52.5 Standpipe piezometers
COMMENTARY ON 52.5
A standpipe piezometer is a device consisting either of a tube or pipe with a porous element on the end, or with a perforated end section surrounded by or wrapped with a filter, which is sealed into the ground at the appropriate level. It is normally installed in a borehole. The basis for distinguishing between a standpipe piezometer and an observation well is how the response zone is sealed into the ground (see Clayton et al, 1995  and Figure 18).
The tube of a standpipe piezometer should be of at least 12 mm internal diameter to allow air bubbles to rise freely and the top of the tube should be open to atmosphere to allow the water level inside the tube to come to equilibrium with the pore water pressure in the ground. Access to the top of the standpipe should be given to allow the water level to be measured. This should normally be done using an electric dip-meter, which gives an audible "beep" or light or both when it makes contact with the water or it may be done by placing a pressure sensor at the bottom of the standpipe and relating the measured pressure to the water level.
|1 Ventilated plastic cap||7 Gravel or sand backfill|
|2 Protective metal cover||8 Sand filter|
|3 Concrete plug||9 Perforated plastic pipe at base|
|4 19 mm — 50 mm ID plastic pipe||10 Low air entry porous ceramic or plastic filter|
|5 12 mm — 19 mm ID plastic pipe||11 Compacted and hydrated bentonite seals|
|6 Grout seal||12 Compacted backfill|
NOTE Adapted from Clayton et al., 1995 .
When using a pressure sensor it should be taken into account that the sensor displaces water during placement and, therefore, the initial water level is out of balance with the pore water pressure. If the sensing part of the pressure sensor is sealed from contact with the surrounding atmosphere, the measurements should be adjusted for changes in the barometric pressure. Problems might be encountered when trying to take measurements in very cold weather due to freezing of the water inside the standpipe at elevations close to the surface. After a standpipe piezometer has been installed, its functionality should be checked by either adding or removing water to the standpipe and observing the subsequent response of the device to the out of balance head. Acceptance of the functionality should be determined on the basis of the observed response when compared to the expected performance of the device in the context of the permeability of the surrounding ground.
NOTE 1 A frequently used standpipe piezometer is the Casagrande-type shown in Figure 18.
NOTE 2 The main advantages of a standpipe are that it is simple and it can be used to determine the permeability of the ground in which the tip is embedded (see Section 7). The disadvantage, however, is the length of time taken to reach equilibrium or to respond to changes in pore water pressure in soils of relatively low permeability (see Figure 17) can be long.
52.6 Hydraulic piezometers
COMMENTARY ON 52.6
Hydraulic piezometers normally consist of a small piezometer tip (a water filled chamber with porous, normally ceramic walls), small-bore water filled plastic tubes and a remote pressure measuring device such as a pressure transducer and electrical readout, or more simply a Bourdon gauge. Hydraulic piezometers are frequently installed directly in trenches, but can also be installed in boreholes.
The most common type of hydraulic piezometer is the twin-tube piezometer (see Bishop, Kennard and Vaughan, 1964 ) shown in Figure 19. In this, the piezometer tip is connected to the measuring device by two tubes, so that water can be circulated to flush out any air.
Hydraulic piezometers usually have quick response times, although the time to respond is affected by the length of the hydraulic tubes (see Figure 17). They can be used for in-situ measurements of permeability.
Twin-tube hydraulic piezometers can be used for measuring a limited range of negative pore water pressures (i.e. suctions). An adaptation of the traditional twin tube hydraulic piezometer is the flushable piezometer (see Ridley et al., 2003 ) shown in Figure 20. This piezometer incorporates a hydraulically operated shuttle valve that is used to isolate the sensor, which is located immediately behind the filter, from the flushing tubes, thereby enabling the piezometer to measure the maximum pore water tension, irrespective of the depth of installation. The hydraulic valve is screwed into the bottom of a plastic tube with a ceramic filter at the bottom of it. If air forms in the piezometer it can be removed by opening the valve and circulating de-aired water around the system. This piezometer also has the advantage that the calibration of the sensor can be checked in situ by comparing the pressure measured by the sensor with the known head of water when the valve is open and all of the air has been removed from the system. If the calibration has drifted the valve and the pressure sensor can be removed and replaced.
Hydraulic piezometers have also been adapted to the measurement of groundwater levels at multiple points within the same borehole (see Black et al., 1986 ) To do this a central access pipe is installed and the levels where pore water pressures are to be measured are isolated using packers placed either side of the measuring point, between the borehole and the access pipe. A mechanical valve located at the level of the measuring point allows fluid to be circulated and air to be purged. A measurement probe attached to a wireline is subsequently lowered to each measurement point in turn and sealed against a measurement port using a mechanical shoe.
If the pressure in the piezometer system drops below atmospheric pressure, air can form in the system causing erroneous readings; therefore the chamber and the tubes should be kept full of de-aired water. Circulation of water through the tubes and the chamber should be done slowly so that the pressure at the measurement point is left as close as possible to the working pressure. The tubing should also be ductile and in service should be able to accommodate any strains (e.g. settlement) that might be generated in the surrounding soil. Frequent (e.g. every 5 m) coils should be included in the tubing when laying it in trenches.
|1 Rubber seals||4 Compression fittings|
|2 Porous ceramic filter||5 Flush de-aired water (in)|
|3 Flush air and water (out)||6 Water-filled chamber|
In strata with high permeability, care should be taken when using hydraulic piezometers to measure the permeability to ensure that the limiting permeability of the porous tip is higher than that of the surrounding ground.
When trying to measure negative pore water pressures (suctions), any hydraulic seals should be able to withstand the suctions without leaking. The level of the sensor relative to the filter should be taken into account when installing hydraulic piezometers in boreholes.
NOTE For every metre that the sensor is positioned above the filter the maximum recordable tension reduces by 10 kPa. Since the maximum pore water tension that can be measured with this type of piezometer is about 80 kPa, a few metres difference in elevation severely restricts their operational range.
In circumstances where the pore water pressure could vary from positive to negative values (e.g. vegetated clay slopes and embankments), a flushable type of piezometer should be used (see Figure 20).
|1 Pump for circulating water and removing air||7 50 mm diameter UPVC pipe|
|8 Hydraulically operated valve|
|2 Flush de-aired water (in)||9 O-ring seal|
|3 Flush air and water (out)||10 Water chamber|
|4 Pressure sensor||11 Porous ceramic (1 bar air entry)|
|5 70 mm diameter fully grouted borehole|
|6 50 mm diameter UPVC pipe|