Overhead High Voltage (HV) cabling has long been the subject of debate and argument. It was at one such debate that I met Mike, business development director at Costain, and we talked about the rationale behind undergrounding HV cables in tunnels. Mike very kindly offered to show me the current undergrounding operations that Costain and partners are building under the streets of our Capital for National Grid and so it was that I spent a very interesting afternoon in North London.
Willesden Junction is not a place I knew in London and after emerging from the Bakerloo line it became immediately clear why. It is a curious mix of light industrial, train and transport depots and some residential streets, but that made it the perfect starting off point for the 7.2km 3m diameter tunnel that would end up somewhere near St John’s Wood and meet a 13km 4m diameter tunnel that had begun in Hackney.
The excellent visitor centre, which was about to be raided by the second school visit of the day, contained some fantastic visualisations of the tunnelling work including this real time drilling chart and full size mock up of the tunnel.
But where the school tour ended was where our tour began and after donning the correct personal protective equipment and having our safety and orientation training, we went on site to witness the production line efficiency of a tunnel boring team, only days from reaching the target ‘breaking through’ point.
Accompanying us was Richard, who made sure we were safe during our visit and after first looking at the impressive muck conveyor belt, which carries tonnes of London clay vertically up the massive main shaft, we then descended the 50 metres or so to the floor of the shaft.
There we caught our train, one of several that continuously run between the tunnel boring machine (TBM) and the main shaft to ferry the precast concrete rings that make up the walls of the tunnel and the tonnes of clay back in the other direction and of course the people that work on this project. These trains take about 20 minutes to travel to the face and run like clockwork to ensure that they pass each other at the correct spot to ensure the TBM is constantly supplied.
The TBM has a crew of about ten, who work with production line-like efficiency. The process of fitting a 1.2m ring of concrete would take longer to describe than it would take to do, so here’s a two minute video showing some of the process (note: the camera shake when the blocks go in place gives you an idea of how big they are!).
It is an impressive thing to watch, and I genuinely felt privileged to have been able to join Mike and the team to watch this tunnel being built in order to futureproof London’s electricity supply. It is an ambitious project and indicative of the sort of large infrastructure investment the UK needs to make as part of the upgrade to our electricity grid, but also to support growth and investment in our construction and engineering industries. It will certainly bring some reality to my next conversation about undergrounding HV cables.