The piezometer, also known as pore pressure meter, is used to measure pore water pressure in soil, earth/rock fills, foundations and concrete structures. It provides significant quantitative data on the magnitude and distribution of pore pressure and its variations with time.
The sensor helps in evaluating the pattern of seepage, zones of potential piping and the effectiveness of seepage control measures undertaken. The sensor also is used to measure water level in reservoirs, wells and boreholes.
What a Piezometer measures
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. Proper evaluation of pore pressure helps in monitoring the behavior after construction and 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 study of pore pressure has following main purposes:
Encardio-rite vibrating wire piezometer is the electrical piezometer of choice as its frequency output is immune to external noise, it is able to tolerate wet wiring common in geotechnical applications and it is capable of transmission of signals to long distances. It has applications in the measurement of positive or negative pore pressure in soil, concrete mass or rock including:
Vibrating wire piezometers are manufactured in various capacities. The vibrating wire and coil magnet assembly is enclosed in a stainless steel body which is electron beam welded to the diaphragm. This results in a vacuum of around 1/1000 Torr inside the sensor resulting in it becoming immune to effect of any ingress of water and other corrosive materials that may be present in the water. As 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 tri-polar plasma surge arrestor 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 oozing 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.
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 = [σ g/ 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 and the Encardio-rite model EDI-54V readout logger is able to display this directly in engineering units.