Working under pressure

Two weeks and a bit. That was the time it took to jack 22 rectangular concrete elements under the busy road Lyngbyvejen. The tunnel ensures that staff at DONG Energy can get safely from the car park to the workplace.

There was pressure in more than one way when MT Højgaard won the main contract in August 2010 to construct a car park and connect it – via a subway – with DONG Energy's head office. The challenge was to provide access for the staff to get from the car park to the workplace, which involves crossing Lyngbyvejen.

Lyngbyvejen north of Copenhagen is heavily trafficked, so it would create chaos if even a small amount of traffic rerouting was required during the construction period. The solution was therefore to make a 55 metre jacked tunnel under the road.
− The project required significant internal team effort since we were pressed for time. Our Design & Engineering department helped us design the elements and provided us with invaluable consultancy concerning the tunnelling itself. In addition, we had great help from virtually every department. Through our joint efforts we were ready to start construction on 20 November, says project manager Morten Pyndt, who highlights the excellent work of our sales and tendering employees, without whom MT Højgaard would not have won the project.

The jacking process

To jack 22 rectangular concrete elements under a motorway is naturally not something you just do. Everything needs to be carefully and thoroughly thought out.
− There must be a absolutely exact plan for the entire process. It might sound obvious, but there is no time to reconsider things once the whole machinery has started, explains Morten Pyndt.
Before jacking of the concrete elements into the ground can start, there is a good deal of preparatory work. Sheet piling and excavation for a starting and receiving pit needs to be done, and a jacking wall and the rectangular elements must be produced. In this case it was also necessary to modify the tunnelling machine with a specially designed steel shield which could be used for rectangular concrete elements.

The jacking itself starts by lowering the tunnelling machine into the starting pit. On the front of the machine there is an electrically powered hydraulic digging arm, which is operated from a small control panel at the front of the tunnel. The digging arm is used to loosen the ground while the four large jacks push from behind. The loosened ground is transported away by means of a small skip that is pulled back and forth. A mobile crawler crane lifts new concrete elements down into the tunnel, when the previous one has been pushed sufficiently far ahead.
Jacking takes time. What actually controls the speed is how fast the surplus soil can be removed. 

Close to perfection

On 6 December the tunnel machine broke through on the other side of Lyngbyvejen, and so completed the jacking of the 22 concrete elements.

− From having been under a great deal of pressure, we are now in a situation where the jacking project has been close to perfection in its execution. In addition we have acquired valuable experience in tunnel jacking so we are very well prepared to carry out similar projects in the future. That is a nice position to be in, says Morten Pyndt.