The Morgan Vinci Joint Venture that is currently constructing the Airside Road Tunnel (along with Laing O’Rourke, Amec and Mott MacDonald), for Heathrow Airport Ltd, has also been contracted to construct all of the bored tunnels (approximately 13.5km) associated with Heathrow’s US$5.8bn Terminal 5 Project. This work consists of a Storm Water Outfall Tunnel, a service tunnel under the A3044, and extensions to the Heathrow Express and the London Underground‘s Piccadilly Line. Although a relatively small part of the overall T5 Project, the timely delivery of the T5 Tunnels is critical to the successful opening of the Terminal. A condition of the Secretary of State’s approval for the project was that the Piccadilly Line and Heathrow Express extensions must be operational for the main opening of the T5 Project. In addition, the Storm Water Outfall Tunnel is critical to the overall project, as it will provide the necessary capacity to take surface water run-off from the new stands and taxi-ways as they are constructed.
Tunnelling within an airport environment, sometimes with low cover, means control of settlement is vital. Tunnels pass under the southern runway, taxi-ways and aircraft stands and, in some instances, very close to sensitive services such as fuel mains. In addition, tunnels cross below the Piccadilly Line tunnel and above the Heathrow Express tunnel.
To comply with the client’s employment philosophy, all labour has to be directly employed. This is quite a challenge for a tunnelling project of this scale. In addition to the challenge of obtaining experienced tunnelling personnel in the numbers required, labour continuity has to be ensured while working within the larger T5 Project environment, where there are many conflicting constraints on programmes for the various tunnel sites.
An integrated team approach
To meet the challenges set out above, an integrated team approach has been adopted for the works, as championed by the client BAA. Right from the concept design stage, both the designer (Mott MacDonald) and the constructor (Morgan Vinci JV) have worked closely together to develop optimum design and construction methodologies. Austrian tunnel contractor Beton-und Monierbau also provided additional sprayed concrete lining (SCL) expertise. This team was co-located in the BAA design offices throughout this design period.
In order to ensure full integration across all the tunnel projects during the construction phase, a co-ordination management team was formed. This is yielding benefits in safety, quality, cost and programme. Lessons learnt from one project are immediately incorporated into the others, providing a consistent approach to safety and quality. The overall tunnels programme has been optimised to achieve continuity for labour and maximum reuse of plant. Much of the tunnelling equipment, including major items, will be reused resulting in significant supply chain benefits.
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By GlobalDataStorm Water Outfall Tunnel (SWOT)
The US$24M Storm Water Outfall Tunnel (SWOT) is required to take the increased surface water run-off from the new Terminal and it’s associated stands and paved areas to an area approximately 2km south of Heathrow. The 2.91m i.d. tunnel is 4.1km in length, and is being driven with a Lovat M131 full-face TBM using pre-cast fibre reinforced segmental wedgeblock lining. The outfall system also incorporates an inlet and outlet shaft, with two intermediate shafts, that are necessary for the maintenance operation of the completed system. These shafts range from 6m to 10.5m in diameter. The shafts are being sunk by caisson method with pre-cast concrete segmental linings through the water-bearing Thames Gravels and keyed into the underlying London Clay. The remainder of the shafts and the tunnel eyes are then constructed using SCL.
The tunnel is being driven from the 10.5m diameter inlet shaft. Various options were considered for segment and spoil handling. The selected option was to use a narrow gauge rail system for delivering segments to and removing spoil from the TBM. A bunkering and conveyor system is then utilised for tipping and spoil removal in the pit bottom of the shaft.
The conveyor system, supplied by Continental Conveyor, includes a chain conveyor and sizer in the pit bottom and a High Angle Conveyor (HAC) to raise the spoil from pit bottom to a spoil heap on the surface. This is believed to be the first time this type of conveyor has been used for spoil removal from tunnels in this type of ground (Figure 2). It has provided a health and safety benefit by reducing the number of crane lifts in the shaft. This arrangement called for the first 40m of tunnel to be constructed oversize using SCL to provide space for the spoil handling system, known as the Front Shunt. This also enabled the TBM and backup to be installed in almost complete configuration. As the shaft crane is now only needed primarily for lowering segments (and not removal of spoil) the potential for extremely good tunnel production was created.
The Front Shunt, and other SCL passages at T5, are being constructed using the TunnelBeamerTM system and LaserShellTM lining. This automated approach to SCL construction was developed primarily to improve safety, as tunnel workers’ exposure to unsupported ground is greatly reduced (T&TI, June 2003).
The TBM was assembled in the Front Shunt and is being driven in a south-easterly direction towards the outlet shaft. The online intermediate shafts are being sunk in advance of the tunnel drive and the TBM is then shoved through them using bolted rings and a steel cradle. Access to the inlet shaft site was gained in September 2002 and sinking the shaft commenced soon after. TBM installation commenced in late February 2003 and tunnelling commenced in March 2003. Once the learning curve was complete, progress exceeded planned rates. At the beginning of August 2003, the best shift saw 45 rings built, the best day achieved 72 rings and the best 7-days 459 rings. Tunnelling is due to be complete by late August 2003.
Sinking of the two intermediate shafts and the outlet shaft followed on from the inlet shaft. All the civil works are expected to be complete by November 2003 when installation of the M&E equipment will commence.
Much of the tunnel route lies beneath grassed areas. However, the route does pass under the southern runway, two taxi-ways, the Duke of Northumberland/ Longford twin river culverts and under a scheduled ancient monument. Therefore surface settlement has to be tightly controlled and closely monitored. Surface settlement along the route was predicted to be 12mm (maximum). To date the actual settlement has not exceeded 6mm and the runway was tunnelled under without any impact on structures or airport operations.
A3044 Service Tunnel
Access to the T5 site from the west for power and other services, is provided by a tunnel under the A3044. The works consist of twin tunnels constructed by pipe jack, from a reinforced concrete launch chamber and two reception shafts. The launch chamber consists of a diaphragm wall box with internal reinforced concrete walls and slabs. Two 6m i.d. reception chambers were sunk prior to jacking the tunnel, between January and March 2003, by the same methods adopted on SWOT. Bachy Soletanche and Laing O’Rourke constructed the launch chamber, handing it over to the Morgan Vinci JV for installation of the TBM in May 2003. The twin 140m long tunnels were driven using a Herrenknecht backhoe TBM with 2.5m i.d. pre-cast concrete pipes. The jacking commenced in early June 2003 and both tunnels were completed in July 2003.
The tunnel alignment was in close proximity to the clay/gravel interface and surface settlement along the route was predicted to be 30mm (maximum). The actual settlement has not exceeded 10mm, with no effect on the road or existing services.
Heathrow Express Extension (HExEx)
The Heathrow Express Extension will form a connection from the existing Heathrow Express line at the Central Terminal Area (CTA) to a new station being built as part of the sub-structure of the main T5 Terminal. It will extend the existing service from London Paddington and will connect to the existing Heathrow Express tunnels via two existing step plate junctions that were formed in anticipation of this extension.
The US$67M HExEx will be a twin-bore tunnel of 5.675m i.d. with a total drive length of 3.2km. Additionally there will be two shafts with connecting passages for intervention and escape, and a ventilation shaft. The bored tunnels will be driven entirely through London Clay by a 6.1m diameter Dosco open face TBM fitted with a roadheader. Tunnel lining will be pre-cast concrete segmental wedgeblock rings. These will be taken into the tunnel on the specially designed Paulo de Nicola rubber wheeled transporters previously used successfully on the Airside Road Tunnel (ART). Spoil removal will be by continuous conveyor along the tunnel feeding to a High Angle Conveyor out of the shaft to reduce rail movements and lifting operations.
Both tunnels will be driven east from open-cut launch boxes to stub chambers previously formed in the existing Hex tunnels. On completion of the first drive, the TBM will be dismantled underground and transported back through the newly completed tunnel to the launch chamber for the second drive. When the second drive is complete, the TBM is again dismantled underground and removed through the tunnel drive.
The tunnels pass just below the existing Piccadilly Line, which runs from the CTA to Terminal 4. In this area extensive monitoring of the existing tunnel will be undertaken, on the surface and within the London Underground (LUL) tunnels, so that real time information can be obtained during the tunnelling activities.
The crosspassages will be driven prior to the running tunnels and will be extended to provide chambers for the TBM to pass through. This arrangement has been developed to provide the safest system of work and for the programme benefits it will deliver.
Close liaison with the T5 Sub-structures Team (who are constructing the TBM launch boxes for both HExEx and PiccEx) has ensured an agreed handover date that is achievable and acceptable to both teams and the programme. Access to the TBM launch box is expected in late November 2003, when the TBM will be installed for the Downline drive. Tunnelling is due to commence in February 2004 and the Upline drive should be completed in January 2005.
Meanwhile, works on the shafts have already begun, the T5C shaft commenced in April 2003 and the set-up for shaft T5D began in July 2003. The T5C intermediate shaft is due to commence in January 2004. The shafts and associated crosspassages are due to complete in December 2004.
Piccadilly Extension (PiccEx)
The PiccEx will form an extension from the existing Piccadilly Underground line near the CTA to a new underground station within the Terminal 5 building, which is integrated with the HExEx station. The US$54M PiccEx will be a twin bore 4.5m i.d. tunnel, with each bore being approximately 1.7km in length. A shaft and connecting passages will be constructed for intervention and emergency egress.
There is no provision for a connection to the existing Piccadilly line so a junction box will be constructed at Mike Cul-de-Sac near the CTA.
The TBM is an existing machine, previously used on the Baggage Tunnel project between Terminal 4 and Terminal 3 in the mid-1990s. The machine is being fully refurbished and up dated to take account of technical advances made in this period. The tunnel bores will be driven using a Dosco open face TBM through London Clay. The lining will be pre-cast concrete segmental wedgeblock rings and will be taken into the tunnel by narrow gauge train (previously used on SWOT). Spoil will be removed from the tunnel on a continuous conveyor and out of the launch chamber to the surface by a High Angle Conveyor (HAC) also previously used on SWOT. Bolted rings will be used at cross-passage junctions as well as at the start and end of the drives.
The tunnels will be driven from the launch box eastwards, and will finish 100m short of the existing LUL line, where a connection will be formed into the existing tunnels. The TBM will be dismantled and removed through the newly constructed tunnel.
The junction box, which will connect the new tunnels to the existing CTA to T4 loop line, will be constructed using diaphragm walls and top down construction. Short lengths of SCL tunnel will be driven from the box to meet with the new and existing bored tunnels. This major structure will be built from an airside site, occupying valuable aircraft stands and requiring the temporary closure of the existing Piccadilly line. It is essential that this construction is completed on time, in order to minimise the effect of these operations.
A number of design/construction options were studied to determine the optimum solution for the Piccadilly Junction. Early studies revolved around fully tunnelled options such as step plate junctions and non-closure options. However, limitations on the position of the box and very limited cover determined the final solution of an open-cut box structure.
Access to the launch box is expected this month, when the TBM will be installed for the westbound drive. Tunnelling is due to commence in November 2003 and will complete in October 2004. The PiccEx T5C shaft commenced sinking in May 2003 and the associated crosspassages are due to complete in August 2004. The PiccEx Junction is due to commence in early 2005 with a 3rd Quarter 2006 completion.
Tunnel segment manufacture
A dedicated facility for manufacturing segments has been built by the Morgan Vinci JV at Ridham Docks in Kent (T&TI, May 2002). This facility was set up to provide surety of supply and quality as well as value for money, and is manufacturing all the tunnel segments for ART, SWOT and both rail extension tunnels.
Manufacture of the segments for ART and SWOT have been completed, and for HExEx and PiccEx tunnels commenced in July 2003.
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