Piezometer installation

2.5.3.1 Terminology

The terms ‘observation well’ and ‘piezometer’ are commonly used to describe drill holes that are used for measuring water levels or pore pressures in the field. The terms are often synonymous although, in many texts and reports, ‘piezometer’ is often used for installations with shorter, sealed completion intervals.

Figure 2.28 shows how observation wells and piezometers may be installed as standpipes or grouted instruments. It also shows how vibrating-wire piezometers may be grouted in place to measure pore pressures at point intervals.

The term ‘monitoring well’ is synonymous with

‘observation well’ but is often used to describe wells installed primarily for water chemistry sampling.

2.5.3.2 Open observation holes

The term ‘open observation hole’ is used to describe an open uncompleted drill hole used for measuring groundwater levels. Open observation holes may have a small-diameter monitoring pipe installed to provide stability against hole collapse, but have no impermeable seal to isolate discrete vertical intervals in the formation.

Measurement of groundwater levels in open

observation holes (with or without slotted pipe installed) can allow rapid characterisation of the piezometric surface, at low cost. The water level recorded in an open hole represents an average of the pressure over the

saturated interval. This is particularly useful in the early stages of project development, where there has been little stress on the groundwater system and where significant vertical head gradients have not yet

developed. Where such gradients are present, open holes may allow the vertical movement of water between intervals. Because of this, some countries have a legal requirement that open drill holes must be sealed within a specified time period.

2.5.3.3 Standpipe piezometers

The traditional piezometer is a standpipe that allows pressure to be measured as a head of water in a pipe. The piezometer consists of a filter tip or perforated interval of pipe of a desired length attached to the bottom of an impermeable casing that rises to the surface. The perforated interval is isolated by an upper seal, typically with grout placed above the seal to the surface (Figure 2.28). The level to which the water rises in the pipe indicates the average pressure over the perforated interval.

In the early stages of groundwater investigation, if piezometers are used in preference to open holes the perforated interval of the piezometer may be long (20+ m), so the average head of a formation is measured. As the investigation becomes more focused, a shorter (6 m or less) perforated interval may installed in subsequent holes to measure the pressure at more discrete depths.

General guidelines on the installation of piezometers are given in Appendix 1, Attachment C.

Figure 2.28: Alternative piezometer installations in core or RC drill holes

2.5.3.4 Vibrating-wire piezometers

Vibrating-wire piezometers consist of a vibrating-wire sensing element in a protective steel housing. The sensing element consists of a tensioned steel wire clamped to both ends of a hollow cylindrical body. The piezometer converts water pressure to a frequency signal via a diaphram, the tensioned steel wire and an electromagnetic coil. When excited by the electromagnetic coil the wire vibrates at its natural frequency. The piezometer is designed so that a change in pressure on the diaphragm causes a change in tension of the wire, altering its natural frequency of vibration. The vibration of the wire in the proximity of the coil generates a signal that is transmitted to the readout device. The readout device processes the signal, applies calibration factors and displays a reading.

Vibrating-wire piezometers are a good method of collecting water level data and defining vertical hydraulic gradients around active pit slopes. Multiple transducers may be set in alternating bentonite/sand packs placed using a tremie pipe (Figure 28b). Alternatively, a string of three or more vibrating-wire piezometer instruments may be grouted directly into the hole without the use of a filter pack.

The grouted-in method is a rapid way to install multiple piezometers in one drill hole, to install piezometers in holes with inclinometers or other geotechnical instruments, or to monitor pressures at a discrete point. A detailed description of grouted-in piezometer installation is given in McKenna (1995).

2.5.3.5 Standpipe vs grouted vibrating-wire installations

The advantage of a traditional standpipe piezometer design is that it allows the water level to be physically measured down the hole, rather than relying on the performance of an instrument. The standpipe piezometer also allows hydraulic testing and water quality sampling to be carried out. The use of grouted-in vibrating-wire piezometers relies on the instrument performing properly.

If a grouted-in vibrating-wire piezometer fails, there is no way of recovering it and measuring the water level.

However, vibrating-wire piezometers typically provide a lower-cost installation, particularly where they are grouted in. It is relatively easy to install multiple vibrating-wire piezometers using one cable, so multiple piezometers can be installed in smaller-diameter holes. Another advantage is that, if access to the wellhead is lost, readings from a vibrating-wire sensor can be taken remotely (provided the cables can be identified).

Where vibrating-wire piezometers are grouted in, the pressure recorded at the piezometer is a discrete pressure at the level of the instrument.

For fractured rock environments, where there is a large difference in permeability between the fracture zones

and the surrounding rock mass, the lag time for piezometer response may be very large if the piezometer is not installed adjacent to a permeable fracture. For pumping tests where a more rapid response is required, it is often better to use sandpack intervals rather than grouted piezometers. Unless the vibrating-wire

piezometer is accurately positioned, sandpacks are often more likely to include permeable fractures and produce a faster response.

Grouted vibrating-wire installations are more

applicable in settings with low permeability. Because there is no standpipe and therefore no water in the completed hole, there are no well-bore storage effects and the initial equilibiration response time of the piezometers is much faster. Figure 2.29 shows a multiple grouted-in vibrating-wire installation where seven piezometers were installed in an HQ diamond hole.

2.5.3.6 Joint piezometer/inclinometer completions At some operations it has been possible to install

completions acting jointly as standpipe piezometers and inclinometers, as illustrated in Figure 2.30.

Figure 2.29: Example of a multiple vibrating-wire piezometer installation in a core hole

Source: Courtesy of Olympic Dam Expansion project

2.5.3.7 Horizontal piezometers

During active mining operations, piezometers can be installed horizontally from the pit slope, often as part a horizontal drain drilling program. However, as for vertical piezometers, the target zone for pressure

monitoring needs to be isolated and sealed. There are two practical ways to achieve this. The first is to use a packer system and to grout the annulus above the packer. The second is to grout vibrating-wire transducers into place, as shown in Figure 2.31. In this process, the grout tube shown in Figure 2.31 must be extended so that the grout is introduced at the end of the hole.

The second option allows multiple sensors to be installed within the hole. An issue with horizontal drilling is that the weight of the drill string often causes the holes to ‘droop’ (deviate downward) during drilling. This would affect subsequent vibrating-wire piezometer readings because the instruments may actually be lower than the target elevation. Therefore, if the intent is to install piezometers, it is beneficial if the hole can be directionally surveyed prior to piezometer installation.

2.5.3.8 Piezometers installed in drainage tunnels The design shown in Figure 2.31 can also be applied to piezometers installed in underground drainage tunnels.

Depending on the capability of the underground drilling set-up, such piezometers can be installed at any angle to target specific zones. Figure 2.32 shows a piezometer installed in a near-vertical upward hole.

2.5.3.9 Westbay piezometers

Another option for multi-level groundwater monitoring is the Westbay piezometer system. Westbay installations allow pore pressure monitoring from a virtually unlimited number of discrete depth zones in a single drill hole. Many existing installations include monitoring of up to 30 depth zones and several monitor up to 50 zones. The system is illustrated in Figure 2.33. It has been used worldwide for over 20 years.

Figure 2.30: Example of dual piezometer/inclinometer installation

Figure 2.31: Vibrating-wire piezometers installed from the pit slope or from underground

A single casing assembly is installed into the hole, with valved ‘ports’ located at each required depth zone.

Hydraulically inflated packers on the exterior of the casing seal the annulus of the hole between the

monitoring zones. Wireline tools are lowered inside the casing to access each valved port, and the pore pressure is measured in situ. Measurements can be made manually, moving a single sensor from zone to zone, or

automatically, by deploying a string of pressure sensors, each ported-in to a different monitoring zone. In low permeability environments, the Westbay system offers a similar quick response time to a vibrating wire

piezometer.

An additional advantage of the Westbay system is that, if required, it also allows pulse hydraulic testing and water sampling to be carried out. Hydraulic tests can be carried out by purging within single zones, multiple zones, or cross hole testing using multiple wells. Such detailed testing can be valuable in identifying flow zones, flow barriers, and compartmentalization in complex fractured rock masses. Water samples can also be collected from each zone.

The PVC Westbay casing, ports and packers are compatible with HQ wireline drilling equipment, which allows the system to be installed inside the drill rods (with the bit removed). The system can therefore be installed to full depth without the risk of the hole collapsing. The drill rods are then withdrawn in stages and the exposed packers are inflated in sequence. This method has allowed the successful installation of the Westbay system to depths ranging from less than 100 m to

over 1200 m, sometimes in poor quality rock. Pressure sensors and/or sampling devices can be retrieved for calibration and repair.

2.5.4 Guidance notes: installation of test