The mapping of subsurface utilities is becoming increasingly valuable, as projects can be planned with confidence and peace of mind, knowing that either the site is clear, or that areas of interest have been inspected and reported on. By John Hughes, Geometric Surveys
Underground utility scanning and mapping
Utility scanning, or subsur face ser vices detection, is the process of identifying and mapping utility mains that are underground. These mains may include lines for telecommunication, electricity distribution, natural gas, fibre-optic cables, traffic lights, streetlights, stormwater drains, water mains, wastewater and sewer pipes. Engineers, project managers, town planners, architects and all other professions involved in the construction industr y need to know where underground ser vices are located before they star t planning and design.
The role of the professional land or technical sur veyor is critical in the supply of accurate spatial information and underground ser vices. It is the task of a land sur veyor to position the underground ser vices that are essential for future design and planning for brownfield projects and the reconstruction of infrastructure.
The best solution is to appoint a registered person to execute the sur vey and utility scanning where the advantage is that the sur veyor can add the detected ser vices seamlessly to their data. The engineer therefore has one data set and is not concerned if the sur face markings of the detected ser vices disappear.
In this respect, the Geomatics Professions Act (No. 19 of 2013) clearly stipulates that a person must be registered with the South African Geomatics Council (SAGC) to per form sur vey work. Anyone who breaches this stipulation is committing a criminal offence. It’s also impor tant to understand that there are various categories of sur veyors: the correct discipline and sur vey categor y must match the specific project outcomes (see www.sagc.org.za or www.sagi.co.za).
Some benefits of subsurface utility scanning include:
- non-disruptive and non-destructive methods
- improved and updated asset register
- assists with the design phase and flags possible utility conflicts that may require redesign
- protects existing services from being damaged during construction
- avoids contractor change orders
- fewer damaged utilities, which means fewer penalties and construction delays
- local communities are not affected by damaged utilities, which could cause health risks as well as take a long time to repair with large associated costs.
Different detection and location methods must be used because of the many different types of materials that go into manufacturing each of the different types of underground services.
Electromagnetic equipment, consisting of a transmitter and receiver, is best suited to locating metal pipes and electric cables. For other types of pipe, such as plastic or concrete, other kinds of radiolocation or modern ground-penetrating radar must be used.
All found services are indicated on the surface by either chalk marks or with spray paint marks for visual inspection by the client. These marks are then captured by land survey and this information is then processed and represented on the feature survey drawing of the site.
General procedures Precision locator/high-power transmitter combination
The precision locator is most effective at locating buried cables and metal pipes. The project area is broken down into sections and the site is covered by walking in a grid formation where the cross-section intervals are approximately 20 m in size. This process helps identify areas that need to be inspected in higher detail.
This is particularly important in more built-up areas where the services network becomes denser – e.g. at a road intersection where there might be stormwater, fibre-optic cables, traffic light connectivity network, electric cables and so on traversing the site. The image of a bowl of spaghetti representing the services network comes to mind.
- Accuracy: ±10% of depth (i.e. 0.1 m if service is 1 m deep)
- Depth: ±5 m in perfect soil conditions, ±3 m can be a reasonable expectation
Ground-penetrating radar (GPR)
GPR utilises a short burst of radiofrequency energy radiated into the ground to detect discontinuities. These discontinuities can be cavities, voids, buried objects, filled areas, non-conductive pipes, etc.
With GPR equipment, the site also needs to be inspected similarly in a grid pattern across the site. In order to identify the service type, a source of where the service is exposed – either by opening an inspection manhole or where the service becomes visible – is required. This is because GPR is unable to detect the service type. Linear obstructions or anomalies will be indicated on the GPR unit’s screen, with the depth clearly visible.
There are limitations with this technology; it is possible that a limited number of services will not be detected. A good example of this is the old clay pipe used to carry outflow wastewater. The clay pipes are not conductive, as the equipment relies on conductivity to be able to detect an anomaly. Coupled with that, the ground in which the clay pipe is planted returns similar signals to that of the soil type and the result is that the service is missed. In cases like this, a sonde needs to be inserted into the pipe, which has good conductivity properties, allowing one to locate the service more easily.
- Accuracy: X, Y – sub 0.10 m Z – In general, it is sub 10% (if service is 1 m deep, the accuracy is 0.1 m).
- Depth: ±8 m in perfect soil conditions, ±3 m can be a reasonable expectation.
If sound survey and solid utility scanning practices are followed, an accurate reliable asset register can be achieved. This allows engineers, architects and all other spatial professionals to achieve their objectives with the least amount of interruptions, and budget overruns, due to services not being damaged.
For further information, please contact the author at email@example.com or +27 (0)11 462 2308 / +27 (0)82 825 8114
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