A piezometer standpipe is a tube with a perforated part, that is placed in the ground in order to determine the groundwater level (i.e. the freatic water table) or to monitor the quality and quantity of the groundwater. The perforated part of the piezometer standpipe is called the filter tube and is in general 1 or 2 meters long. The 'blind' or imperforated part of the piezometer standpipe is called the riser tube.
The diameter of the standpipe is large enough to ignore the capillary rise inside the tube. This implies that the water level inside the piezometer standpipe indicates the position of the freatic water table provided that the groundwater has had sufficient opportunity to enter the tube and to stabilise. The time it takes to stabilise and to consider the data as correct, depends on the purpose of the measurements and soil type.
By placing a perforated tube in the ground it is possible to take either groundwater samples, to determine the groundwater level, to sample and to measure the thickness of floating and sink layers and / or to determine the permeability of the ground (at the height of the filter).
The piezometer standpipes can be placed in the ground in different manners dependant on the purpose of the measurements and tests. The position and the length of the filter tube may vary. In principle two different filter positions are possible, i.e.:
- The filter tube intersecting the groundwater level
- The filter tube below the groundwater level
When the filter tube is placed such, that it intersects the groundwater level, it will be made possible to locate a floating layer containing hydrocarbon particles.
A nylon filter stocking placed around the filter tube combined with coarse sand or gravel deposited around the filter tube prevent that soil particles pour through the slot perforations and cause clogging of these slots. A bentonite plug positioned at the right depth(s) above and possibly below the filter tube guarantees that only groundwater from the targeted soil layer is extracted.
The piezometer standpipes are traditionally placed using drilling techniques. It is much more efficient and economical to place these standpipes using the direct push capabilities of a CPT penetrometer pushing rig. This can be done using 55 mm casing tubes and a so-called lost tip. These casing tubes and tip are pushed to the desired depth. Subsequently the piezometer standpipe, composing of a filter and riser tubes, stocking and bottom plug, is lowered into the string of casing tubes. The string of casing tubes is pulled up the same moment the void between the string of casing tubes and piezometer standpipe is filled out with coarse sand. As soon as the string of casing tubes is fully retracted the hole can be finished of with soil and a protective cylindrical case or street covering hood placed to protect the standpipe.
In general the exact position of the piezometer standpipe is determined using a theodolite and this position is logged in a (triangular) coordinates grid in order to store the measured data in a Geografic Information System (GIS). Multiple piezometer standpipes can be part of such a measuring grid.
Groundwater level measurements can either be done manually using one of the groundwater level meters or automatically by means of submersible piezometer dataloggers (Divers). The latter enable more frequent measurements, e.g. every single hour or several times per day.