26 Groundwater monitoring and sampling
COMMENTARY ON 26.1
The determination of groundwater levels and associated water pressures is extremely important, because these have a profound influence on the behaviour of the ground during and after the construction of engineering works. Various strata, particularly those separated by relatively impermeable layers, can have different groundwater pressures, some of which might be sub-artesian or even artesian.
The location of permeable water-bearing strata and the measurement of water pressure in each stratum is particularly important where deep excavation or tunnelling is required, since special measures might be necessary to control the groundwater.
Accurate groundwater pressure measurement usually requires the installation of piezometers. The groundwater pressure might vary with time owing to seasonal (rainfall), tidal, anthropogenic or other causes, and it might be necessary to take measurements over an extended period so that such variations are investigated.
The efficient design of drainage works often requires determination and use of the contours of the water table or piezometric surface to ascertain the direction of the natural drainage, the seasonal variation and the hydrological controls.
Information on groundwater quality is relevant to the design of concrete foundations, and to assessments of the extent of any groundwater contamination and the risks it might pose to the water environment.
The groundwater regime should be determined as part of an investigation; this might require in-situ permeability testing (see Section 7), the installation and monitoring of groundwater measuring instrumentation (see Section 8) for use in the measurement of groundwater levels and direction of flow, and the assessment of aspects of groundwater quality.
Sampling and monitoring of groundwater quality should be carried out in accordance with, as appropriate, BS 8550, BS EN ISO 5667-1, BS EN ISO 5667-3, BS ISO 5667-11 and BS ISO 5667-22.
Investigations of groundwater in connection with the assessment of potential risks to the water environment should be carried out in accordance with BS 10175, which provides guidance on the development of suitable sampling and monitoring strategies and how to implement them including the location and design of monitoring wells, how to take samples and their conservation and analysis.
26.2 Observations during drilling boreholes and excavating pits
Water level entries into boreholes (by whatever drilling method) and excavations (trial/observation pits and trenches) should be recorded.
Boreholes are not necessarily completed in a single continuous operation; they can take several days to reach the required depth and progress is suspended overnight. The water level in the borehole should be measured and recorded, along with the details of any casings, at the beginning and end of each shift.
The level of water measured in a borehole during or shortly after the completion of drilling is often used to ascertain the groundwater pressure; this should, however, be treated with great caution, because frequently insufficient time is allowed for the water level to stabilize, particularly as the drilling might have disturbed the pressures in the ground around the borehole. Moreover, the
levels from which the water is entering the borehole might not always be known, because these are affected by the use of temporary borehole casings. For most conditions, an observation well or piezometer should be used in preference (see 26.3 and Section 8).
All observations during drilling should be reported together with the datum used for groundwater level measurements.
NOTE Water level and casing observations also form part of the required information given in the sampling and in-situ testing standards.
26.3 Observations in installations
Following drilling or excavation, observation wells or piezometers should be installed as appropriate to provide the opportunity for longer term monitoring to establish the groundwater levels under a range of conditions (e.g. long term, seasonal and tidal variations or during construction). A programme of regular measurements should be established, which might extend well beyond the period for intrusive investigation activities, and should be continued until the groundwater levels and variations of groundwater levels or pressures have been established with confidence.
NOTE The design and construction of the monitoring installations is covered in Clause 52.