Buildoffsite visit to the Align tunnel precast segment supporting the construction of the Chiltern Tunnels
Align JV is the joint venture of Bouygues Travaux Publics, VolkerFitzpatrick and Sir Robert McAlpine responsible for delivering Package C1 of the HS2 high speed rail route between London and Birmingham. It is arguably the most technically complex of the route packages with a 3.37km viaduct and 16.04km of twin-bored tunnel. The tunnels are being driven south to north with the major centre of site activity being located at the South Portal of the tunnel.
The South Portal site covers an area of about 135 acres, but it is immediately noticeable that such a large site is not visually prominent in the landscape, as it sits in a dip in the topography. It is clear from a tour of the site the level of consideration and planning that has been applied throughout what is HS2’s largest construction site.
As a construction site the South Portal has organised and structured planning embedded throughout; from construction vehicle access being only to and from the M25, therefore avoiding local roads and heavy traffic issues with the local community, to segregated fenced and gated walkways around the site, with ‘traffic light’ warnings where people cross internal access roads. It is clearly apparent that very careful planning went into the site layout. At the heart of which are two precasting factories, one of which produces tunnel lining segments and the other the precast concrete deck segments for the viaduct.
Visiting and having the opportunity to tour the tunnel segment factory exemplifies the principles of planning which are evident across the South Portal site. In fact, the tunnel segment factory is not one single factory, it is two parallel factories which occupy a building which is U-shaped in plan. A concrete batching plant is located between the two outstanding ‘legs’ of the building. This plant is dedicated to supplying concrete only to the tunnel segment factories and is managed by Tarmac through a sub-contract. The batching plant is designed to enable it to supply concrete to both segment precasting factories simultaneously, or in case of a mixer breakdown, 1 mixer is able to supply both factories by alternating the batches between factories. Two other batching plants at the South Portal supply concrete to the viaduct factory and the wider construction works.
The tunnel segment factories are factories in the truest sense, operating 24hrs/day, 5 days per week; automation is used throughout the process and robots have been introduced where they can benefit the quality of the product. Just 15 people work in each factory at any one time, principally overseeing individual processes, installing all the cast-in gaskets, and inserts , checking conformity and activating specific automations. The casting process is a series of discrete stations linked by mechanised conveyors.
Identified accountability and traceability are critical on the project and to help achieve this Align JV ensures that every tunnel segment has an individual barcode cast into it. This data against each barcode is logged through the factory and then all the way to the tunnel where the segments are installed, by the two tunnel boring machines (TBM). This data is then exported to the tunnel BIM model, providing in depth ‘as constructed’ data and a BIM model of true value for future operations, inspections, and maintenance.
The casting process commences with the demoulding of the previously cured segment. The empty mould is then cleaned and oiled by the mould preparation robot. The mould is then closed by the team, before the cast in fittings are installed into the mould. This includes the segment sealing gaskets, which unlike those commonly seen on precast tunnel segments are not glued to the completed segment but clipped into the mould and cast into the segment, there by resulting in significantly less potential for failures and faults. The mould is then checked for cleanliness and inspected for the completion of all cast in items, before it is transported into the casting chamber. Here the automatic cover is clamped onto the mould leaving an opening sufficient for the concrete to be poured. The concrete is constantly transported between the batching plant and the casting chamber via a 2m3 flying hopper, where it is discharged into the 4m3 holding hopper. The concrete is then poured from the holding hopper into the mould, which requires approximately 3.5m3 of concrete per segment. As the concrete is poured it is vibrated to achieve compaction. Immediately the concrete pour/compaction is complete the automatic cover lifts and slides to an automatic cleaning station to be prepared ready for the next pour. The cast segment is moved on to the station where most of the manual working is carried out – the edges of the mould are cleaned to steel and any excess concrete is removed from the surface of the segment. Following this stage the segment passes to finishing – undertaken by a concrete finishing robot, which has been designed to produce a surface finish matching the curvature of the design. The overall dimensions of the segments are required to be within such tight tolerances that they are required to be 3D scanned, to ensure compliance with the specification. After finishing, the segments pass into the curing oven, where they are steam cured at 45 oC for a duration of approximately 6 to 7 hours. Following this, the segment leaves the oven sufficiently cured to be demoulded. The moulds are opened mechanically using pneumatic torque guns to unbolt the control screws that are integrated within the mould construction and the segment is demoulded using a vacuum lifter until mechanical support arms extend under the unit to provide support. From this point the segment is automatically flipped over and quality checked, before moving along another automated conveyor, and lifted by another vacuum lifter to the internal pre-storage for 24hrs to cool. Cooling undercover for the additional 24hrs avoids any risk of thermal shock which could exist if the segments were immediately moved whilst still at 45oC from the ovens to the outside storage yard. Finally, the mould which has been split open is returned to the start of the process to be cleaned and prepped.
Each of the 10 working line work stations takes approximately 10 mins to complete and with approximately 6 to 7 hrs curing in the ovens, the time to produce a segment from start to finish takes approximately 8 hrs overall.
It is clear that throughout the design and the operation of the tunnel factories, quality was and is a primary driver. For a facility delivering fluid wet concrete, pouring this, and vibrating it to achieve a high-quality result it is incredibly clean. Production levels are set with sufficient time to allow each operation to be undertaken in a way to reduce the potential for poor quality to be introduced. Rest periods for the personnel working in the factories are built in with all of the work team being able to undertake all the different operations in the factory that provides flexibility into the heart of the operation. All the cast in items are stored in separated and clearly marked areas avoiding any likelihood of incorrect items being introduced into any segment. To ensure there is always a sufficient amount of tunnel rings available to ‘feed’ the TBMs a 3 month supply of all materials, consumables and cast-in elements is continually maintained.
The outcome of this extent of initial thinking and its subsequent embedding across all processes is a paired factory producing approximately 200 segments per day(1000 segments per week). The external storage yard holds over 13,000 cast tunnel segments (1,900 tunnel rings) ready to be transferred to the tunnel boring machines on demand. However, these high-level numbers mask other as equally important outcomes. The amount of waste produced in the manufacture of the segments is negligible in real terms. The number of rejected segments is again extremely small, if one does occur it is crushed and the concrete used as base for roads and working areas. Every segment is fully traceable and the data is incorporated into an evolving BIM model to benefit the long-term management of the tunnel asset. All these outcomes show that the investment in the design of the manufacturing process and its operation has very clear economic and sustainability benefits not only to the Align JV but also to HS2 for the long term.