What's the diff between what's specified & what's engineered?

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?

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.