For location of services we use the following techniques:
- Radio Detection scanning
- Signal transmitters (sonde/flexitrace)
- Ground Penetrating Radar
- Visual Inspection of access chambers
- Statutory Authority Searches (Stats Desktop Study)
Electromagnetic Locators (EML)
Electro Magnetic Locators are more commonly referred to as Radio Detection (RD) and Cable Avoidance Tool (CAT). These detectors can be used in power, radio detection and generator modes. Where a physical connection can be made to a conductive service, a signal generator is used to introduce a signal along it allowing the service to be traced. If a physical connection is not possible, induction methods can be adopted. Using these methods, RD should detect live electrical services (including Telecom) and conductive services, although in congested areas it may not be possible to resolve closely spaced services.
All detected services are marked up insitu on site with chalk or spray paint. If required, a total station can be used to record the information and the services plotted on a CAD drawing.
Signal Transmitter (sonde / flexitrace)
Where good access to an open service is available (eg. drain), a transmitter (sonde or Flexi-Trace) can be threaded along the service and traced using conventional Radio Detection techniques. For cable runs a clamp can be attached to the cables, to induce a signal, and the cable traced. This enables accurate plotting of deeper drainage runs which may be out of range of GPR.
Ground Penetrating Radar
GPR is a method which is capable of detecting anomalies in the ground. When these anomalies can be plotted into a continuous line, this may indicate a cable, pipe or duct. GPR is therefore capable of detecting clay and plastic services; something that more conventional techniques can’t do.
Although the GPR data can be interpreted in real time, this method is not recommended as services can easily be missed, particularly in congested areas. Our preferred methodology is to scan the area on a regular orthogonal grid; the spacing of scanlines is adjusted to suit site conditions, but typically it is at 1m centres. The GPR data will be saved electronically and processed and analysed off site using specialist software. This provides far superior results to analysing the data on site in real time
Typically a GSSI SIR3000 GPR system will be used with an antenna of suitable frequency. This is most likely to be 400MHz or 270MHz. Although low frequency antennas provide better penetration depth, the resolution and detail of GPR scan data decreases with lower antenna frequencies. It is therefore desirable to use the highest frequency possible, while still meeting the objectives of the survey.
A big advantage of GPR is that it will also provide additional valuable information about other sub-surface obstructions such as foundations, storage tanks, culverts, basements and other obstructions within the survey area.
Visual Inspection of access chambers
Where possible all covers in and immediately adjacent to the survey area are lifted and detailed. For each inspection chamber the following information can be recorded (depending on your requirements):
- Type of cover and services within.
- Location of services entering the chamber.
- Depth of services.
- Diameter of service.
- Invert levels for drainage runs.
By recording all physical data, the accuracy of the survey is improved. Depth information can also be used to carry out insitu depth calibration of the GPR.
Statutory Authority Searches (Stats Desktop Study)
We can also undertake a desk study comprising statutory undertakers survey report for the nominated area and all services leading to it. We contact utility companies on your behalf and obtain copies of available records. The received reports outline features below the surface such as gas and oil pipelines, electricity cables, telecommunication cables, mains water, sewers and fibre-optic cables.
We can append this information onto a single CAD drawing, giving you all the results at a glance. No more thumbing through reams of paper and drawings, all at different scales!
Presentation of Results
Sandberg offer clear and concise reporting of utility survey results. Client’s reporting requirements vary and we can offer anything from a site mark-up to a comprehensive and detailed report with CAD drawings. Reports are issued in hard copy and electronic format, so they are easy to distribute and environmentally friendly too.
Detection Rate
There is a perception held by many that all buried pipes, cables and ducts can be detected and mapped irrespective of their size, duty, depth, location, material type, geology and proximity to other utilities. A well designed and executed survey should be able to detect up to around 95% of utilities but it may not be possible to achieve a 100% detection rate.
Statement taken from: The Essential Guide to Utility Surveys, issue 4 October 2011 by The Survey Association.
