EDF Energy called in VolkerInfra to help bring 500MW power to thousands of new customers in Enfield, north London, including the new north London printing plant of News International, publisher of The Sun and The Times newspapers.
Having conducted an initial feasibility study in 2004 VolkerInfra was awarded the responsibility of actually laying the underground power lines. It consisted of a double 132kV circuit carried by 12 aluminium cables each needing to cover a distance of 5.5km between Brimsdown and Waltham Cross.
For much of the route the cables could be laid in trenches, where ducts are first installed and then the cables put in place using both pull and push rollers to minimise strain. But in this case the job was more involved because the journey also required the cables to cross railway lines and the M25.
VolkerInfra operatives would also be required to carry out work in three substations, two owned by EDF Energy and the other owned by National Grid.
There were other physical constraints to consider too. The team were working in a largely residential area, moving staff and plant through narrow streets. When drilling under the railway line the six-person team needed to position some substantial machinery.
And then, as if to add a new twist to an already challenging brief, the route had to be changed after the planning phase. "Before we can do directional drilling under a railway we have to prove to Network Rail that there is no safety risk. It is a very slow and difficult process," explains VolkerInfra's Fred Mastop, who managed the contract.
"The original route proved unfeasible. There were issues to do with consent because there were already some cables laid in the vicinity. So we had to come up with a new route," says Mastop. "We had to change around 70% of the route. We had to plan this re-routing, design and build all at the same time."
"We determined that there were three alternative routes the cable could take," says Mastop, "One of them proved to be significantly more attractive than the others, but it involved taking alternative railway crossings."
Under the new scheme the route would be the same for the first two kilometres, Mastop explains. However, whereas the original plan involved drilling three holes installed with ducts carrying four cables under the railway, the new one had three locations where two cables would cross the tracks in parallel and two more where three cables would make parallel crossings. "It was only slightly more expensive and there was no need to increase the cable lengths," Mastop says.
But for all its advantages the new route still needed to pass through the Network Rail approval process: "Getting permission to cross the railway tracks extended the time we needed to complete the project by six months," Mastop says.
The approval process was made a little more time consuming because one of the new proposed drilling locations was not far from a stanchion, a post which supports overhead power cables. In the end, however, VolkerInfra was successful in proving the benefits of the new scheme to Network Rail.
Despite the scale and complexity of this major power project, VolkerInfra completed it in April 2007.