We employed the same tunnelling methods as T5 for the Heathrow Express and Piccadilly Line extension tunnels. Historic-al factors meant that there were differences in the ways each job started and finished, however.

When the Heathrow Express was built the construction of T5 was already on the horizon and stub tunnels were constructed, enabling tunnel boring machines to drive straight off with-out disrupting services to Heathrow Terminals 1, 2, 3 and 4.

To construct the new twin Piccadilly Line bores, Morgan Vinci first had to excavate a box from which to launch its tunnel boring machines (TBMs). Meanwhile, to connect the new tunnels with the existing Piccadilly Line at Terminals 1, 2 and 3 our team had to construct a new junction box. This has a larger cross-section than the adjoining tunnels, extending close to water-bearing gravels overlying the London clay through which the tunnels themselves were driven. We selected top down construction over the alternative of driving bored tunnels all the way and then enlarging them to make space for the connection.

Top down construction involves installing contiguous piled walls and then casting a reinforced concrete slab on the ground. Once the concrete has cured and gained strength, ground beneath the slab is excavated – the slab becomes a roof. Once final depth has been reached a base slab is cast. The technique was selected to eliminate the risk of hitting the water-bearing gravels and enabled Morgan Vinci to use an open-face TBM rather than a more expensive earth pressure balance machine, which would have been essential to deal with ground water.

Bachy Soletanche installed diaphragm walls for the junction box through the existing Piccadilly Line tunnels at the start of a 20 month line closure and possession, allowing the box to be excavated. The bored tunnels stopped just shy of the box – the TBMs retreated the way they had come so that the drives would not risk destabilising the existing tunnels. We created short lengths of hand dug, sprayed concrete-lined tunnel to make the connection.

Both lines terminate in a shared cut and cover station box, nestling under the main Terminal 5 building. This is equipped with six platforms in all, in anticipation of high-speed rail links from west of Heathrow and/or the arrival of Crossrail.