Abstract
The core principles of collecting good quality data from site for creating CCTV inspection reports including some explanations of how to avoid common mistakes.
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The PLR Naming Convention
Most global drainage inspection standards include a system for naming pipes based on either the upstream manhole ID or a combination of both the upstream and downstream manhole IDs. The Manual of Sewer Condition Classification (MSCC) defines the naming convention for WRc pipe and sewer inspection using the Pipe Length Reference (PLR) system at all times:
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Rising up through the manhole from the bottom (will be explained why this makes sense in the next paragraph) as shown in the diagram above, so the Y PLR is the next highest one from the X, and the Z PLR is the highest one.
Going clockwise around the manhole from the X position (in the example above, the Y and Z PLRs would be swapped over).
Random selection. This is not recommended, because all that matters is that every pipe in the project has a unique PLR, and contractors should be consistent with how they manage and deliver their data across all projects and customers.
What if there are more than 3 outgoing pipes?
The chances of this are extremely rare but not impossible. At the time of writing this, the author has only ever come across a manhole with 4 outgoing pipes once (after 30 years in the business), and yes it was on a highways project. There is known to be a surface water manhole in the North West of England which has 7 outlets. All 7 outlets go to the same next manhole through a syphon to the other side of a river, and the 7 pipes in the upstream manhole are all along one wall and at ever increasing heights from the bottom of the chamber so that the rain water flows first through pipe 1, then through pipe 2 etc as the level of rainfall increases or pipes have failed.
The MSCC simply advises the user to use whatever character they wish after Z. The issue we have here is that under MSCC rules, this can only be one character (because the xml data format has a limit of 11 characters on the PLR value - 10 for the upstream node plus the PLR Suffix).
Then additionally, we have the very old (bit still regularly used) STC25 manhole reporting system which specifies incoming pipes as A, B, C, D etc, so using A as a PLR Suffix for an outgoing pipe may be misleading.
It is recommended to go back to the letter W and then start going backwards from there for outgoing pipes 4 and upwards (i.e. X, Y, Z, W, V, U etc).
Why is the highways drainage system shown above been constructed this way?
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This is great idea which includes built in resiliency into the drainage network and usually works well, except that during normal operational conditions, we are only alerted to a problem when all three pipes are blocked or failed, so the remedial works will be much more significant than if there is only 1 outgoing pipe.
HADDMS Pipe Referencing
The HADDMS inspection standard follows the same PLR Suffix naming convention of Upstream Node plus PLR Suffix, except that it does not X, Y and Z for the PLR Suffix. Instead, it uses a dot and a numeric incremental counter like .1, .2, .3 etc so the MH1X pipe in the diagram above would be MH1.1, MH1Y is MH1.2 and MH1Z is MH1.3.
Other than that, the logic is exactly as described here.
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Good Site Drawings
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Understanding STC25 Manhole References
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