What is a Piezometer?
The Piezometer, also known as pore pressure meter, is used to measure pore water pressure in the soil, earth/rock fills, foundations and concrete structures. The definition of Piezometer does not end here. They are also used to provide significant quantitative data on the magnitude and distribution of pore pressure and its variations with time.
The geotechnical sensor helps in evaluating the pattern of seepage, zones of potential piping and the effectiveness of seepage control measures undertaken. The Piezometer is also used to measure water level in reservoirs, wells and boreholes.
What does a Piezometer measure?
The pressure experienced by water contained in pores of earth materials, concrete structures or rock is generally called pore water pressure. In any instrumentation scheme for geotechnical or geo-structural study associated with large civil engineering structures like tall buildings, dams, underground tunnels etc., measurement of pore water pressure (also known as piezometric level) plays an important part. Piezometers measure the pore pressure that helps in monitoring the after construction behaviour. It indicates potentially dangerous conditions that may adversely affect the stability of the structure, its foundation and appurtenant. It also provides basic data for design improvement that will promote safer and more economical design and construction.
The use of a piezometer is essential as the study of pore pressure has the following main purposes:
- The effect of water in pores of soil or rock is to reduce the load-bearing capacity of soil or rock. The effect is more pronounced with higher pore water pressure which might lead to a total failure of the load-bearing capacity of the soil.
- To determine the level and flow pattern of groundwater
- Determine flow pattern of water in earth/rockfill & concrete dams and their foundations and delineate the phreatic line.
The Application of Piezometers
A piezometer is used for the following purposes:
- Construction control, stability investigation and monitoring of earth dams, embankments, foundations, shallow underground works and surface excavations.
- Uplift and pore pressure gradients in foundations, embankments, abutments, and fills.
- The hydrological investigation, groundwater pressure study and water supply operations.
- Pore pressure studies concerning waste and environmental applications.
- Monitoring of pore pressure for soil improvement & stability and slope stability
- Reclamation of land in high population density areas in coastal regions for building highways, multi-storied structures, port trusts or recreational facilities.
Why are vibrating wire piezometers the best?
The Vibrating Wire Piezometers measure the pore water pressure in fully or partially saturated soil and rock. Encardio Rite's vibrating wire piezometer frequency output is immune to external noise. It can tolerate wet wiring which is common in geotechnical applications and is capable of transmission of signals to long distances. It also has applications in the measurement of positive or negative pore pressure in the soil, concrete mass or rock.
The piezometer construction involves a sensitive stainless steel diaphragm, magnetic high-tensile strength stretched wire, ceramic low air filter, thermistor, glass to metal seal, and a stainless steel body with resistance to rusting or corrosion.
Vibrating wire Piezometer - Operating Principle
Here's how the vibrating wire piezometer works. It 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 proportion to applied pressure. Any change in the pressure deflects the diaphragm proportionally which 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 = [σ g/ ρ]1/2/ 2l Hz
where σ = tension of the wire
g = gravitational constant
ρ = density of the 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-54V readout logger can display this directly in engineering units.
Encardio Rite’s Range of vibrating wire piezometer
Vibrating wire piezometers are manufactured in various capacities. The vibrating wire and coil magnet assembly is enclosed in a stainless steel body and is an electron beam welded to the diaphragm. This results in a vacuum of around 1/1000 Torr inside the sensor, making it immune to the effect of any ingress of water and other corrosive materials that may be present in the water. As a piezometer is of stainless steel construction, it is not affected by normal chemical corrosion at locations in which it is used.
A thermistor is provided integrally in each piezometer to monitor temperature and if necessary, to make the temperature correction in the zero reading. A tripolar plasma surge arrester inside the transducer housing protects the vibrating wire pluck and read coils from electrical transients such as may be induced by direct or indirect lightning strikes.
A low air entry value ceramic flat filter with a thickness of 3 mm and a grain size of 40-60 microns is normally provided. The water oozes through internal pores or seams in rock formations of dam foundations, mass concrete of structures, foundation soil of structures, reclaimed land soil etc. percolates through the filter to pressurise the diaphragm. Depending upon the application, filters with different porosity and air entry values are available.
- Encardio Rite Model EPP-30V is an electron beam welded robust vibrating wire piezometer suitable for monitoring pore pressure in heavy structures like earth/rockfill dams and mass concrete during construction, and for long term monitoring after construction is over.
- Encardio Rite Model EPP-40V Slim Size Piezometer (19 mm diameter) is specially designed to monitor pore pressure in small diameter boreholes during construction activity. It also has a glass to metal solder pin connector for easy cable connection.
- Encardio Rite Model EPP-60V Low-Pressure Piezometer is made to monitor low pore pressures. It is extensively used for settlement measurements and water level/water pressure measurement in boreholes.
- Encardio Rite Model EPP-50V is a push-in type piezometer that measures pore water pressure in soft soil/clays and landfills. It has a pointed cone at one end and drills rod threads at the other end.
- Encardio Rite Model EPP-20V uplift pressure sensor incorporates the latest vibrating wire technology to provide a remote digital readout of fluid and/or water pressure in standpipes, boreholes and embankments.
- Encardio Rite Model EPP-10 is a porous tube piezometer consisting of a porous tip covered with geotextile cloth, PVC standpipe, suitable adaptors and an end cap.
- Encardio Rite Model EPP-10SP open standpipe is used for measuring groundwater level and its variation with time. It consists of a pipe that is sealed along its entire length and is installed in a borehole.
- Encardio Rite Model EPP-10/6 water level indicator is used to measure the depth of groundwater in standpipes, boreholes, and wells.
- Encardio Rite Model EPU-20G uplift pressure measuring device consists of a perforated/non-perforated pipe of 50 mm dia of adequate strength.
Features of Encardio Rite Piezometer
- Reliable, accurate, low cost, and simple to read.
- Protected against lightning spikes.
- Easy installation in standpipes, pressure vessels, and ideal for underground work.
- Hermetically sealed under a vacuum of 0.001 Torr; stainless steel construction.
- Thermistor provided for additional temperature measurement.
- Can measure negative pressure. Not limited to the depth of water being within 5 m from the observation station as is in the case of twin-tube piezometers.
- Very small time lag.
- Transmission of the signal as a frequency over long cable lengths.