Essential to industry, ’Dial before you dig‘ provides a great service to contractors involved in rehabilitation works. It brings information from all the underground asset owners together. However, plans from the asset owner may not be 100 per cent accurate. Let me explain this. Pipes and cables are often not laid perfectly straight, as they could be displaced during the backfilling process or through ground movements during heavy rain seasons.

Is there anything else we can improve in the current procedure? Yes, 3D utilities mapping. This mapping provides the solution to the above mentioned challenges.

3D Mapping Process

The following section summarises a typical 3D utilities mapping process. Referencing

This is the most important process to achieve an accurate mapping of the underground utilities. In order to provide accurate local co-ordinates or the WGS coordinates referencing, a systematic approach to referencing each scan has to take place. All this can be easily achieved by mounting a prism on the GPR equipment, utilising a robotic total station to track the exact position of the system. Alternatively, GPS (RTK) receivers could be installed onto the GPR system.

For contractors on a tight budget, there is another method of acquiring accurate co-ordinates. In the case of the RIS Hi-Mod system from IDS, a starting point can be marked on the side curb, which allows easy referencing to control points by resection or back-sighting with a total station. The RIS system can be easily set up to perform scans referenced to the marked point and curb, and automatically increases and stores the co-ordinates after each scan. Acquisition

The acquisition process refers to the scanning of the road using GPR systems. Apart from the starting point, some additional ‘guide’ points will be required at set intervals to help the operator steer GPR systems in the correct direction. These guide points also serve another purpose, to fit the scans correctly and accurately onto a bending road in CAD or GIS. To acquire all underground utilities in all different directions, longitudinal (x-axis) and transversal scans (y-axis) have to be performed by the GPR. Post Processing This is the process where all the underground utilities are identified and labelled. This process can be further sub-divided into the following categories:

* Post processing for radar maps (raw data); * Cable/Pipe detection process; * Data verification; and * Export of data to CAD/GIS.

In the case of GRED Utilities post processing software, post processing is just a click away with the pre-defined macro files. Cable/pipe detection is aided with high quality Radar Maps, 2D/3D topographic maps and automatic pipe detection tool kits. Acquired underground utilities are seamlessly exported into CAD/GIS. Case study

In January 2009, Ground Control (Australia) Pty Ltd was contracted by Queensland Department of Transport and Main Roads to perform a full 3D Utility Mapping on a one kilometre section of the Bruce Highway, located near the town of Mackay. The purpose of the survey was to identify any and all services located underground in the area designated by Main Roads. Of particular interest was the first 600 mm of depth below the road surface, as this will be removed at a later date, and services to this depth must be located to enable safe excavation to occur.

Challenges

• To map one kilometre of highway using single antenna GPR took too much time.

• Closure of a reasonably high traffic flow highway posed traffic disruption. Minimum disruption to traffic would be ideal.

• The requirement to map the underground utilities accurately (in X, Y and Z axis).

• Working through a rainy season.

• All underground assets in different directions had to be mapped and exported to update existing Autocad drawings.

Solution

The IDS ‘RIS MF’ is proposed for the 3D utility mapping of the one kilometre length of road. The RIS system efficiently provides a scan width of 2 m, maximising productivity whilst minimising road closure times. This is made possible with its multi-array, dual frequency antennas (200 and 600 MHz). Most importantly, the RIS system provides adequate ground penetration during the rainy season. In addition, RIS provides a complete and accurate 3D utility mapping solution, from acquisition to post processing, from 3D drawings and 3D topography maps to CAD/GIS. In essence, ‘real’ location/depth of underground utilities is detected, metallic or non metallic.

3D utility mapping process

The mapping job requires surveying to be done in different sections of the Bruce Highway, both north-bound and south-bound. After an initial site inspection, it was decided that it would be easier to do the survey in four different sections; minimising disruption to traffic flow as well as easier referencing with separate ‘starting points’. The next task was to mark out a grid pattern. This assists the operator in aligning the RIS system accurately with reference to the starting point, using the curb as the reference line. Over a period of two nights, four separate acquisitions were completed.

After the scanning of the area was completed, the next stage of the project was to process the acquired radar scans. GRED 3D Utilities software was utilised for the post processing: Data processing * The four acquisitions were separately post processed.

Post processing for radar maps (raw data)

* Post processing was performed using a preset Macro file.

Cable/pipe detection process using the following tools:

* Parabola fitting is performed to correctly adjust the depth of utilities. * C-scans to perform vertical slicing to locate the cable/pipes. * Multiple B-scans to mark the exact position and depth of utilities.

Data verification * Acquired utilities were compared to the plans from the Department of Main Roads. * Depth verification was performed with parabola fitting feature in GRED software and by the actual depth of pipes/cables in man holes. * 3D modelling in GRED was used to perform a 3D view of the pipes, performing ‘sliced’ view of the pipes in X, Y and Z plane.

Export of data to CAD/GIS

* All acquired utilities were exported to Autocad with the IDS_GCAD toolbar. Results

* Pipes and cables were exported into Autocad Benefits of 3D mapping Cost

* No requirements for repetitive cable/pipe locating (so long as new assets are updated in the CAD drawings or GIS) * Less frequent and shorter duration of road closures for future pipe locating * Less likelihood of damaging underground utilities, which equates to lesser repair bills and down time.

Technical

* 3D utilities map in CAD/GIS provides an accurate 3D view of the underground utilities for rehabilitation or introduction of new underground utilities * Provides a referenced co-ordinate (eg. WGS84 or local co-ordinates) of the underground assets, accurately defines the exact position and depth of the assets * Provides contractors with an accurate 3D map prior to excavation.