Piezometers are used for measuring pore pressures in ground.
Pore pressure has fundamental importance to understanding ground behaviour, before—during—and after construction.
The simplest instrument for measuring pore water pressures in ground is an open standpipe. This is a small (usually 19mm) diameter plastic pipe with a porous section at the bottom. The pipe is installed inside a borehole and the porous section is positioned at the depth where the pore water pressure is to be measured. The annulus between the porous filter and the borehole is filled with sand, the top and bottom surfaces of the sand are sealed with bentonite and the rest of the borehole is filled with a cement/bentonite grout.
The pressure of the ground water pushes water into and up the standpipe until the level of water inside the standpipe (h) is equivalent to the pore water pressure in the ground at the elevation of the porous filter.
Electric piezometers consist of a deflecting diaphragm and a porous filter separated by a small reservoir of water. Deflections of the diaphragm are detected using a vibrating wire or a strain gauge and are converted to an equivalent pressure using a suitable calibration.
The piezometer is inserted into a borehole and the annulus between the porous filter and the borehole is filled with either sand or cement/bentonite grout.
Water from the ground forces its way into the reservoir and causes the diaphragm to deflect until the pressure inside the reservoir is the same as the pore water pressure in the ground at the elevation of the porous filter.
If a piezometer is installed above the prevailing ground water table, the pore pressure in the soil could be negative and the water in the piezometer will tend to be drawn out of the reservoir. If this happens air can eventually form inside the piezometer and it will not function reliably.
Can a vibrating wire piezometer measure negative pore water pressure? It’s a simple question… or is it?
Hydraulic piezometers consist of a porous filter enclosing a reservoir of water, which is separated from a pressure gauge by flexible, water filled tubes. The tubes are used to circulate water through the system, removing air and ensuring that the reservoir remains full of water.
Geotechnical Observations’ flushable piezometer is a hydraulic piezometer that can measure positive and negative pore water pressures.
Flushable piezometers are installed in fully grouted boreholes and the grout acts as a secondary filter, helping to maintain saturation of the piezometer under the influence of suction. If air forms in the piezometer it can be removed by circulating water through the hydraulic tubes.