Following the well documented collapse during construction of the Heathrow Express Tunnel (HEX) in 1994, mention of the words ‘Heathrow’ and ‘Tunnel’ in the same sentence, conjured images of lengthy and intensive Health and Safety investigations, huge company fines and the vilification of the NATM in the UK. But time moves on, and BAA plc is constructing a new tunnel at Heathrow, the $88.5M, 1.3km long, 8.1m i.d. twin-bore Airside Road Tunnel (ART), which is an integral part of a US$195M project designed to provide all weather road vehicle access from Heathrow Airport’s Central Terminal to the western aircraft stands. It is also a project that BAA hopes will finally lay to rest the events of 1994.
“Any tunnelling project at Heathrow is inevitably linked to events in October 1994,” Andrew Wolstenholme, BAA Group Construction Director told T&TI, “It has long been our view that it is important to draw a line under this event and its associated issues which happened eight years ago. The key learning points arising from the Heathrow Express incident have long been incorporated into BAA’s systems. We completely changed our processes and systems for construction and we continue to monitor systems to ensure these are fully incorporated.”
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It comes as no surprise then to find that the twin bores will be constructed by TBM with a bolted reinforced precast concrete segmental lining, with each 1.7m long x 350mm wide ring consisting of seven segments and a key. The TBM in this case is a Herrenknecht 9.2m diameter machine capable of functioning in both EPB and compressed air mode.
The tunnel’s vertical alignment has been dictated by the vertical gradient (maximum ± 5.3%) and the tunnel depth (12m at the deepest point to the crown and 17m to ground level) needed for the structure’s operational criteria. Critical to the designed depth is the location of the HEX tunnel that cuts across the ART almost at the halfway mark (see diagram). Its location has forced the ART to be placed above the HEX as tunnelling at greater depths to bore under it, to increase cover, would have made the operational criteria impossible to achieve. Combining HEX and the strict gradient required, pushed the ART upwards leaving a shallow cover of just a few metres that precluded the use of NATM or sprayed concrete lining (SCL).
The project and the players
The tunnel construction contract for the ART was won by the Morgan/Vinci JV through a procurement process six years ago. The contract form is similar to the partnering ethos, in that work is carried out in a non adversarial environment with a closely integrated team operating from a single office. Mott MacDonald is undertaking all civils, tunnels and M&E definition design with AMEC carrying out the M&E detailed design and installation. Laing Civil Engineers is responsible for the heavy civil work at each portal and for managing the enabling works involving service diversions.
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By GlobalDataThe project consists of, from the east, a 140m long two-way open carriageway that leads down to an 80m cut and cover twin cell box that passes into the twin 1.3km long bored tunnels. The tunnels will be equipped with emergency cross passages every 130m and end in a 60m long cut and cover twin cell box structure that takes the vehicles to a T junction at the westerly side of the airport. The junction allows access to the remote aircraft stands and safeguards access to the planned Terminal 5 at a later stage.
Work started on-site on the 7th January 2001 with construction of the western portal, which is also the TBM launch chamber. Activities began at the eastern portal site on 21st October the same year. This will act as the reception chamber for the first eastbound drive. Upon completion the TBM will be disassembled and transferred back to the west portal and reassembled ready to start the westbound drive.
Portal considerations
Both cut and cover sections of the portals were piled and built using the top down construction method. Firstly the top slab is created followed by excavation to the base level and concrete laying of the base slab. At the launch portal (western) the 9m diameter shield has a mere 5m of ground cover. The area’s geology from surface down is 0.5m of pavement quality reinforced concrete; 4.5m of water bearing Terrace Gravels and finally 1-2m of weathered London Clay overlying good quality London Clay. The TBM crown is within 0.5m of the Terrace Gravels, with only a tunnel diameter to the overlying aircraft stand. The team recognised the potential difficulties of launching a TBM in this location and so extensive ground treatment was carried out, involving the installation of a 20m deep x 30m long x 0.5m wide slurry bentonite cut-off wall outside the launch shaft.
TBM assembly began in April this year within the launch chamber. At the eye, a 9.5m diameter circle was formed within the reinforced concrete wall. Immediately outside the launch chamber eyes, the London Clay was replaced with a 2.8m thick cement/bentonite wall extending down to the invert of the tunnel forming an area of stabilised ground. This wall acted as ground support when the reinforced concrete bored piles were broken out prior to the installationand assembly of the TBM. Following removal of the piles the slurry block was also sprayed with shotcrete to prevent the material drying out.
Monitoring points were installed in the face and the slurry was monitored for movement during TBM installation. No movement was detected.
The tunnels, TBM and lining
The tunnels will run through London Clay, but locally the excavated face will come close to the layer of Terrace Gravel which overlies the Clay. Some thin layers of claystone may be encountered within the London Clay with typical compressive strength of 20-30MPa. In order to provide confidence in the clay/ground interface, CPT’s were undertaken along the entire tunnel route at a maximum of 40m centres. Also, there is no significant water flow expected, the water table is stable and stationary with the permeability of the Terrace Gravels estimated to be 4.9 x 10-3 m/s.
The anticipated boundary of the clay and the Terrace Gravels has had a major bearing on the choice of TBM. The preferred mode of operation will be using compressed air support through the London Clay. To enable air pressure to be introduced to the TBM face as soon a possible, a circular steel can has been bolted to the inside of the face of the reinforced wall of the launch chamber. This can incorporated a double lip Phoenix seal for the shield and additionally inner and outer double skin Bullflex grout filled seals, for sealing against both the body of the tapered shield and the segments. In the short tunnel lengths where the competent clay cover is reduced, the team believes that the alternative EPB operation will be affective in controlling the ground and minimising settlement.
The TBM has 14 pairs of shove rams, positioned to accommodate the 14 possible locations of the ring’s key. When the rams are operated at 350 bar, the TBM’s maximum thrust reaches 69272KN. The segment erector has been fitted with a vacuum suction plate for lifting the 5.6 tonne segments. It is remote controlled allowing full control over ring building. The remote also controls the segment feeder and the shove rams.
The segments will be delivered to the TBM by Paulo de Nicola rubber tyred vehicles, with a full ring and the necessary grout carried per trip. Both ends of the vehicles will have an operator’s cab to enable complete driver vision in either direction.
The alignment of the machine and segments will be in the hands of a VMT guidance system.
The tunnel spoil will be transferred from the cutterhead via the screw conveyor, sizer and Putzmeister pump over the static conveyor on the TBM gantries. The muck will then be loaded onto a continuous conveyor, fixed to the side of the tunnel wall, that is extended as the tunnel advances. At the portal the conveyor discharges onto an inclined conveyor which transfers the muck to the site stockpile for removal from site.
Settlement minimisation
Historically, open faced shields previously used on the Heathrow Express achieved face loss of 1.2%. For the ART, analysis of surface settlement on the Airport Infrastructure has been based on achieving 1% face loss (around 30-40mm surface settlement). The Morgan/Vinci team expect that this figure can be improved. The TBM was designed to limit surface settlement by pressurising the face, the annulus around the shield and also pressure grouting through the tailskin. As said earlier, the preferred mode will be compressed air, as this will enable the London Clay to be excavated in its virgin state, whilst maintaining face pressure without the use of additives. The challenge was to develop a system that could achieve this, as the conventional screw could not be used to form a plug to maintain air pressure at the face. This was solved by adopting a twin barrel 750mm bore Putzmeister pump, a further development of the basic principle of the concrete pump. The excavated spoil is extracted from the face of the TBM through the screw conveyor. A sizer is located at the discharge end of the screw and the Putzmeister pump is located below this. Both the sizer and the pump are therefore fully supported by the screw and are independent of the trailing gantries. As the excavated London Clay is discharged from the screw it passes through the sizer into the pump where it is compressed by the pumps piston. Here it forms a continuous plug that retains the compressed air, ensuring constant air support at the face.
Extensive surface settlement monitoring instrumentation has been installed to monitor actual settlement in the extremely sensitive airport construction site. These include surface, sub-surface and deep arrays. Access to the surface is limited for survey at night only due to the limitations that exist within a live operational airport.
Schedule
At the time of T&TI‘s visit to the site last month, the first ring had been erected and the TBM was being prepared to start boring. The first bore is scheduled to reach the eastern portal in five months, with the JV hoping for a 10m per day advance, working around-the-clock. Once the TBM has been brought back to the western portal, boring of the second tube is expected to get underway in December 2002 and is scheduled for completion in June 2003. M&E installation is set to start in February 2003.
The finished tunnels will have an exposed segmental lining and contain a 6m wide carriageway designed to allow a vehicle to pass in the event of a breakdown. If the schedule proves realistic as the JV believes, the ART project will be open to public and transferring passengers in February 2005.
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