The 1.2km, twin bore airside road tunnels (ART) are, at 8.8m diameter, the largest of the five new T5 tunnels and required particularly delicate tunnelling. The ART crosses the existing Piccadilly Line tunnel with 3m clearance and the Heathrow Express tunnel with 5m clearance. Excessive movement of either existing tunnel would endanger rail-way operation and could force suspension of services.

The airside road tunnel also passes beneath taxiways. Ground movement resulting from tunnelling had to be limited to ensure the safety of BAA’s operations. This was achieved in a number of ways.

We tuned the vertical and horizontal alignment of the ART to minimise the influence of tunnelling. In plan, the tunnel is curved to avoid as far as possible taxiways and apron areas, while in elevation it undulates, with a
W-shaped profile.

Near the portals at either end the twin bores of the ART are extremely shallow. Risk assessments showed that if excavation broke out of the stable London clay stratum into the overlying gravel layer, there was a danger that settlement could result. Therefore, before tunnel driving
started, we excavated the gravel within 2m of the tunnel crown adjacent to the location of the portals and replaced it with concrete.

From the portals the tunnel bores descend and run for a length at depth, well within London clay which, though subject to swelling, is a stable material meaning that risk of ground movement is reduced. The airside road tunnel rises to clear the Piccadilly Line and Heathrow Express tunnels, but drops in between them to maintain maximum depth.

To minimise ground movement we designed a bespoke tunnel boring machine (TBM), allowing pressure to be maintained on the tunnel face even while the ground was being gouged away. Earth pressure balance TBMs are well established for soft and variable ground but had never previously been used in London clay, which is stable and generally considered an ideal material for tunnelling.

The decision to go for an earth pressure balance machine was taken because risk assessments identified the potential for settlement, over the shallower sections of the drive, of as much as 150mm using a conventional TBM.

German TBM supplier Herrenknecht designed a machine that would maintain pressure equal to that of the surrounding ground through use of compressed air. This prevented ground collapsing ahead of the TBM as it advanced, so keeping ground settlement within tolerable limits. Grout was injected into the gap left between the ground and the tunnel lining as the TBM advanced in order to prevent relaxation of the ground causing settlement.

Settlement was closely monitored during excavation through the use of instrumentation within the rail tunnels and on the surface, and surveys
for distortion.

Airside road tunnel portals

At the airside road tunnel portals the transition from tunnel to ground level
is made in cut and cover boxes – we constructed piled walls, excavated the ground between the walls and constructed the base and roof slabs. Conventionally the piled walls would have been propped with massive temporary steel struts to resist overturning forces imposed by the surrounding ground as excavation advanced. But careful analysis of the piles combined with a carefully planned construction methodology enabled us to eliminate the struts, saving 31 weeks on the excavation programme and greatly reducing the safety risk associated with handling and erecting temporary steel struts.

The piled walls were deemed to be strong enough to work in cantilever mode during excavation, but were closely monitored as excavation advanced. We carefully orchestrated the excavation sequence itself, removing soil in 500mm bands and monitoring levels using laser surveying equipment.

We provided temporary support at intermediate levels as work advanced by pouring concrete slabs directly onto the base of the excavation – the slabs acted as struts, bracing the piled walls. They were progressively broken out as excavation to full depth advanced and the permanent base slab was cast.

Meanwhile, if movement was detected, additional slabs could quickly be installed by our team to provide support and prevent excessive deformation. We designed for the job a special concrete mix that cured and provided strength quickly.

In fact, no unplanned propping was needed.