Tunnel lining segments for BAA‘s Airside Road Tunnel (ART) Project at Heathrow Airport, are being constructed by the Morgan Vinci Joint Venture at a new purpose built factory at Ridham Dock. The facility will subsequently be available to manufacture linings for the JV’s rail and drainage tunnels on BAA’s Terminal 5 project.

Whilst a number of locations for the factory were considered, the site at Ridham Dock was finally chosen for its good road access, the adjacent port facilities and connections to the railway network via an existing siding. Following the granting of planning permission for the permanent facility in March 2001, clearance and piling of the 11.7 acre site started almost immediately.

A 4000m3 steel frame factory building was erected to house the manufacturing and curing facilities. The building was designed to accommodate two carousel production systems although, initially, only one production line and curing chamber has been constructed. The second production system is to be installed as and when required. The requirements for the carousel production system result from the pre-cast team’s previous experience and incorporate a pallet base system that facilitates both horizontal and vertical casting of segments. The carousel production system was designed and manufactured in France by CBE to meet this specification.

Whilst the types and sizes of the segments for the ART and T5 Projects were taken into account, a broader view of the potential market requirements was also taken into consideration during the design of the plant. Consequently, the plant has facilities for producing traditional steel cage reinforced, steel fibre reinforced or un-reinforced segments with or without the addition of polypropylene fibres.

The production line

The carousel production line has 40 No. pallets on which are currently mounted five rings of horizontally cast moulds to produce the ART ring. The ART ring is a bar reinforced bolted tapered ring, 8.1m i.d. x 8.8m o.d. x nominally 1.7m wide. Horizontal casting was selected on the basis of the size of the individual unit and the requirement that the segments have a very high quality intrados surface finish, which is to be left as built in the tunnel. The close tolerance machined moulds were also supplied by CBE and are fitted to individual pallets. The moulds are designed for quick and easy operation using a minimum amount of labour.

Concrete is placed in the moulds at the concreting station. This is located in an acoustic chamber in which the mould and pallet are raised onto a pneumatic anti-vibration isolation frame. The moulds are fitted with external pneumatic vibrators that are used to compact the concrete, which is discharged from a filling hopper. Concrete is delivered to the filling hopper from the Skako batching plant using an overhead bullet skip. At the final station on the production line, the cover plates are lifted and the extrados trowel finished before the mould is transferred across into the curing tunnel.

The curing chamber is heated by steam in a radiator system in separate compartments to give a predetermined temperature profile. Steam is also injected to maintain humidity levels. Temperature and humidity are monitored and automatically controlled by the computer control system. The chamber contains thirty pallets and moulds in three separate rows that progress through the chamber during a five hour period.

Following transfer from the curing chamber to the production line, the moulds are opened and inserts released prior to arrival at the de-moulding station. At this station the segment is stripped from the mould using a vacuum lifting frame and placed on a table frame. Gaskets are fitted using a sprayed glue and vacuum clamping frame system. The extrados vacuum lifting frame is then used to transfer the segments onto the trailers where the self-adhesive bitumenous packers are fitted prior to delivery to the stockyard. The moulds then continue along the production line where they are cleaned and oiled prior to casting.

Quality assurance tracing

The all welded prefabricated reinforcement cages are manufactured off site by Rom-Tech and are delivered on a just-in-time basis to a storage area on site. A bar coding system is used by Rom-Tech to identify each cage manufactured and this same bar code is used to identify the unique cage used in each segment. Other suppliers of components are being encouraged to provide products with bar codes to enable the system of identification and traceability to be automated as far as possible.

Once the reinforcement cages and the cast-in inserts are placed in the moulds, the moulds are checked and closed prior to entering the concreting station.

The production process is controlled from the cabin adjacent to the concreting station and is automated as far as possible. The computer system automatically recognises the pallet and mould prior to its arrival at the concreting station and orders the correct amount of concrete for the mould from the batching plant. The system also automatically controls the concrete discharge and vibration cycle.

A data collection system collates all the information regarding the concrete mix, reinforcement cage, inserts, gasket and curing profile so that when the unit is struck from the mould and given its unique identification number/barcode, full traceability of constituents and components is achieved.

Concrete batching plant

The Skako Concrete Batching plant is a 2m3 counter current mixer fed from three 100t elevated aggregate storage bins, two cement silos, one PFA Silo and two admixture dispensers. The plant also has facilities for adding polypropylene and steel fibres when required. Polypropylene fibres are supplied by Fibrin in 150Kg tows which are automatically cut to length and discharged by compressed air into the mixer drum. 20mm and10mm Glensander granite aggregate and fines are delivered by Foster Yeoman in 1000t barges from their Isle of Grain facility adjacent to Ridham Dock and then transferred to large ground level storage bins by trucks.

In order to minimise the volume of process water discharged, all wash-down is re-cycled by the use of a filtration system and agitator tank and re-used together with clean water in the concrete mix.

Stockyard facilities

The stockyard area constructed, covers 11,000m2 and is serviced by two 40m span, 12.5t double cantilever gantry cranes. This allows the full area between the crane rails to be used for storage whilst loading in and out can be carried out under the crane cantilever sections. A second smaller stockyard area will be constructed when the second carousel system is installed or if additional capacity becomes necessary.

The ART segments are delivered to the stockyard area, in the as cast position, on trailers. A vacuum lifting frame is used to lift on the extrados face of the segment and then rotates it through 90 degrees before stacking the units on edge in the stockyard.

A vacuum lifting frame designed to lift on the intrados segment face will be used to remove the segments from the stockpile and rotate them through 90 degrees so that they can be delivered to site intrados face up. Deliveries will initially be by road transport direct to the construction site.

During loading, each segment identification/barcode will be scanned by a barcode scanner and the information downloaded into the main database, ensuring accurate control of stock and delivery details.

Current operations

The facility is currently operating on a single day shift, for a five day working week, which is adequate time to produce the 38 rings per week of segments required. If, however, production needs to be further accelerated then a second shift will be introduced.

Delivery of the first rings of segments to the ART site will start later on this month. Construction of the 1,295m long 8.1m i.d. twin bore road tunnels is due to start in early June. These will run from the Central Terminal Area to remote aircraft stands on the west side of the airport and are programmed for completion during August 2003.