9 Groundwater measuring stations and piezometers

9.1 General

9.1.1 Groundwater measuring stations In order to obtain data on the magnitude, variation and distribution of the groundwater heads or pore pressures in the ground, appropriate groundwater measuring stations shall be installed. The type and arrangement of groundwater measurements shall be specified in accordance with EN 1997-2. When drilling for piezometers, flushing additives should be avoided. When flushing additives are used, the effects on the filter and the ground shall be considered and, if necessary, special measures shall be taken.

9.1.2 Piezometers Open or closed systems can be used to conduct groundwater measurements. The choice between open or closed systems should be made depending on the permeability of the ground, the rate of change in pore water pressure and the required precision and duration of the measurements.

NOTE 1 In open systems, a piezometer pipe is used to measure the groundwater head at the installation point of the filter in the ground. For hydrostatic pressure distribution, the groundwater head in unconfined aquifers corresponds to free groundwater surface and in confined aquifers to groundwater pressure. In closed systems, the pore water pressure in the ground is measured directly by a pressure transducer. The transducer is therefore an integral part of the measuring system.

NOTE 2 In confined aquifers, the measurements of the water level in the piezometer pipe can be considerably attenuated or subject to a time lag compared with the variations in groundwater pressure, depending on the permeability of the aquifer. When using open systems in confined aquifers, the measurements of the water level in the piezometer pipe may be subject to attenuation and time lags compared with variations in pore pressure. The groundwater flow required for filling and emptying the piezometer pipe depends on the ground permeabilityand the pipe’s cross section surface. The water level measured in the piezometer pipe of an open system corresponds to the mean head of the groundwater potential in the filter zone. In homogeneous aquifers with an approximately horizontal groundwater flow, the filter zone can extend over the entire depth of the aquifer as, in this case, the head of the groundwater potential is virtuallythe same along the filter zone. Very different groundwater potentials can occur over the depth of stratified aquifers and in the proximity of groundwater flows with pronounced vertical flow sections. In this case, filtering should only be carried out over a relatively short vertical section of the aquifer for which the head of the groundwater potential is to be determined. In both systems, a filter should beinstalled in theground at the location at which the groundwater head or the pore pressure shall be measured. The filter shall prevent ingress of soil particles into the measuring system. All components and equipment intended for installation in the ground shall be sufficiently resistant to mechanical loading and chemical attack by constituents in the groundwater. Any reactions between the materials used and the ground, in particular the formation of galvanic effects, shall be prevented.

NOTE Galvanic effects may cause modified pore pressure. This effect emanates from gases generated by electric currents from galvanic cell created by using different metals or alloys in the piezometer tip. Groundwater measuring stations shall be positioned and secured in such a way that third parties are not at risk. Appropriate measures shall be taken to avoid any risk to the groundwater measuring station due to contamination, flooding, traffic or frost. Measures to protect the installation during the observation period shall be carried out as requested in, e.g. national regulations, see Annex E.

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