In June 2002 tunnelling on the first of two bores for the Airside Road Tunnel (ART) project commenced under Heathrow Airport’s infrastructure and over the Heathrow Express running tunnel. One year later, in June 2003, the second bore was successful completed, and significantly, meeting the challenges of achieving minimal ground movement and zero unplanned impact on the operation of the airport and the Heathrow Express service.

Project background

Heathrow Airport Limited (HAL) strategic planners identified a major opportunity to further enhance the effectiveness of Heathrow if a fixed, all weather road link was provided that linked the remote aircraft stands in the west to the Central Terminal Area (CTA). After numerous studies a road tunnel was considered to be the most effective all round option. From this, the ART project was created.

ART is a twin-bore 8.1m i.d. tunnel 1.3km long, connected by escape cross-passages with two sumps at the low points. The eastern portal, located at the CTA, consists of a 140m two-way open carriageway leading down to an 80m long cut and cover twin-cell box and connects into the bored tunnels (Figure 1). The Western Portal, consists of an open T-junction, allowing vehicle access from the remote aircraft stands and an access to the future Terminal 5 (T5).

The ART team consists of five companies working within an integrated team framework based on a contractual agreement that holds behavioural commitments central instead of traditional contractual obligations. The companies include BAA (client), Morgan Vinci JV (tunnel constructor), Laing O’Rouke (civil constructor), Amec (mechanical and electrical design and installation) and Mott MacDonald (tunnel and civil design and mechanical and electrical concept design). The Management Team consists of members from all the companies, selected on their ability rather than company allegiance.

The challenges

A balance had to be struck between the needs of the user, HAL, and the buildability of the scheme. To meet operational requirements the position of both portals was fixed. The tunnel needed to be as short as possible, with a 6m wide carriageway and uni-directional travel.

This resulted in the need for two 8.1m i.d. parallel tunnels. Vehicles intending to use the tunnel could only operate at gradients up to 1 in 19. This, therefore, fixed the vertical alignment constraints. As the tunnel would pass beneath the infrastructure of one of the busiest airports in the world, the main challenge was maintaining ground movements caused by the tunnelling to a level that would not affect the airport infrastructure, services and facilities. A further constraint was that ART would have to pass beneath or over the Heathrow Express running tunnel.

Meeting the design challenges

Although extensive records of sub-soil conditions were available from the previous construction in the area, further ground investigation to accurately establish the London Clay/gravel interface was considered critical. The vertical alignment of the tunnel would be very shallow and hence in close proximity to the clay gravel interface at some locations along its length. Static cone penetrations were undertaken at generally 5m-10m intervals along the route and at closer centres in the vicinity of the Heathrow Express crossing, where the clay cover was shallowest, to provide additional confidence of the depth of the clay/gravel interface. The ground conditions in the area consist of Made Ground, typically 2m thick overlying 4m-6m of water-bearing Terrace Gravels, and underlain by London Clay.

Given the vertical gradient constraint and the location of the portals it was concluded that the only feasible vertical alignment was that ART had to pass over the Heathrow Express tunnel. In order to reduce the settlement and minimise the construction risks the tunnel was deepened to maximise the clay cover over most of the length with the exception of the crossing over the Heathrow Express and the portals. This resulted in a vertical alignment that took a ‘W’ shape with low points mid way between each of the portals and the Heathrow Express crossing. At the crossing point over the Heathrow Express a balance of ensuring sufficient clearance between the ART and Heathrow Express (3m), and also adequate clay cover above the ART (3m) had to be achieved. The final vertical alignment gave a clay cover to the crown varying from 0m to 11m (5m-16m cover to surface).

While the end points were fixed, there was some flexibility in the determination of the horizontal alignment. To meet the challenges of minimising impact on the airport infrastructure an alignment was selected that would, where possible, pass beneath the ‘grass islands’ between the taxiways. By adopting this alignment it minimised the impact of the surface exclusion zones on airport operations and significantly reduced the aircraft stand ‘outages’.

Meeting the construction challenges

The challenges posed were to build two shallow, large diameter tunnels, with minimal clay cover while maintaining low ground movements and without impacting on the airport and Heathrow Express. The selection of the TBM and its performance criteria would be crucial. It was therefore imperative that the selection of the excavation method ensured that risks were reduced to an acceptable level and minimised the likelihood of damage and disruption to the airport operations (T&TI, July 2002). In addition, there was an aspiration that the excavated London Clay be retained in a condition that it could be used within permanent works at a future time on the T5 development.

The Team, with Herrenknecht, developed a TBM to meet these challenges, capable of operating in Earth Pressure Balanced (EPB) mode and as a closed face TBM, providing face support by mechanical means in association with compressed air. Neither of these principles had previously been used in London Clay.

To provide confidence that the systems were feasible, large-scale trials were undertaken at Herrenknecht’s factory in Germany. The trials demonstrated that the London Clay, with the appropriate conditioning, could be turned into a ‘paste’ for operation in EPB mode but this was found to be difficult and the energy consumed in this process was very high. Trials were also carried out on the compressed air support proposal. This determined that the conventional screw conveyor used by the TBM in the EPB mode could not form a plug of virgin clay that would maintain the necessary air pressure at the face.

The Team then had the challenge of developing a mechanical system that would permit the excavation of the clay in its virgin state, but still be able to maintain the face support either mechanically or with compressed air. The solution was to incorporate an adapted Putzmeister double piston pump into the excavation removal system at the outlet of the screw conveyor. The pump was specified to meet anticipated TBM advance rates of 50mm/min. This resulted in the pump needing a capacity of 400t/hour and a piston diameter of 750mm, the largest pump of its type and unique for this type of operation. The trials demonstrated that both modes of operation were feasible with the dual mode TBM, and would fulfil the criteria that had been established.

The key features of the TBM were that it had approximately a 70% closed-face cutterhead in order to provide face support. The annulus around TBM was filled with bentonite to provide ground support and retained by the cutting bead at the front of the skin. Continuous tailskin grouting interlocked to the TBM advance. The tunnel was lined with 1.7m long, 350mm thick steel reinforced concrete segments. The ring was tapered to aid with steering and alignment.

TBM operation

Extensive data was gathered in the early stages of the first drive to verify if the original design settlement predictions were realistic. This allowed the TBM parameters to be modified after the first 300m of the drive, to control ground movement to a very tight range. While the TBM had the capacity to operate in open or closed mode for the majority of both drives the TBM was operated in the semi-open mode. Ground movement was controlled with air pressure in the plenum chamber varying from 0.5 to 2 bar depending on the ground cover. The bentonite pressure around the annulus and the grouting pressure were also varied and linked with the face pressures. These parameters were planned in advance by the Team. Continual monitoring of ground movement and TBM performance enabled fine tuning of the parameters.

As the TBM advanced, the tapered shield and the injection of the bentonite into the annulus around the shield provided immediate ground support. The segments were backfilled with a cementitous grout, injected through six grout tubes located at the rear of the tailskin. Each grout tube is equipped with pressure sensors and is operated independently via pumps and interlocked to the TBM thrust rams, to avoid the possibility of the TBM advancing without the void being filled which would otherwise induce settlement.

TBM performance

The eastbound tunnel drive commenced on 22 June 2002 at the eastern portal and was completed on 16 December 2002 at the western portal. The construction was progressed on a 24 hour and 7 day basis per week. As the dual-mode TBM was a relatively new concept, and with the level of sophistication of the TBM, it took considerable time for the operators to become familiar with its operation. After the installation of the full TBM back-up and the tunnel conveyors for removal of the spoil to the surface, the TBM was able to achieve an advance rate of 12m/day with a proven capability of regularly achieving 20m/day.

Following the completion of the first drive, the TBM was lifted out of the reception chamber, using a 1,200t crane, in one 600t lift that is believed to be the largest TBM-lift ever carried out. This was an alternative to removing each of the four sections of the TBM, and the central drive unit. It resulted in an overall saving to the programme. The TBM was then transported across the airport to the launch chamber, and lowered onto the launch cradle, ready for the second drive. The TBM and its associated equipment were transported across the airport on a low-loader vehicle between 23.00hrs and 05.00hrs when there are no flights into the airport. The whole process was highly successful.

The Westbound drive commenced on 23 February 2003. A major improvement in start up time compared to the first drive was achieved, as knowledge had been gained from the operation and performance of the TBM during the first drive. The performance matched the programmed rate. Following the installation of the complete TBM back-up and conveyer, advance rates exceeded those forecasted. Best progress in a day was 25.5m and for a week was 142.8m.

Approximately 170,000m³ of London Clay was excavated and remained suitable as fill material, due to the method of excavation developed for the dual-mode TBM. All of the clay was utilised within the T5 development as an engineering material.

Ground movements – was the challenge met

A robust approach to risk mitigation and management was adopted for the ground movement prediction and monitoring regime, from which detailed contingency measures based on pre-determined trigger values were established with the relevant stakeholders or asset owners. In the design stage a conservative design volume loss of 1.0% yielded predicted settlements of approximately 40mm. A trough width factor of 0.45 was also adopted giving a zone of influence of some 15m either side of tunnel centreline.

Tight permissible movement tolerances trigger limits were imposed on key structures. For the aircraft stands and taxi-ways the maximum operation slope differential in any direction of 1 in 60 and 1 in 100 respectively, was adopted to ensure adequate drainage, and safe operation of aircraft was maintained. For the Heathrow Express, running tunnel trigger limits were prepared for track geometry and tunnel lining deformation. For deformation the maximum trigger value set was a change in tunnel diameter of 34mm.

Damage to the fuel hydrant system, supplying the main supply for the airport, had the potential to severely impact airport operations and spillage contamination would be a serious environmental concern. The pressurised fuel pipelines are about 30 years old and were not designed to accommodate significant movements. The fuel mains are normally laid to gradients of 1:500 to allow the removal of accumulated water at drain-off low points as an essential safety feature. Any induced movements would potentially alter the fuel main profile and render the drain-off points un-operational. Based on the settlement predictions the fuel companies had recommended various advanced works to protect the fuel pipelines. However, it was agreed that the TBM’s performance would be observed over the initial stages of the tunnelling operations to determine if these works would be required. As a result of the TBM’s performance only a small proportion of works to the system had to be carried out.

Precise levelling points were used to monitor ground movements along and perpendicular to the tunnel centreline, to monitor the surface and near surface movements. To alleviate concerns regarding the formation of voids below the 450mm thick concrete pavements, which could provide an arching effect, ground anchors isolated from the concrete surface were installed to record movements below the pavements.

Real time movements at hourly intervals, using a robotic total station, were also gathered to evaluate in detail the response of the ground from the advancing TBM. This allowed full optimisation of all TBM parameters to achieve minimal ground movements. The performance of the TBM enabled a high degree of confidence to be established; to undertake the tunnelling operations under the operational airport, the pressurised fuel mains and over vital services with minimum impact. A key risk mitigation measure was the use of moving ‘Exclusion Zones’ co-ordinated with the airport operations team. The purpose of this was to exclude aircraft movement directly above the TBM thereby reducing the consequential effect if major ground movements did occur. This procedure had minimal effect on the movements of aircraft.

The actual maximum heave and settlement recorded was about 15mm. This equates to a worst volume loss of 0.35%. The average volume loss was about 0.2%, which is significantly less than the conservative design volume loss of 1.0% adopted at the design stage. Over the length of the tunnel 63% of the route was within +5mm and –5mm, 89% was within +10mm and –10mm, with a maximum settlement of 15mm.

The alignment of ART crossed perpendicular over the existing Heathrow Express tunnel. Real time monitoring was installed in the tunnel to ensure the track and tunnel condition remained within operational tolerances allowing the unrestricted operation of train services on the Heathrow Express system while the TBM was driven above. The overall movement to the Heathrow Express tunnel was 2mm. There was no noticeable change to the cant, track-twist or gauge of the track. Trains on the Heathrow Express operated as normal throughout the tunnelling, and any associated work for monitoring was undertaken within normal maintenance closures.

The tunnel passed under the fuel pipelines in four locations without any disruption to the service and with no remedial works required

Key achievements

A TBM was designed to meet the challenges of constructing approximately 2.4km of large diameter tunnel with limited clay beneath Heathrow’s infrastructure, strategic pressurised aviation fuel pipelines, and twice crossing over the Heathrow Express Tunnel with only 3m clearance. The overriding success of the TBM has been the extremely impressive settlement control, while still maintaining the integrity of the excavated London Clay. The challenges were met and the result was zero impact on the integrity and operation of those structures. This was a major tunnelling achievement representing the commitment, the technical competence and the innovation of all those involved working in an integrated project team managing risk to reach a common goal.

Related Files
Cross section of the ART tunnels and cross passage
Map showing the ART alignment