9.3.1.3 Closed systems

9.3.1.3.1 Prior to and during the installation, the filters shall be saturated and thesystem shall be calibrated (see 10.1.3).

NOTE The preferred method for saturation is boiling or vacuum boiling.

9.3.1.3.2 Any contamination (e.g. by oily or greasy substances such as by touching) and insufficient saturation during storage and transportation, which effects the permeability of the filter shall be avoided.

Depending on the ground conditions and installation depths, there are the following methods to install closed systems:

  • by pushing or driving down to the installation depth;
  • by pushing in after pre-drilling;
  • by placing in a borehole.

When selecting one of the above methods, consideration shall be given to ensure that there will be a good contact between the pore pressure and the transducer.

9.3.1.3.3 Piezometers with press-in tips fitted below the filter shall be used when installing a piezometer by pushing.Extension pipes are used to push the piezometer into the ground. The diameter of the pipes over the lowest metre shall be equal to or larger than that of the press-in tip to prevent leakage to the filter.

9.3.1.3.4 If it is intended to reuse the pushed piezometer (e.g. after short-term measuring operations), the pipes are left in the ground until the entire piezometer is subsequently retracted. The extension pipes should otherwise be retracted after the piezometer has been installed to avoid the ground above it being affected, particularly in the case of ground susceptible to settlement (e.g. embankments). A section of pipe with a length of at least around 1 m should be left in the ground above the press-in tip and filter, to act as a seal. When retracting the extension pipes, the resultant cavity shall be sealed by filling it up to the ground surface with a suitable slurry with a permeability lower than that of the original soil.

9.3.1.3.5 In low-permeable soils, pushing can generate considerable local excess pore pressure. There is thus a risk that the transducers used in electrical systems, in which the pore pressure acts directly on the membrane, can be damaged (e.g. zero shift due to irreversible stretching of the membrane). Overloading should be checked by measurements or other means. Excess pore pressure during pushing can be reduced by lowering the rate of pushing. Overloading of the membrane can also be avoided by using a pick-up transducer during insertion in order to control pressure.

9.3.1.3.6 Piezometers installed in holes drilled down to below the measuring level shall be installed in a saturated filter pack. A seal shall be placed above the filter pack. In artesian groundwater conditions, drilling with sufficiently high water pressure in the casing is necessary. The piezometer and seal can then be installed in the casing after displacement of the groundwater. Where the groundwater inflow is low owing to a low permeability of the layer with confined groundwater conditions, the piezometers should preferably be installed by pushing. (Figure 8).

Dimensions in millimetres
Closed system with filter pack and sealing in a borehole

Key

  • 1 measuring instrument
  • 2 protection cap
  • 3 bentonite plug
  • 4 piezometer
  • 5 electric cable
  • 6 sand filter
Figure 8 – Closed system with filter pack and sealing in a borehole

9.3.1.3.7 Incorrect measurements obtained by closed groundwater measuring systems with pneumatic transducers can be caused by droplets of condensation in the tubing system. The droplets can be removed by flushing the tubing with dry gas. Furthermore, there is a risk of overloading the measuring membrane, on which the air exerts a back-pressure, if the pressure in the supply tube is increased too rapidly. In low- permeable soils in particular, excess pore pressure corresponding to the air pressure is initially generated at the front of the membrane. The membrane can be over-stretched when the excess pore pressure decreases, resulting in distortion of the values measured.

9.3.2 Checking installation

9.3.2.1 General

Function controls shall be performed during installation if possible and immediately after installation to ensure the proper function of the groundwater measuring system. All groundwater measuring stations shall be marked indelibly. A record of installation shall be prepared for each piezometer (see 12.1.7).

9.3.2.2 Open systems

The filling shall be checked by control soundings. The function of open piezometers shall be tested prior to commissioning. After reading the stable water level, the water level in open piezometer pipe shall be raised or lowered and the fall/rise rate shall be measured and recorded. Flushing until water runs clear at the measuring station should also be performed.

9.3.2.3 Closed systems

9.3.2.3.1 In case of closed groundwater measuring equipment, no direct function control by means of the read-out device shall be made after reading.

9.3.2.3.2 In pneumatic systems, the air pressure shall be applied to the supply tube after installation and the stabilisation process monitored until a constant pressure is obtained at the specified flow. The process should be repeated after releasing the air pressure in the supply tube for control purposes. The pressure should be plotted against time to check if the piezometer is functioning.

9.3.2.3.3 The hydraulic system and belonging read-out device shall be insulated against frost and thermal variations.

9.3.2.3.4 The hydraulic systems shall be flushed for gas bubbles before being connected.

9.3.2.3.5 All pipes, tubes and cables in closed systems shall be protected from mechanical damages between the measuring point and the read-out device, e.g. in excavated and refilled trenches.

9.3.2.3.6 After connecting the read-out device, the response shall be observed and recorded.

9.4 Maintenance

9.4.1 To ensure the correct function of piezometers, maintenance controls shall be performed regularly during lifetime depending on the purpose and when clogging of the filter is suspected.

9.4.2 Functioning checks for open systems shall be performed accordingto 9.3.2.2 and for closed systems according to 9.3.2.3. The results shall be compared to the earlier checks.

9.4.3 If the results of the functioning checks of the open piezometer differ considerably from the earlier ones, the following measures shall be taken:

a) total inside length of the piezometer shall be measured in order to determine the amount of sludge which shall be removed, if possible;

b) the revitalisation of the piezometer can usually be done by flushing the piezometer pipe with clean water or air, using for instance a rigid hose pushed to the bottom of the pipe; flushing shall be continued until the up-coming water is clear;

c) after revitalisation of the piezometer, an additional functioning check shall be carried out and the results shall be compared with earlier checks.

9.4.4 If revitalisation fails, a new installation shall be considered.

9.5 Decommissioning

The piezometers shall be de-installed when required and the borehole shall be back-filled according to 5.5.

ISO 22475-1 Sampling by drilling and excavation and groundwater measurements. Part 1: Technical principles of execution