What are the Piezometer Uses, Types and Overview?
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What is a Piezometer?
Large civil engineering projects or infrastructures like high rise buildings, tunnels, dams, etc. require measurement of pore water pressure as an integral part of geotechnical monitoring services. The pressure experienced by water contained in pores of earth materials, concrete structures or rock is generally called pore water pressure/Piezometric Level.
The pore water pressure is measured by piezometer sensor which is also popularly known as ‘pore pressure meter’. Different types of piezometers are available for geotechnical monitoring services, to suit specific application and project requirements.
The simplest type of piezometer is casagrande piezometer consisting of a casagrande tip connected to lengths of extension pipes. Using a dip meter, the water level inside the pipes is measured.
The electric piezometer, generally vibrating wire type, is most commonly used, as it provides hassle-free, accurate and real time data.
What Data Does Piezometer Give?
Piezometer provides significant quantitative data on the magnitude and distribution of pore pressure and its variations with time that helps to understand ground behavior before-during-after construction. The piezometer data is studied for following main purposes:
- Effect of water in pores of soil or rock 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.
- Determine level and flow pattern of ground water
- Determine flow pattern of water in earth/rock fill & concrete dams and their foundations and to delineate the phreatic line.
- Provides basic data for design improvement that will promote safer and more economical design and construction.
- Proper evaluation of pore pressure also helps in monitoring the soil behaviour after construction and indicates potentially dangerous conditions that may adversely affect the stability of the structure, its foundation and appurtenant.
Sample data from vibrating wire piezometer
Why Vibrating Wire Piezometer is Used?
The vibrating wire piezometers are most common as they use vibrating wire technology that has following advantages:
- It’s frequency output is immune to external noise
- It is able to tolerate wet wiring common in geotechnical applications
- It is capable of transmission of signals to long distances
Vibrating Wire Piezometer Overview
The vibrating wire piezometer incorporates the latest vibrating wire technology to provide remote digital readout of fluid and/or water pressure in standpipes, boreholes, embankments, fully and partially saturated natural soils, rolled earth fills and the interface of retaining structures. The sensor mainly consists of:
- Filter (Ceramic filters for low air entry and brass filters for high air entry filters)
- A sensitive stainless steel diaphragm
- A magnetic, high tensile strength stretched wire, one end of which is anchored and the other end is fixed to a diaphragm (vibrating wire).
- Magnetic coil assembly
- Thermistor for temperature readings
- Glass to metal seal with four terminals for cable connection
- Cable joint housing with suitable cable gland for connecting and securing cable
- Stainless steel body with resistance to rusting or corrosion against several kinds of dissolved impurities found in water under field conditions. For saline water application, a special sensor with additional protection is provided
Component details of vibrating wire Piezometer
How Does a Vibrating Wire Piezometer Work?
In geotechnical monitoring services, piezometers are installed either in borehole or in earth-fills. The surrounding water applies pressure on the diaphragm. Any change in pressure deflects the diaphragm proportionally.
This deflection cause change in tension and resonant frequency of the stretched wire (vibrating wire), one end of which is anchored to the piezometer body and the other end is fixed to the diaphragm. Thus any change in pore pressure, directly affects the tension in the vibrating wire.
The vibrating wire is plucked by the 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 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. Any vibrating wire read unit or automatic data logger will convert and provide the data directly in engineering units which can be kPa or MPa.
Piezometer overview
Vibrating Wire Piezometer Application Areas
- 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.
- Hydrological investigation, ground water elevation study and water supply operations.
- Pore pressure studies in relation to waste and environmental applications
- Monitoring of pore pressure for soil improvement & stability and for slope stability
