52 Groundwater measurements
52.2 Observation wells installed in boreholes and excavations
COMMENTARY ON 52.2
Determination of the unconfined groundwater level can be made by observation in an open borehole or excavation. Water level observations in an open borehole or excavation are frequently not indicative of equilibrium conditions as discussed in 26.2 and a considerable period of time might be required for the water level to reach an equilibrium unless the ground is reasonably permeable.
Observations of water level in an open borehole or excavation should be made at regular intervals until it is established that the water level has reached an equilibrium.
Where the time to reach an equilibrium is likely to be more than one day or there is a likelihood that the borehole or excavation could collapse, an observation well (sometimes called a simple standpipe) should be installed into the borehole or excavation. Observation wells usually comprise a perforated tube, often 50 mm diameter, but sometimes of the same construction as a standpipe piezometer (see 52.5). The perforated section should cover the zone of ground where the unconfined measurements are required. The annulus between the porous/perforated section (which might not be the whole length of the pipe) and the walls of the borehole should be filled with a granular filter material. When there is a possibility of fine particles (e.g. if 10% of the surrounding strata is finer than 2 mm) an inert geotextile material should be wrapped around the perforated section of the tube. The effects of using a screening material on the subsequent measurements (e.g. permeability) should be assessed. Grout should be used around unperforated sections of the well to prevent water entering the well from strata that are not being investigated.
If water samples are required to be taken from the well (see 26.4), it should be constructed from materials that do not react with the ground and that do not release or absorb contamination.
If gas samples are required, a rubber bung or tight fitting plastic cap with gas tap(s) should be fitted at the top of the tube. Wells intended for the monitoring and sampling of ground gases should be constructed in accordance with BS 8576 and BS 10175.
Observations in open boreholes and excavations (with or without a properly constructed observation well) should not be relied upon when the depth of the borehole or excavation spans more than one hydrogeological unit (e.g. where perched water is separated from a deeper aquifer by a lower permeability material). In such circumstances, observations should be made in separate observation wells, terminated and sealed to prevent the ingress of water from depths other than the depth of interest.
NOTE 1 An alternative to the use of independent observation wells is to drill a larger diameter hole down to the bottom of the low permeability strata and construct an impermeable plug of suitable material and sufficient thickness (e.g. bentonite/cement grout with a thickness of at least 1 m). This could necessitate some preliminary trials to confirm that the selected material is effective. The plug is allowed to set before continuing the borehole by forming a smaller diameter hole. In this way a seal (to prevent the downward migration of contamination) is created. The borehole is grouted, as the casing is withdrawn in order to complete the seal.
Following installation, and before the start of the monitoring programme, the observation or monitoring well should be fully "developed" to maximize the rate at which groundwater flows into the well from the surrounding strata. Various methods can be used to develop wells (see BS ISO 14686), although perhaps the most appropriate for monitoring wells is by "purging" (i.e. the water standing in the well is removed by bailing or pumping so as to induce replenishment with groundwater). This development should also settle the granular surround and ensure free flow of liquids through the well screen. The rate of pumping should be substantially greater than that proposed for subsequent purging or sampling.
Development should continue until the water is visibly clean and/or of constant quality (e.g. in terms of its electrical conductivity). This operation should be continued until three well volumes plus the volume of water added during drilling, frequently taken as five well volumes of water (see BS ISO 5667-3), have been purged. Adequate provision should be made for the disposal of contaminated water from monitoring wells resulting from well development and purging operations.
NOTE 2 The use of cement/bentonite or similar materials in monitoring well construction can affect the water chemistry (e.g. pH). A sufficient equilibration period and the use of alternative materials minimize the likelihood of any such effects occurring.
Once installed, sufficient time should be allowed to elapse after completion of the borehole for conditions to stabilize so that the unconfined groundwater level (and any subsequent variations with time) can be monitored. If samples of groundwater are required, at least 14 days should be allowed for an equilibrium to be reached, but when this is not possible the time allowed should be as long as practicable.
When all monitoring work has been completed and there is no further need for the observation well, it should be sealed by grouting, ensuring that the grouting is effective above and below the water table.
NOTE 3 Further guidance on the decommissioning of boreholes can be found in the Environment Agency guidance document Decommissioning Redundant Boreholes and Wells .
The level of water measured in a borehole with or without an observation well should not be used as an indication of a pore water pressure because the levels from which the water is entering the borehole are unlikely to be known. A piezometer should be used if the groundwater pressure is required.
52.3 Methods of measuring groundwater pressure
The selection of an open (unconfined) or closed (confined) system to undertake groundwater measurements should be based on the hydrogeological conditions at the location. Open systems rely on the measurement of a free unconfined groundwater surface (i.e. at atmospheric pressure), typically in an open pipe. In closed systems the water pressure in a confined section is measured, generally by an electrical transducer or less commonly an hydraulic or pneumatic piezometer.
52.4 Response time
COMMENTARY ON 52.4
All methods of groundwater pressure measurement require some flow of water into or out of the measuring system before the recorded pressure can reach equilibrium with the actual groundwater pressure.
The selection of the measurement method should be made on the basis of the anticipated ground and groundwater conditions. Figure 17 should be used to select the most appropriate method of measuring groundwater pressure based on the anticipated ground and groundwater conditions.
NOTE For an excavation or a borehole, a large volume of water might flow before the water level reaches equilibrium with the groundwater pressure. On the other hand, some types of piezometer require only a very small change in the volume of water for the groundwater pressure to be read. The rate at which water flows through the soil depends on the permeability of the soil. The time required for a measuring system to indicate the true groundwater pressure is known as the response time and depends both on the quantity of water required to enter the system (including all pipes and tubes) to operate the pressure measuring device, and on the permeabilities of the ground (including any medium, e.g. sand or grout, placed between the pressure measuring device and the ground as part of the installation) and the piezometer. The selection of a suitable method for measuring the groundwater pressure is largely determined by the response time (see Terzaghi and Peck, 1948 ). Piezometers generally respond quicker in soils with large compressibility (see Gibson, 1963 ).
|1 Diaphragm piezometer||4 Casagrande open piezometer|
|2 Closed Hydraulic (Bishop) piezometer (with 2,5 m long tubes)||x Time for 90% response (days)|
|3 Closed Hydraulic (Bishop) piezometer (with 300 m long tubes)||y Coefficient of permeability for ground (m/s)|