9.2 Piezometers

9.2.1 Open systems

9.2.1.1 Open systems can be divided in two groups as follows (see Figure 6):

• a) open standpipe;
• b) open pipe with inner hose.

a) Open standpipe		b) Open pipe with inner hose

Key

• 1 seal
• 2 filter
• 3 tube
• 4 filter pack
• 5 indicating instrument

9.2.1.2 The piezometer in open systems shall consist of a filter and a piezometer pipe which extends up to or above the ground surface and permits equilibration with atmospheric pressure.

NOTE 1 In stable soils and rocks, groundwater observations may be made in open holes.

NOTE 2 The groundwater is able to oscillate freely in the piezometer pipe. Groundwater measurements in piezometer pipes can be conducted either by determining the water level, or by measuring the water pressure in the piezometer pipe, at a specified depth below the water surface. In open systems, the pressure is determined in relation to the actual atmospheric pressure at the ground surface.

9.2.1.3 Measurements shall be recorded either manually (e.g. by an electric contact gauge) or automatically (e.g. by a pressure transducer).

9.2.1.4 Depending on the design, open systems should be used for measuring the groundwater heads in medium to high permeable soils or rock. In general, they should not be used for determining groundwater heads in soils and rock with very low permeability or for measuring rapid changes in pore pressure in low permeable soils and rock.

9.2.2 Closed systems

9.2.2.1 General

9.2.2.1.1 The piezometer inclosed systems shall consist of a robustcasing which is installed in the ground with a filter at the lower end (filter tip) and a water-filled chamber, behind which the water pressure is transmitted to the measuring device. Filters with sufficiently high air entry values shall be used. The pressure measurements can be performed using measuring systems as illustrated by Figure 7:

• hydraulic measuring systems;
• pneumatic measuring systems;
• electrical measuring systems.

a) Hydraulic system		b) Pneumatic system
c) Electrical system

Key

• 1 pressure transducer
• 2 flow regulator
• 3 pressure supply tube
• 4 return tube to atmosphere
• 5 valve for flushing
• 6 membrane
• 7 measuring instrument
• 8 electrical transducer
• 9 filter tip
• 10 filter

9.2.2.1.2 The pore size and air entry value of the filter shall be selected on the basis of the in situ soil and the expected pore pressure so as to prevent ingress of air bubbles.

9.2.2.1.3 Pore pressures shall be expressed as the pressure in relation to the atmospheric pressure at the ground surface. When using an absolute pressure transducer, both the absolute pore pressure at the location of installation in the ground and the actual atmospheric pressure at the ground surface should be determined simultaneously.

9.2.2.1.4 Closed systems can be used to measurepore pressures and thus to determine the distribution of the groundwater potential in all types of soil. In particular, closed piezometers are required when determining pore pressures in soils and rock with very low permeability, measuring rapid changes in pore pressures and in artesian conditions. Electrical measuring systems should be used when performing measurements of rapid changes in pore pressure and for continuous data recording. Closed systems may only be used for long-term applications (i.e. over several years) when an adequate level of redundancy is available or if the system can be checked and repeatedly calibrated.

9.2.2.1.5 The required precision of the measurements for a certain project shall be decided in advance so that a proper equipment for the project can be chosen.

Taking into account all possible sources of error and the compensation for the atmospheric pressure, the precision of the measurements should normally not be less than 1 kPa in the range of 1 kPa to 100 kPa, and 2 kPa for values greater than 100 kPa.

9.2.2.2 Hydraulic systems

The pore pressure shall be transmitted by a fluid-filled pressure tube to a pressure transducer on the ground surface. The system shall allow the removal of entrapped gas bubbles. It shall be protected against frost.

Hydraulic systems shall not be used if the geodesic difference in levels between the pressure transducer at the ground surface and the groundwater surface or groundwater pressure surface exceeds about 7 m for water-filled systems and about 9 m for oil-filled systems, in order to avoid cavitation in the pressure tube.

9.2.2.3 Pneumatic systems

Pneumatic systems shall have a membrane placed behind the filter and two tubes (one supply tube and one return tube) connecting the back of the membrane with the measuring and control instruments on the ground surface. A flow meter and a flow controller shall ensure a constant flow of compressed air in the supply tube for all measurements. Dry gas shall be used to prevent condensation in the tubes. The readings may not be taken until the measured values remain constant with respect to the required purpose.

NOTE The membrane closes the connection between the supply tube and the return tube before the supply tube is pressurised. In order to perform the measurement, the air pressure in the supply tube is increased until the air pressure at the back of the membrane is equal to the pore pressure acting on front of themembrane and the membrane lifts, resulting in a connection to the return tube. The slight excess pressure required to open the membrane therefore also remains constant. The pressure is measured by a pressure transducer in the supply tube after the specified constant air flow has been set. Because of the open return tube, pneumatic systems always measure the pressure in relation to the actual atmospheric pressure at the ground surface. In order to prevent the membrane being overloaded, the air flow is increased gradually so that the membrane is lifted only slightly. As the membrane is closed before the supply tube is pressurised, loading is virtually independent of the pore pressure. Pneumatic systems are therefore largely unsusceptible to drift. It is not possible to check either the membrane or the filter directly during the operating period.

9.2.2.4 Electrical systems

Electrical systems measure absolute or relative pore pressures using an electricaltransducer behind the filter. When measuring absolute pore pressure, the atmospheric pressure at the ground surface shall be measured simultaneously.

NOTE If the pressure transducer on the side not exposed to the pore pressure is fitted with a means of equalizing the pressure with the atmospheric pressure (e.g. a venting tube), the pressure in relation to the atmospheric pressure at the ground surface is measured. Electrical data recording systems are comparatively robust as the filter, membrane and electrical sensor are placed in a common sturdy housing andthe data is transmitted to the ground surface through electric cables that are relatively unsusceptible to disturbance. Data can be transmitted and recorded either by a readout device or continuously by a logger. However, electrical systems are very sensitive to hydraulic overloading as the pore pressure acts directly on the membrane. The constant movement and tension also subject the membrane to a high level of loading, which affects its long-term performance. It is not possible to check the filter, the membrane or the electrical pressure transducer directly during the operating period unless a pick-up pressure transducer system is used.