The US$7.3bn Terminal 5 (T5) scheme at London Heathrow Airport, in the UK, is among the largest infrastructure projects currently being undertaken in Europe. The site of the new terminal is hemmed in by the world’s most heavily utilised runways to the north and south, the M25/M4 junction (Europe’s busiest motorway interchange) to the west, and existing airport terminals to the east. It is therefore unsurprising that large portions of the works are related to tunnels, which will form part of a transportation system to the terminal.
Six tunnel projects, consisting of nine separate tunnels totalling 13.5km, are included in the scope for the integrated rail and tunnels team carrying out the work. In June last year, the 8.1m i.d. twin-bore Airside Road Tunnel (ART) was completed (T&TI September, p14). Also successfully completed is the 4.1km long, 2.9m diameter Storm Water Outfall Tunnel (SWOT), which was handed over to client BAA in January this year. The twin-bore, 3m diameter, 140m long pipejacked service tunnel carrying 33kV cables and water pipes under the A3044 road was completed in August 2002. The fourth tunnel project is the 600m long, cut and cover Track Transit System, leaving the balance of work underway in the remaining two tunnel projects, the Heathrow Express Extension (HexEx) and the Piccadilly Line Extension (PiccEx).
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The US$67M HexEx will extend the Heathrow Express rail link to T5 from the existing station at the central terminal area. It consists of two 5.7m i.d. bored tunnels, each 1.6km long, as well as a ventilation shaft and two emergency egress shafts. The down-line drive (away from London) was started at the end of February and by the end of May had driven 850m, leaving just 700 rings to completion, planned for early July. The up-line drive of the HexEx is due to start in August, with completion by November.
Budgeted at US$54M, the PiccEx will similarly extend London Underground Ltd’s (LUL) Piccadilly Line from the central terminal area to a new underground station within T5. The PiccEx station will be integrated within the HexEx station at T5. Twin bores of 4.5m i.d., each 1.7km in length, form the running tunnels and there is also a shaft and cross passages to facilitate emergency egress. Both PiccEx tunnels are driven eastwards from a launch box on the T5 site, near the existing Piccadilly Line loop that serves the other terminals. The existing loop has no provision for an additional connection, so a new junction will be formed using diaphragm walls and top down construction. The westbound running tunnel was started on 3 November 2003 and excavation was completed ten days ahead of schedule on 16 March this year, with advance rates consistently bettering 50 rings per day being boasted. The eastbound drive was started on 20 May and the 4.8m diameter open-faced Dosco TBM was just being launched when T&TI visited the site at the end of May. It is scheduled for completion in mid-September.
T5 Agreement
The designer for the T5 tunnels is Mott MacDonald, with a Morgan Vinci joint venture taking the role of lead contractor. Civil works such as the drive shafts have been carried out by Laing O’Rourke, supported by Beton-und-Monierbau for the SCL works. M&E design and installation is the responsibility of Amec for the ART. Such distinctions are, however, largely superfluous considering the innovative way BAA has structured the contractual arrangements and its associated influence on site management, the “T5 Agreement”.
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By GlobalDataBAA has a capital investment programme in the region of US$3.6M a day, which firmly positions it as one of the UK’s principal infrastructure developers and one of the UK construction industry’s largest clients. Sir John Egan, head of the task force that produced the “Rethinking Construction” report in 1998, was chief executive of BAA from 1990 to 1999. The report highlighted the UK construction industry’s lack of consistency and value for money, advocating the application of best practice to improve performance.
This philosophy continues with the way BAA procures projects and uses its major client status to effect change in UK construction industry standards. Put simply, BAA looks for “value for money” and has used the T5 Agreement as a mechanism, whereby multi-disciplinary teams that may comprise individuals from discrete companies and organisations are incentivised to solve problems and add value to project solutions, whilst moving away from a culture of recriminations for actions and decisions. Speaking about the Agreement, Ian Williams, T5 tunnel delivery manager, said the “results are on our actions” and it removes the road blocks, allowing staff to focus on building the tunnels to a high standard.
The T5 Agreement is an integrated approach to partnering and the management of risk. From the outset BAA decided that it would carry all risk associated with the T5 development and move away from traditional contractual arrangements, which it felt quickly degenerated into an “unproductive culture of blame and confrontation” when challenged by difficulties. Instead, the emphasis is on finding solutions to problems; as contractors already know BAA is liable for the risk, their efforts are best served solving the problem and receiving a share of the incentive fund. All contracts have been let according to the terms of the T5 Agreement and the expectation of the client is that the philosophy will be passed down the supply chain.
Suppliers and contractors have been involved since the planning stage, allowing for comment on constructability and alternatives. The integration of individuals into the project teams mirrors the contractual arrangements of the T5 Agreement partners on a smaller scale. This has facilitated identification of potential problems before designs were finalised and made it easier to add value within the targets.
Suppliers are required to quote transparently, without any waste built in, any risk payments are scheduled into the incentive fund pot. This is used if problems occur; if the budget is bettered, the balance is shared amongst the partners. First tier contractors are reimbursed according to actual cost and a defined profit range. Depending on the commercial model within the agreement, execution plans and sub-tier agreements, there are also fixed payment components. Excessive costs are challenged and managed through the planning process and disallowed costs are defined in the actual cost agreement.
PiccEx
The PiccEx tunnels are being constructed through London Clay using a 4.8m diameter Dosco boom-in-shield type TBM. The TBM has been drawn from JV partner Morgan Est’s fleet, having previously been used on a baggage tunnel project between Terminal 3 and Terminal 4 at Heathrow about ten years ago. The valves and hydraulics have been upgraded and the whole system refurbished and modified to comply with health and safety standards. A radio controlled vacuum erector has been installed to reduce risk to personnel by keeping them out of the build area. In a power-off test for commissioning, it was commented that the erector vacuum held for over eight hours.
At the time of the visit in May, the first PiccEx tunnel had been driven and a temporary set-up was being used to start the second tunnel from the eastbound drive shaft. Unusually, this 15m diameter segmentally lined shaft was built from the bottom up, as part of the civils works in the area as the ground was backfilled. The close co-operation between parties on the T5 project was amply demonstrated by George Jackson, construction manager for the Morgan Vinci JV, when he described how the 40m backshunt at the shaft was incorporated into the design to assist the tunnel drives.
In addition, the civils contractor carrying out the backfilling incorporated a foam concrete block at the headwall, also at the tunnellers’ request, due to concerns that, as the ground was just compacted backfill, there could be a risk of settlement as the TBM broke away from the shaft. When tunnelling is complete, the drive shafts will be capped below surface and backfilled, since the area will eventually encompass a taxiway and apron.
When the site was visited, the first ring had just been built. The initial 16 rings out of the shaft are bolted, to ensure stability until the shield is out of the headwall block and into the clay, after that expanded precast segmental lining will typically be used for the remainder of the drive. The rings are dowelled to further anchor them and the annulus at the headwall is filled with pumped concrete. The temporary set-up utilised a 7.5t Schöma narrow gauge locomotive for spoil removal to pit bottom using skips, which are then hoisted to surface by a crane. Shove rings and a frame are used to transfer shove loads into the shaft plug to maintain the shaft lining geometry. After the tunnel has been driven for 60 rings, the full gantry can be moved in and spoil removal is by means of an 800mm stitch jointed belt to pit bottom. Every 150 rings, a belt extension is required and on the first PiccEx drive this generally took only two hours. With the switch to the belt set-up, the muck is no longer hoisted by skip, but is instead brought to the surface by a High Angle Conveyor (HAC) supplied by Continental Conveyor. Working on a two belt system, the spoil is transferred onto the bottom belt where it is captured by the top belt. At the end of the lift, the belts bifurcate and the spoil is loaded onto the tip conveyor and used as backfill elsewhere in the T5 works. The HAC unit had previously been used on SWOT, as had the locomotive system, which is then used for supply trains and transporting rings into the TBM. Typically, two rings at a time are transported on four flatcars.
The 16 ram TBM is laser guided by the ZED Basic System, which reads to the back of the machine and gives a predicted forward position based upon no further action being taken. The TBM is also equipped with the ZED Global Co-ordinate System version 1.25, which does not require the laser beam to be set to the designed alignment. Keys benefits are that offsets are no longer required to be calculated or entered, reducing the potential for errors and the position can be downloaded to a portable terminal referenced to a global co-ordinate system. The driving tolerance for the PiccEx tunnels are +/-25mm for both line and level, and at the time of the visit the tunnels were said to have remained within these tolerances.
The PiccEx tunnels are driven to within 3.5m of the existing Piccadilly Line loop, which serves the other terminals. On completion of both drives a junction will be constructed using a diaphragm wall box from the existing tunnels to the new works. The ends of the new tunnels are sealed with a sprayed concrete headwall until the junction works are complete. This work is scheduled to take place during the loop closure, which starts in January 2005 for 20 months.
HexEx
The 5.675m i.d. HexEx tunnels are also being excavated through London Clay. A Dosco open-faced TBM with a roadheader boom is again the contractor’s choice and this machine also has a history at the airport, having been used on the original Heathrow Express tunnels. Both tunnels are driven eastwards from cut and cover launch chambers, which will eventually be capped and backfilled in a similar manner to the PiccEx drive shafts.
The HexEx TBM has also been upgraded and modified in terms of its hydraulics and again has had a radio controlled erector installed for personnel safety. The increased size of these tunnels has allowed the use of rubber tyred transports previously used on the ART, supplied by Paulo de Nicola, for segment delivery of two rings at a time and other supplies to the TBM. This travels along the tunnel invert directly on the rings and it was noticeable how clean the invert has been kept without sleepers to snag material or hinder clearance. Spoil removal is again by conveyor through the tunnel and a HAC system at pit bottom.
Typically the works are constructed on a five-day shift pattern, but at the time of the visit continuous working had been adopted. This was a planned measure for passing critical infrastructure such as the existing Piccadilly Line loop with only 6.5m of cover, followed by an aviation fuel main and then holing through to the sprayed concrete cross passage at the T5D shaft.
The cross passages along the tunnels are for ventilation and emergency egress at the shafts. They are all constructed with a sprayed concrete lining by the tunnels team supported by Beton-und Monierbau using the Tunnel Beamer system and LaserShell lining (T&TI June 2003, p38). A robojet sprayer is used to build up the wet-mix steel fibre reinforced shotcrete, which also includes polypropylene fibres for fire damage mitigation, in single-pass overlapping layers. The cross passages are constructed in advance of the TBM passing the particular shaft, an enlargement of the cross passage dimensions at the end allows the TBM to pass through and install a temporary bolted lining, with final connections made when the drives are complete.
With advance rates of 23 rings per 12 hour shift being achieved on the HexEx, wear to the cutter head has been characterised as “only having to change the odd pick”. The tunnel is to be driven to an existing stub chamber that was formed when the Heathrow Express tunnels were constructed. The TBM will then be dismantled and transported back through the tunnel to the launch chamber to start work on the second drive.
Safety and welfare
Safety on the T5 project is characterised by the “One In A Million” campaign that aims for a maximum of one reportable accident per million manhours worked. Again, this involves BAA attempting to impose a culture of change, with the workforce encouraged to value safe working practices themselves rather than being disciplined with codes and rules. The entire tunnel workforce is directly employed by the JV and continuity of work has been built into the programme, which allows for teams to become established and the reinforcement of safe practices and lessons learnt.
Management described a “zero tolerance to unacceptable behaviour”; however employee turnover rates on tunnels are very low, indicating the regard in which the workforce holds safety. The works are recognised by many UK tunnellers to provide a good rate of pay, the canteen and facilities are truly top class; all part of the holistic approach adopted on site. The words of construction manager, George Jackson, are echoed by the enthusiasm and results achieved by the workers, the “T5 Agreement is the way to go.”
Related Files
Map showing the tunnelling works required for the Terminal 5 project
