A very interesting recent academic study from the University of Birmingham investigates the effectiveness of the British PAS128 standard for underground utilities. Four different utility survey specialist firms were invited to locate (without potholing) underground utilities in the same area. They were asked to assign a PAS 128 quality level to each utility piece of underground utility equipment detected. The researchers concluded that the agreement between the different locate companies was very good, although some equipment was not detected. With respect to PAS 128 test pits revealed that achieving a high vertical accuracy was problematic. While using multiple technologies for underground remote sensing increased the confidence in what was detected it did not increase the accuracy.
The survey area was a 15 m by 20 m asphalt-covered car park surrounded by grass on two sides. The site was chosen because it was sufficiently complex below ground to require distinction between utility networks and not subject to traffic. Four companies carried out the geophysical survey and returned their results with respect to a local grid. Following the analysis of the commercial subsurface utility surveys, a number of locations were suggested for trial pit excavations. The aim of these excavations was to reveal the physical nature of the assets detected by the subsurface scans and to evaluate whether the features detected were both complete and accurate. All data were captured in three dimensions – that is, plan locations and depths were recorded as related to a site grid. The trial pits were specifically located to confirm the presence or absence of recorded features and identify key feature types.
Comparing the results from the different companies it was found that there was good agreement for the different utility types especially for the water and telecoms networks. The biggest differences observed were for the drainage and electricity utility networks. An electricity cable and a drainage pipe remained undetected by most of the companies. The electricity cable was detected by only one of the survey companies, apparently because the cable carried a significant electrical load on the day that that company conducted its survey. Missing a drainage pipe is probably not hazardous for the public and workers, but hitting an electric cable during excavation can have very serious consequences. However, for an industry where the rate of detection is roughly 70% and decreasing as new types of buried equipment are being introduced, this is not a surprising result.
The researchers also assessed the PAS 128 quality level standards by digging test pits to verify location accuracy and identification of the type of utility equipment. As a reminder PAS 128 Bx quality levels are defined as
XY Z
B4 Undefined Undefined Undetected
B3 +/- 500mm Not determined One geophysical technique
B2 +/- 250mm +/- 40% of detected depth One geophysical technique
B1 +/- 150mm +/- 15% of detected depth Two geophysical techniques
When comparing the results reported by the survey companies with the location and type of utility found through test pits it was found that the vertical accuracy corresponding to QLB1 was rarely achieved. Having two different technologies providing a strong signal indicating a buried feature increases the surveyor’s confidence in the detection of this asset, but does not increase the accuracy because each technology has its own accuracy depending, for example, on frequency, soil type and conditions, asset size and material. I expect that the revision of PAS 128 currently underway will take this research into account.
Comments