3.6 Groundwater measurements in soils and rocks
(1)P Groundwater measurements shall conform to 2.1.4.
(2)P The determination of the groundwater table or pore water pressures in soils and rocks shall be made by installing open or closed groundwater measuring systems into the ground.
NOTE Subclause 3.6 applies to the measurement of positive pore water pressures relative to the atmospheric pressure. Measurements of negative pore water pressures are not considered.
3.6.2 Planning and execution of the measurements
(1)P When relevant, groundwater measurements and sampling shall be conducted in accordance with EN ISO 22475-1.
(2)P The type of equipment to be used for groundwater measurements shall be selected according to the type and permeability of ground, the purpose of the measurements, the required observation time, the expected groundwater fluctuations and the response time of the equipment and ground.
(3) There are two main methods for measuring the groundwater pressure: open systems and closed systems. In open systems the piezometric groundwater head is measured by an observation well, usually provided with an open pipe. In closed systems the groundwater pressure at the selected point is directly measured by a pressure transducer.
(4) Open systems are best suited for soils and rock with a relatively high permeability (aquifers and aquitards), e.g. sand, gravel or highly fissured rock. With soils and rocks of low permeability they may lead to erroneous interpretations, due to the time lag for filling and emptying the pressure pipe. The use of filter tips connected to a small diameter hose in open systems, decreases the time lag.
(5) Closed systems can be used in all types of soil or rock. They should be used in very low permeability soils and rocks (aquicludes), e.g. clay or low fissured rock. Closed systems are also recommended when dealing with high artesian water pressure.
(6)P When very short- term variations or fast pore water fluctuations are to be monitored, continuous recording shall be used by means of transducers and data loggers, with any types of soils and rocks.
(7)P In cases where open water is situated within or close to the investigation area, the water level shall be considered in the interpretation of the groundwater measurements. The water level in wells, the occurrence of springs and artesian water shall also be noted.
(8)P The number, location and depth of the measuring stations shall be chosen considering the purpose of the measurements, the topography, the stratigraphy and the soil conditions, especially the permeability of the ground or identified aquifers.
(9)P For monitoring projects e.g. groundwater lowering, excavations, fillings and tunnels, the location shall be chosen with respect to the expected changes to be monitored.
(10) For reference purposes, measurement of the natural fluctuations in groundwater should be made, if possible, outside the area affected by the actual project.
(11)P In order to obtain measurements reflecting the pore pressure at the intended point in a soil or lock layer, provisions shall be made, according to EN ISO 22475-1, to ensure that the measuring point is adequately sealed off with regard to other layers or aquifers.
(12)P The number and frequency of readings and the length of the measuring period for a given project shall be planned considering the purpose of the measurements and the stabilisation period.
(13) The criteria adopted should be adjusted after an initial period of time, according to the actual variations of the readings being observed.
(14)P If it is intended to assess groundwater fluctuations, measurements shall be taken at intervals smaller than the natural fluctuations to be characterised and over a long period of time,
(15) During the drilling process, the observation of the water level at the end of the day and the start of the following day (before the drilling is resumed) is a good indication of the groundwater conditions and should be recorded. Any sudden inflow or loss of water during drilling should also be recorded, since it can provide additional useful information.
(16) During the first phases of site investigations, some of the boreholes may be equipped with open perforated pipes protected with filters. The water level readings obtained during the following days yield a preliminary indication of groundwater conditions, but are subject to the limitations mentioned in 3.6.2 (4). The dangers associated with the connection of different aquifers should be taken into account, as well as any relevant environmental regulations.
3.6.3 Evaluation of results of groundwater measurements
(1)P The evaluation of groundwater measurements shall take into account the geological and geotechnical conditions of the site, the accuracy of individual measurements, the fluctuations of pore water pressures with time, the duration of the observation period, the season of measurements and the climatic conditions during and prior to that period.
(2)P The evaluated results of groundwater measurements shall comprise the observed maximum and minimum elevations of the water table, or pore pressures and the corresponding measuring period.
(3)P If applicable, upper and lower bounds for both extreme and normal circumstances shall be derived from the measured values, by adding or subtracting the expected fluctuations or a reduced part of them, to the respective extreme or normal circumstances. The frequent lack of reliable data for extended periods of time of this type of measurements will necessitate the derived values being a cautious estimate based on the limited available information.
(4) The need for making further measurements or installing additional measuring stations should be assessed during the field investigations and in the ground investigation report.
NOTE In Annex C, an example is presented of a statistical method to evaluate the ground water conditions, when long term measurements in a reference pipe in the region arc available and a short measuring period on the actual site has been carried out.