rock. This type of piezometer comes with different parts that are

• It has a sensitive stainless steel diaphragm, a magnetic high-tensile-strength stretched wire, one end of which is anchored and the otherfixed with the diaphragm.
• Ceramic low air filter and thermistor used for measuring temperature
• It has a glass-to-metal seal with 4 pins for making a cable connection and a stainless steel casing that has corrosion resistance and impurity resistance found in under-field conditions.

 Piezometer working

• The vibrating wire piezometer comes with a magnetic high-tensile-strength stretched wire where one side is anchored and the other is fixed with a diaphragm that deflects when pressure is applied.
• Piezometer working is based on pressure variations. The changes in pressure deflect the diaphragm in direct relation, and this deflection affects the tension in the stretched wire. Changes in pore pressure have an effect on the tension of the wire.
• Wire plucked with coil magnet: according to the proportion of tension in the wire, it resonates at frequency f that can be measured as

 

 

f = {[σg/ρ] ^1/2}/ 2l Hz

•  σ = tension of the wire
• g = gravitational constant
• ρ = density of the wire
• l = length of wire

Resonant frequency that helps wire to vibrate, induces AC in coil magnet. that define the piezometer equations

Pore pressure has a direct relation with the square of frequency

PIEZOMETRIC MEASUREMENT Techniques

Using a probe

• The measurements from piezometric are taken care of by a process that needs high accuracy and reliable working. The basic method is to use a manual piezometric probe, which is used for spot measurements.
• It is a simple but less effective method that measures the height of water with the use of a differential pressure sensor. It is good for periodic measurements that measure the value of the water table at a point in time.