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Wed 25 Jun 2008 04:00 AM

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What lies beneath

The ground beneath our feet is becoming increasingly congested, with many services crammed in.

Martin Parsons takes a look at the latest developments in location techniques for underground utility services.

While there may be records of where underground services such as power, water, gas and so on are located, these may not always be entirely accurate.

This means that when faults occur, or when extension work needs to be carried out, time and money can be wasted trying to find a specific cable, pipe or joint.

The ground beneath our feet is becoming increasingly congested, with many services crammed into the same space.

However, developments in new underground location technology are bringing extremely accurate underground location within the reach and budgets of today's utilities, contractors and construction companies.

Inaccurate mapping

Most utility companies and other organisations that manage or own underground services, do keep underground records, but it is hard to ensure their accuracy, partly because many of these services were installed decades ago, when accurate records were not always kept.

Today's utilities may have an approximate idea of a service's location, but not the precise spot.

Furthermore, it is not hard for records to become out of date. Changes in the above-ground layout, such as a housing estate or business park being built, can mean that the relation between the underground service and above-ground landmarks change.

Although above-ground markers may exist for these utilities, they may be moved or even removed during construction work.

Digging holes in the wrong place is an expensive business: a hole can cost upwards of £500 (US$987) to excavate. In addition, utilities and their contractors are under pressure to minimise the amount of time carrying out streetworks and face penalties if they exceed the granted time limits.

The problem is exacerbated by the fact that the ground beneath our feet is becoming increasingly congested, with many services crammed into the same space.This can make it extremely difficult for any contractor, engineer or surveyor working in that area, because there is the potential to inadvertently damage the pipes or cables of another organisation, which obviously costs money and could also lead to delays in completing projects.

Limited technology

Techniques for recording and finding underground services have been around for years, but have all had their limitations.

Although above-ground markers may exist for these services, they may be moved or even removed during construction work.

Until recently, the location-finding technology available relied on considerable effort and manual input on the part of site workers. As a result, many utilities and their contractors have made minimal investments in location-finding equipment.

Starting at the least high-tech end of the range, there are the good old-fashioned above-ground markers that we are all used to seeing: cheap and visible, but basic and easy to move.

Moving up a step, there are various radio frequency-based methods that have been around for some time. These falls into two main categories: passive and active radio detection.

Passive detection

Passive mode is based on the knowledge that any metallic pipe or cable naturally picks up and re-radiates a signal, so it is easy to track an existing pipe along its course.

The drawbacks are that it depends on metal content and a sufficiently strong signal, which is not always the case. For instance, fibre-optic and cable television services are notoriously difficult to find.

In addition, this approach is dependent on the operator knowing roughly where the cable or pipe is located and does not provide any means to record more detailed information once found.

Active detection

Active radio detection involves applying a transmitter unit onto a pipe or cable, or holding it above the ground on the point where it is expected to be.Someone then walks along holding a receiver, which detects the signal and emits a beeping sound that increases in relation to the signal's strength, or proximity to the cable or pipe.

This system is relatively straightforward, but is limited by the fact that it will detect any other pipes or cables in the immediate area, potentially creating confusion.

Again, active radio detection depends on knowing the approximate whereabouts of the pipe or cable and does not provide a simple mechanism for recording location data.

Rather more sophisticated is the passive electronic location method. Instead of depending on the pipe or cable to emit a signal, small markers are attached to the pipe or cable during the installation process, whether at regular intervals or simply at key junctions.

The markers are usually ball-shaped, but can take other forms and contain a fluid that ensures the marker is always horizontal and optimally positioned to respond to an electronic signal.

This information is then stored on a handheld device, which is then used on future occasions to find the pipe or cable.

Markers can be allocated different colours and radio frequencies, to distinguish between types of services.

Detailed records

All the techniques described so far focus on finding cables and pipes, rather than helping utilities to build up accurate, detailed records in the first place. However, there is a new kind of underground location technique emerging: ID markers.

ID marker systems are an extension to the existing concept of passive electronic location, but have added intelligence.

Each marker ball contains an ASIC (application specific integrated circuit) and are used in conjunction with more advanced handheld locators.When the ball is placed into the trench, the locator's LCD interface is used to record the ball's unique 10 digit serial number, plus up to 256 bits of data.

This might include: when the marker was installed, by whom, the name of utility, the depth and the nature of the junction or joint it is marking, among other crucial details.

The handheld locator can be connected to GPS satellite systems as well, this means that the exact coordinates of each marker ball can be correctly recorded.

The GPS networks may in turn be connected to databases and CAD drawings held by the utility, so that over time, a set of detailed records can be created. When further work needs to be carried out at a later date, the information is downloaded back onto the handheld locator.

If the details relating to a particular marker ball are found to have changed in anyway, the data can easily be updated and modified using the device.

ID marker balls can be read to a depth of 1.5 m. This is usually sufficient for most utility installations, but not all, such as sewerage pipes.

In these instances, the hole needs to be backfilled until the ball is at the right level. There also needs to be 10 cm between the ball and a metal pipe, but ways are being found around this, as the development of location technologies continues to progress.

Investing in the equipment starts at less than €1,500 (US $2,332). Compared to the overall costs of most excavation projects, this represents a small investment and one that could provide a rapid pay-back.

There is a small amount of additional effort required by the contractor or utility's on-site staff, but it will save time in the long-run. It can even be seen as a way for contractors to differentiate themselves in a highly competitive market.

Accurate underground location is achievable. Each time a hole is dug, the opportunity is there. Valid reasons for not having accurate underground location are disappearing, while the benefits are clear.

Martin Parsons is marketing manager for 3M Electro & Communications Business.

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