When we look at some of the major projects that have gone sideways, it often can be attributed to the lack of planning for underground infrastructure. This is a global issue and we know that the best way to manage the risk from underground infrastructure is to map it before construction begins. For example, the Sydney light rail project in Australia was problematic with a serious budget overrun because there was so many underground unknowns discovered during construction. Another example is a hospital project where what was thought to be a complete inventory of underground utilities missed a 20 inch gas pipe and a six inch gas pipe that were encountered on the project site during construction. Neither was documented and they were not on the asset owner's records. The challenge is to find these assets before construction in the early stages of planning and design so as to be able to mitigate any project impacts and safety concerns. Very few major projects move forward without an environmental investigation and a geotechnical investigation, so it's not clear why there's resistance to doing a subsurface utility investigation when the immediate risk to the project and the public is considerably higher.
Legacy data
Legacy data is often proprietary asset owners' "as-planned" engineering records. Relying solely on these legacy utility asset records for location information is unreliable, but augmented by GPR and other remote sensing data they provide valuable information.
Modern GIS also cannot be relied on to always provide reliable data. This image is from a GIS with an xref in the background. The middle of the street is where the asset owner believes the water line to be, but a subsurface survey actually found lines on the top and the bottom side of the street, which is nowhere near the placement according to the asset owner's records.
Even more disconcerting is that some asset owners have stopped updating their records. In one example we encountered, the last update and revision to a telecom owner's documents was in 1998. Obviously by 2022, we know there's a lot more utilities added into these streets.
The challenge for practitioners is how to find this information. Fortunately, advanced technologies, such as multi-channel, multi frequency ground penetrating radar, can augment the available legacy and EM data to provide a complete, accurate and current picture of the subsurface.
Underground locating combining traditional and advanced techniques
At DGT Associates our typical workflow starts with collecting and compiling legacy maps. We collect all legacy data that is relevant including client information and all the asset owners' maps. We add our institutional knowledge from decades of experience with construction in the particular region where we're working. We then perform our traditional EM pipe locating and add that data to the legacy data we have compiled. Together this provides the background for our ground penetrating radar scanning.
Advanced ground penetrating radar
This is a picture of DGT Associates' new ground penetrating radar setup, a drivable rig with a mobile mapping unit. This is a 24 channel system, so it captures 24 slices every time we scan. It is also a multi frequency system. Low frequencies penetrate deeper with less resolution, while higher frequencies have better resolution but don't penetrate as deeply. This system can collect about 10 miles of data per day with 24 channels for a total of 240 miles of GPR data. With Boston's soil conditions this system is collecting vertical slices down to about 8 feet. In addition with multiple frequencies we are simultaneously collecting horizontal slices.
Value of GPR in subsurface surveys
GPR is not magic and is not replacing any of the previous techniques. What is the value of including advanced GPR in locating and mapping subsurface utilities ? The first thing is safety. With our rig we do not require boots on the pavement. Secondly we get fast data collection compared to traditional locating. Thirdly, we can determine depths with GPR really well. When we see the top of the pipe, we're able to trace it in the cross section. GPR is also good at confirming the location of facilities shown on legacy maps or detected by EM. But GPR can't see everything, For example, we often can see a trench where there is likely a utility even if we can't see the actual pipe or cable. We typically need to determine from other techniques, legacy drawings or EM scanning, what type of utility we are seeing.
A major strength of GPR is finding unknowns. Going through the depth slices, we can see each one of the lines representing a utility. The type of utility is derived from legacy and EM maps and is colour coded according to the standard with blue being water, orange telecom, purple unknown, and so on. Something that's really striking in this GPR scan is the purple utility buried only a foot and a half deep, crossing diagonally across the street. Nobody knew about this utility before we went out and scanned it with the GPR.
One of the best examples of how combining GPR with legacy data enables us to detect unknown utilities, determine what they are and confirm they are still present is in Cambridge. We had compiled comprehensive legacy background information before we went out to do a complete GPR scan of the site. Among the things we found were these unique rectangular pits, shown as purple circles (colour coded for unknown). Each pit is about one foot by two feet wide, and they're regularly spaced at 12 feet. If all we had was the GPR data, we would have no idea what these were.
But because we had the legacy background maps, we were able to see that these pits line up with old legacy data showing an abandoned gas pipe. Without the background map, we would have had no idea what the GPR scan was showing. The GPR scan confirmed the location of the abandoned gas pipe and showed that it was still there.
A typical subsurface survey deliverable is colour coded according to American Public Works colour coding. All of the lines are annotated to conform to the ASCE 38-02 quality standard. Every line is represented not only by its annotations for reliability, but also includes the number of conduits and other measures of capacity, and asset owner information. Together it provides as complete a picture of the underground assets for the project site as can be assembled from existing data.
Conclusion
The light rail in Sydney went from a $2 billion to a $3 billion job because of the lack of a comprehensive subsurface survey during planning and early design. There's plenty of proof including ROI studies that there are very good reasons to use SUE surveys. There are good practitioners all over the world who can carry out subsurface surveys. Just as with above ground surveys knowing accurately the location of underground utilities and other infrastructure enables good designs because you can make informed decisions based on information collected by professionals who know what is underground and where it is.
What we see in the coming years is that subsurface surveying is going to become even a bigger business and it's going to become more lucrative. We expect to see more practitioners and more investment in innovative technologies for detecting and mapping underground utilities.
This post is based on Mitchell Lidell's & Michael Twohig's (DGT Associates) talk at the Subsurface Utility Mapping Strategy Forum (SUMSF).
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