Construction of foundations for Terminal 5C threw additional challenges at our design team. By the time foundations for T5C came to be built tunnelled extensions to the Piccadilly Line and Heathrow Express were already in place.

Alignment of the tunnels was dictated by the need for physical proximity between stations and arrivals/departure halls, and by the layout of the site, which was boxed in by the M25 motorway and Heathrow’s north and south runways.

Sensitive excavation of the basement was required just metres from the new underground structures. Swelling of the clay had potential to affect the tunnels with destabilising affects. Tunnel linings are composed of precast segmental concrete rings. In the same way that gravity holds together the blocks making up a Roman bridge, tunnel lining rings are supported by the compressive force of earth pressure.

Vertical or horizontal squeezing would have caused the tunnel to deform, compromising the space within the tunnel needed to safely accommodate the movement of trains; longitudinal or torsional deformation would have interfered with track geometry affecting ride quality and potentially track safety.

Meanwhile, as swelling occurred earth pressure on the tunnel would decrease. Compression within the tunnel lining would normally stand at 1300-1500kN/m2. So we had to maintain a minimum of 50kN/m2 during and following excavation. If compression fell below this there was a danger of the tunnel lining becoming unstable and collapsing. Concerns were focused particularly on the Heathrow Express tunnel, the crown of which was a mere 3m from the bottom of the excavation.

We carried out analysis of the tunnels to assess what movement was likely to occur and to design a construction sequence that would result in least ground movement. The design process also set in place trigger levels at which mitigating measures would be taken to halt ground movement.

We staggered excavation to minimise the amount of unloading at any one time. Our team also mounded on adjacent areas to maintain loading as excavation was carried out. We installed huge holding down beams connected to anchor piles above the Heathrow Express tunnel. These acted like giant staples.

We installed movement sensors in the tunnels and surveyed them in real time during excavation of the foundations. Throughout the construction process none of the trigger levels for mitigation were reached.