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Services Buried

Services Buried

This week I have been spending most of my evenings musing on buried services. This is not part of a new and desperate technique for getting to sleep, but the result of some interesting night shifts. In my short experience as an engineer I think that few things change as much between design and as-built as the route of new buried services across existing.

In an ideal world you’ll want to dig a full length trench, exposing all the services in their as-built location. This would give you the opportunity to plot an optimised route through all the intersecting existing buried services; matching the specifications of all involved.

As a designer your first hint of what is underground is given through a C2 or C3 survey. Essentially this just means writing to every conceivable statutory utility supplier in the area and asking them to provide information about any buried services across the site. Aside from highlighting that there may be something there, this information is no where near as helpful as it sounds, or perhaps should, be. There will be no indication of depth, the location shown is indicative at best, and apparently fabricated at worst. More over it is entirely possible that some of the services shown won’t be there, or worse; services not shown will.

In attempt to manage this considerable risk further CAT and GPR scans are carried out. Effectively sonar for the street, these provide are more refined view of what is going on. Locating services, however, is a bit of a black art- and although we now put identifying tape down, the majority of services are unmarked and don’t appear clearly enough. This is especially the case when one line runs under another.

Eventually, however, you’ve got to come to some conclusions and provide a drawing that shows the route this new buried service will take crossing the road. This means taking into consideration the guidance off all third parties and ensuring that there is adequate cover to protect the new pipe. Finally a route will become apparent, and a contractor will send out a team to install the new buried service. This is where it all becomes more interesting.

In an ideal world you’ll want to dig a full length trench, exposing all the services in their as-built location. This would give you the opportunity to plot an optimised route through all the intersecting existing buried services; matching the specifications of all involved. The nature of our infrastructure means, however, that the majority of this work is undertaken in limited (~6 hour) night shifts, and such a luxury cannot be afforded. The best you’re likely to get is the trench being dug in 1-2m advanced lengths.

Considering that the surrounding live services necessitates hand digging: The guys actually doing the work will, of course, want to keep the trench as shallow as possible- both to get the job done quickly and limit the need for temporary support to the excavation. This ideal minimum needs to at least provide adequate cover, which is normally stipulated at 750mm. Additionally it needs to keep without the influence of other foundations (normally assumed to spread at 45 degrees), which is easier for shallow excavations.

As a designer you want to protect the pipes as much as possible. Although many ducts and pipe forms are designed for buried loading, few can sustain the knife-edge loads associated with traffic. The aim is therefore to ensure that the surrounding provides enough distribution to prevent this. A sure fire way of doing this is to specify concrete, but that means waiting for it to cure. Using a dry mix can help to address this, and allow more immediate compaction. A good depth of quality fill will also go some way to providing a similar distribution- especially in conjunction with a solid bitumen road repair.

Typically gas mains want 150mm of sand between them and any hard spots created by the new services. Similarly electric cables, etc. specify 300mm. This is covered by adding a specification to instruct minimum covers in the likely case of unexpected services or locations. In the scope of digging a trench by hand accommodating this is quite a significant ask, with the nominal depth easily falling to 1-1.5m.

It is not, however, unthinkable that two services will be found; positioned such that these blanket specifications simply simply can’t be accommodated within the minimum bend, and the need to dig in short lengths means that ducking under will require significant back dig. Diversion is always an option, but that involves a lot of risk- what are you diverting in to, and how are you going to get back?

Perhaps the best approach comes in understanding the difference between what is specified and what is engineered.

For example, dropping the cover to 600mm can be justified if the backfill will provide enough distribution. Similarly getting closer to existing services might be considered allowable if the surround is lost and a finer material, such as sand, is used to bed the difference.

Ultimately, like a lot of engineering, it will all come down to experience.

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Comments

  1. Helen

    I’ve always struggled to understand why the way we install services has not been standardised to some sort of integrated slotted system and we still effectively chuck them into the ground and wrap a bit of coloured sticky tape around them.

    It is not uncommon to find that an uncharted gas main flies through your carefully designed drainage. Water and foul drainage aside, why can’t everything else follow the same line?
    Cost saving on time/labour vs capital cost of additional cable plus slot frame would be a reasonably simple exercise.

  2. Like railway toughing for roads? Sounds an interesting proposition, and would solve the majority of issues that surround getting cables into highways; although it might be a bit too limiting considering the number of branches coming off your average residential utility line.

  3. Helen

    Aha! Good point there Thomas. Looking at a residential street I cannot see how using this ‘troughing’ would make any economical sense to use all the way to the doorstep. However you could certainly use it for future developments to contain the main spread of cables over the main road. Line the troughing with a detection cable so you can trace the lot in one go and maybe even line it on two sides, one of which would have breaks in the line so you can see where a group of services branch off towards a house. In my mind this works.

    The installation of services is not the time consuming activity. Detecting them is.. and then digging is another nightmare altogether. Time consuming, costly and dangerous.

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