The challenge of cabling up a city

Work is continuing on the project to run new main electrical supply cables into the centre of Derby.

It’s not a project that starts tidily at one end and follows a path to the other, but is more like doing a very long thin jigsaw.  By the end there will be several thousand lengths of new cable ducting running under Derby, with major junction boxes every few hundred metres where the lengths of cable will be joined.

At present they are working at three sites:

One set of cable ducts have been pulled through - the second is still being pulled out following the reamer which has cleared their path through the hole.

One set of cable ducts have been pulled through - the second is still being pulled out following the reamer which has cleared their path through the hole.

On the edge of the city I watched as directional drilling contractors PBU finished pulling the second cable bundle into place last Saturday.  Using their specialist equipment, they had drilled and reamed a pair of 70m long tunnels under the Hell Brook.  For them it was a straightforward job – and much less disruptive than digging a trench through the brook, even through the dry weather meant the brook was quite low.  Next, this section is due to be extended back towards the last pylon in the nearby field with open trench digging.

The ends of just-installed cable ducts are to veer right to avoid the water pipe which was expected to be in the verge.

The ends of just-installed cable ducts are to veer right to avoid the water pipe which was expected to be in the verge.

On Burton Road the team from Balfour Beatty were having a harder time.  They were finding more existing pipes under the road than expected – and some they were expecting were not where they expected.  The end result has been having to dig their trench wider, or to vary the levels of the ducting to avoid obstacles.

Further towards the centre of Derby progress seems to have been faster, where a stretch of open digging along the verge from Lyttelton Street to Kingsway was fenced off.

The work area along the foot/cycle path from Lyttelton Street to Sainsbury's.

The work area along the foot/cycle path from Lyttelton Street to Sainsbury's.

Companies operating major public services like these are known as ‘statutory undertakers’ and have a right to use the highways for their equipment.  This is overseen by the local highways authority – in this case Derby City Council.  

Finding space under Derby’s roads is not easy; it is surprisingly crowded under the tarmac, with sewers, gas pipes, water pipes, electricity cables, telephone cables and more.  However the costs of using other land may be more. 

For example, the fields just beyond the Hell Brook and Heatherton are not yet developed and would be easy to dig.   However, these fields are due to be covered with new homes.  Running cables through here would sterilise areas – houses could not be built over them in case there were a fault – and developers are not required to provide routes like this, so will charge large sums for the use of their land.

Thus the already congested spaces under our streets are seen as preferable… 

Ground radar can give a decent picture of what to expect, but takes time, money, skill and equipment.  Digging pilot holes can also enable better planning.  Some of this has already been done – and more is due to happen.

Only a few years ago the, then new, Royal Derby Hospital had new supply cables installed.  This included running two cables along different routes to a sub-station, to minimise risk of both cables being damaged by the same fault and the hospital losing power.

At present we have just two cable sets providing power to most of Derby, running side by side through Derby.  The replacement cables will also follow a common (but new) route.  Obviously the costs would be a lot higher if different routes were chosen for each set of cables, but it would be interesting to see the risk assessments and cost estimates that led to the decision to use a single route.

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