An observation well is an instrument installed in the ground at a specific loca-tion to allow measurement of the groundwater level or pore water pressure.
When determining groundwater levels observation wells have a number of advantages over observations during boring:
1 Because they are long-term installations, appropriately designed obser-vation wells can allow obserobser-vation of equilibrium groundwater levels, even in very low permeability soils.
2 Observation wells can be installed as piezometers to record water lev-els or pore water pressures in a specific stratum.
3 Observation wells can be monitored for long periods of time, to observe the variations in groundwater level at a site.
The two most commonly used devices for monitoring groundwater levels in permeable soils are standpipes and standpipe piezometers.
The simplest from of observation well is the standpipe (see Fig. 6.3). This consists of a small diameter pipe, of which the bottom section (usually at least 1 m in length) is perforated or slotted, with the base plugged. The pipe
is installed in the centre of a borehole and sand or gravel placed around the pipe, if necessary tamped into place. Backfilling should cease at a depth of about 0.5 m below ground level and the remainder of the hole sealed using puddled clay or bentonite/cement and capped off with concrete, to prevent surface or rainwater entering the borehole. It is advantageous to haunch the concrete to help to shed the surface water. Unless a special protective cover is required the pipe should project about 0.3 m above ground level and be pro-vided with a suitable cap or threaded plug. In urban areas it is essential that the cover or capping arrangement is secure enough to resist vandalism. Some designs of covers (known as ‘stop-cock’ covers) can be installed flush with the ground surface. These covers are sometimes preferable in vandal-prone areas because they are unobtrusive and may not attract the attention of vandals.
Plastic tubing (such as PVC) is an ideal material for standpipe tubing.
Typically supplied in 3 m or 5 m long pieces it can readily be sawn to desired length and joined using PVC couplings and solvent cement. The perforated lengths of pipe are usually supplied in 1.5 m lengths and are drilled or pre-slotted. If necessary the plain pipe can be slotted on site using a hacksaw but it should be noted that the total area of perforations should be at least twice the cross-sectional area of the standpipe. The water level in a completed standpipe can be measured using a dipmeter (see Section 14.2). The preferred internal diameter for standpipe tubing is approximately 50 mm; this enables water samples to be taken and allows a small airline to be used to flush out the standpipe if it becomes blocked. Smaller diameter tubing is sometimes used, but the minimum acceptable internal diameter is usually 19 mm, because this is the smallest size down which many commercial dipmeters can pass.
A standpipe is simple and cheap to install, but it is only a basic instrument.
The standpipe will respond to pore water pressures in water-bearing strata along its entire depth. This is acceptable if the standpipe is used in a simple
Figure 6.3 Typical standpipe installation.
unstratified unconfined aquifer, where the total head is constant with depth.
However, if a standpipe is installed in a layered aquifer system water can enter from more than one water-bearing layer. If the groundwater levels are different in each layer (e.g. a main water table and a perched water table) the standpipe will show a ‘hybrid’ water level, between the two true water levels.
Standpipes are not suited to use in layered or complex groundwater regimes. In such cases it is necessary to use a ‘piezometer’ where the instru-ment is sealed into the ground so that it responds to groundwater levels and pore water pressures over a limited, defined, depth only. The most common type of piezometer is the standpipe piezometer.
Figure 6.4 shows typical construction details for standpipe piezometers. The aim is to produce a ‘response zone’ of sand or fine gravel at the level of the stratum in which the groundwater level is to be observed. Rigid PVC tubing is installed in the borehole in a similar way to a standpipe, with a ‘piezometer tip’ located in the centre of the response zone. Grout seals above and below the response zone ensure water can only reach the tip from the desired stra-tum. As with a standpipe, water level readings can be taken with a dipmeter.
Figure 6.4 Typical standpipe piezometer installations. Two piezometers are shown, each with its response zone and piezometer tip in a different water-bearing stratum.
Installation of standpipe piezometers is more complex than for standpipes, and should be carried out with care. It is essential that the seals are effective, otherwise water may leak into the response zone from strata above or below.
If clay backfill is used it must be adequately compacted (e.g. with the drill rods or shell) to reduce the risk of later settlement. Where grout is used to backfill parts of the boreholes it should be cement-bentonite grout of the appropriate consistency. A layer of bentonite pellets should be placed between the grout seal and the sand filter in the response zone, to avoid the sand becoming contaminated with grout (if pellets are not available, ben-tonite balls will have to be made up by hand). Once the lower benben-tonite seal is in place, and has had time to swell, it is good practice to flush out the dirty water in the borehole and replace it with clean water before installing the sand filter.
A ‘generic’ specification of the grading of sand filters is not possible.
However, for guidance a filter consisting of a clean well-graded sand and gravel with only a small proportion of fine to medium sand, is suitable for soils with some clay or silt content. For a fine sand soil, the filter should con-sist of coarse sand or coarse sand and gravel with not more than a few per cent medium sand. Local material may have to be used, but it is essential that the filter material is free from clay and silt. Bentonite pellet and grout seals are installed above the sand filter. The tubing should be capped off at ground level with a secure cover or headworks.
The ‘piezometer tip’ typically consists of a porous plastic element or a porous ceramic element (sometimes known as a ‘casagrande element’); tips are generally a 150–600 mm in length. It is good practice to soak the filter sand and ceramic element (if used) in water prior to installation – this helps avoid any air being trapped in the system, and speeds up the process of equilibra-tion between the piezometer and the natural groundwater level.
It is possible to install two or more piezometer tips (each in its own response zone and separated by grout seals) in one borehole. If this is being contemplated, it is essential that it is carried out by experienced personnel and is carefully supervised. This is awkward work, in all but the largest boreholes and there is always the risk that the installation of the second tip and seals will affect the piezometer already installed. If possible, single piezometer installations in each borehole should be used, purely because the water level readings will be easier to interpret, with no worry of water leak-ing between response zones.
In many investigations for groundwater lowering projects piezometers will be installed in relatively permeable soils, where the water level inside the piezometer will respond rapidly to changes in the pore water pressure in the soil. However, piezometers in soils of moderate to very low perme-ability may respond slowly to changes in pore water pressure. This is because a finite volume of water must flow into or out of the piezometer to register the change in pressure. This leads to a ‘time lag’ between changes
in pore water pressure in the soil and the registering of that change in the piezometer. The time lag is greater in soils of lower permeability, and is greater for piezometers where larger volume flows are needed to register pressure changes.
In a standpipe piezometer the prime factor controlling the equilibration rate is the internal diameter of the tubing; the smaller the diameter the shorter the time lag and the quicker the piezometer will respond to pressure changes. In soils of low to moderate permeability, it is normal to specify the internal diameter of the tubing as small as possible (19 mm is the lower practicable limit to allow monitoring by dipmeter). However, in permeable soils such as sand and gravels, the equilibration rate will tend to be rapid, and 50 mm diameter tubing could be used, allowing greater flexibility for sampling or flushing out of the piezometer.
A defining feature of observation wells is that (provided they are ade-quately protected from damage or vandalism) they can be used to observe groundwater levels long after the main ground investigation is complete.
This can allow natural changes in groundwater levels to be determined.
However, this requires the instruments to be monitored for extended peri-ods, and the practicalities of this are sometimes overlooked. If readings are to be taken manually this will have to be included in the site investigation plan and associated costings. In remote or inaccessible sites it may be appropriate to use datalogging systems (see Section 14.5) to record ground-water levels, to reduce the cost associated with regular visits by personnel.
6.5.3 Other methods for determination of groundwater levels