Brick, cast iron, poured concrete and shotcrete have all historically been used to line tunnels. Today, prefabricated concrete segments are widespread. Among their advantages are the consistency and improved quality control that comes from off-site manufacture.
“Prefabricated concrete is one of the essential materials of construction,” says Bruno Lemiere, in charge of customer relations at Consolis, which is a major supplier in this field. “It is both highly technical and highly value-added. Using it, construction time is faster, the products are of better quality, and fewer raw materials are used because the factory setting allows optimisation of the upstream calculations. Precise quantity control is possible.
“Precast tunnel linings are particularly popular in northern countries because concrete cannot be poured when temperatures are below zero, but they are gradually spreading to all western European countries. For tunnels or railway infrastructure it is the technology par excellence.”
Nevertheless, precast segments present difficulties. They are heavy and awkward to handle. And they must be transported lengthwise, which means they need to be turned through 90° at the tunnel face – an operation that is typically performed in cramped and tight conditions.
Even outside the tunnel, transporting them is problematic. Transportation by rail requires a line close to the site, while road is expensive and unpopular with authorities – the trucks are necessarily large and many. The solution to build the segment factory at the tunnel portal is common but it requires storage space to allow segments to cure and to await transportation to the tunnel face – and tunnel portals are frequently in regions where large storage areas are not available. The logistical challenge of handling segments and getting them efficiently to the tunnel workface without disrupting the excavation is therefore considerable.
Making The Segments
Lemiere describes the key steps of production. Steel reinforcement – if there is any – is placed in the mould; an alternative is steel fibre – short lengths of wire added to the concrete mix. The concrete is poured, followed by floatation of the surface to ensure a smooth finish. Once the concrete has set – usually around seven hours – the segment is removed from the mould and turned so that the concave side is upwards for storage and further setting. “The segment can then be finished by removing the burrs and adding a little more mortar where necessary,” he says.
Stefan Medel is managing director of Germany-based Herrenknecht Formwork Technology. “The way that segments are handled depends on many factors,” he says. “Among them are the curing stage that the segment has reached; the type of production that has been used – whether it is stationary or carousel – and the space that is available for segment production and storage.”
Some curing takes place in the mould, which continues after the segment has been demoulded. The concrete is still in a delicate condition at the demoulding stage. “For demoulding of segments, it is recommended to use a vacuum lifter to avoid damaging the young concrete,” says Medel. “Most vacuum lifters today are also equipped with mechanical safety clamps to avoid safety issues should the vacuum fail when transporting the segments to storage after demoulding.”
Frans Heilman is director of Dutch company Aerolift, which manufactures vacuum lifters for segments. “In the past, mechanical clamps were used to demould,” he says. “The moulds themselves are hinged, with lids that have to open to remove the set segment. Mechanical clamps need the lids to open more fully, so that they can grasp the segment at an edge or a cast-in lifting point. Vacuum lifters can grip the smooth surface, so they need the lids to open less fully, to around 10°, rather than 30°. That means the moulds are much less expensive to make, and much less sensitive to vibration.”
He continues: “Using suction means that there is no damage, to the segment or to the inside of the mould, which is a risk with mechanical grips. Avoiding that is a big benefit. Nor is there any need to embed lifting points into the segment. There are always internal stresses in any lifting, but, of course, vacuum lifters can grip any smooth surface, and the cups deform to exactly the same shape as the load, which means the segment can be lifted from points that stress it least.
“Customers frequently want to add protection ropes or webbing underneath for safety, or even hydraulic clamps. We are happy with that but there is no need for it. Vacuum these days is very safe. It does not drop things.”
Heilman points to another benefit of vacuum technology: “Drying is quicker using vacuum lifting. It can lift the load while it is at a more delicate stage because it does not pick it up by the edge which is still fragile; the multiple vacuum cups spread the stress more evenly. So you can lift it earlier from the mould. For similar moulds, you can achieve a 15% saving in drying time. That is a lot of saving.”
A single segment can weigh anything from 2-3t up to 15-18t for a large-diameter tunnel. The vacuum lifter deposits the piece on a tray, called a ‘turner’, which pivots it to set it concave side up for finishing. “Or we can demould, lift and turn with the same device,” says Heilman. Then the segments are stored, in ‘families’, typically of six segments plus key that will form the future rings of the tunnel.
“For handling of the segments after pre-storage, mechanical clamps for one or several segments are mostly used,” explains Medel. “They are adapted to the static requirements of the single segments or stacks. Either vacuum lifters or clamps can be used with cranes or forklifts, depending on the overall layout of the segment production plant. To cover longer distances, the segments are often transported on specially-designed segment trolleys or scissor lifts which run on rails.”
For HS2, to minimise transport costs and emissions, the segment production plant for the main tunnel is being set up on land at the portal by the M25 motorway, but that type of solution is not always possible. There are advantages in permanent factories, and Consolis, for example, is currently supplying five different rail tunnel projects from three manufacturing sites.
It builds at its permanent facilities at Conflans-Sainte-Honorine in the western suburbs of Paris, and from plants at Bruz, in Brittany, and Cinq-Mars-la-Pile, in central France.
One of the sites being supplied is the Grand Paris project, which is the largest construction site in Europe. It involves building 200km of metro lines in the Ile-de-France region for a cost currently estimated at €35bn (US$39.8bn). Consolis is one of the major suppliers.
Tunnel diameters are in general between 7.75m and 8.70m; the segments, seven per ring, are being produced at Conflans-Sainte-Honorine mostly on new carousel lines and are being delivered by barge along the River Seine, and then by rail. “The segments weigh around 8t each, but we are making them with a precision of a tenth of a millimetre,” says Lemiere.
For any tunnel project the portal is a major logistical bottleneck. The right segments must be available at the right time to feed the TBM for installation. “The overall concept of the segment production plant must be considered for a just-in-time delivery of segments to the TBM,” says Medel. “All the components (the batching plant, concreting system, handling equipment) and all the work steps (curing, turning, finishing, transport and storage ) must fit and intertwine to be able to supply the right segment to the TBM at the right moment. Besides the main equipment and processes, it is important also to provide reliable infrastructure. You need sufficiently large and fortified routes and spaces for segment transport and storage, as well as a reliable supply of electricity, water and compressed air.
“Focusing on segment delivery to the tunnel, a deliberate logistic concept has to be worked out right at the beginning of planning. It must consider, among other things, the type of stacking [arrangement] of segments – whether by ring or segment type – to be able to supply them to the TBM in the correct sequence required for the tunnel.” If the tunnel is large enough, multi-service vehicles (MSVs) can be used to carry them. Rail lines are an alternative.
On the TBM, vacuum lifting again comes into play. “The vacuum segment erector lifts it in two directions and rotates it slightly, to optimise the accuracy of its positioning,” says Heilman of Aerolift. “The erector has three suction pads. For a normal section, all three are used. But the key segment is smaller; for that, only the central suction pad is active. To give access to the bolt holes of the key segment for fixing it, the central suction pad is designed to extend and retract. Key segment recognition ensures that all three suction pads are activated when lifting a normal-sized segment.”
Key segment recognition also depends on software that digitally identifies each segment in the entire project. “In recent years, customised segment production and logistics management systems software are being used to link and track the individual segments of the production,” says Medel (see box on opposite page). “Nevertheless, an urgent recommendation is to set up redundancy in the system for the tunnel supply. Without it, whenever one of the components has a breakdown, all tunnelling work comes to a stop. That has to be avoided, or at the very least, a risk assessment for it must be in place.”
Medel continues: “When it comes to installing the rings in the tunnel, exactness of segment dimensions is decisive. The concrete segments must comply with tolerance requirements of up to a tenth of a millimetre, even though they weigh several tonnes. Therefore, besides an adequate geometry of the segments and the ring, maximum precision and dimensional stability of the moulds is a basic requirement for ensuring smooth installation.”
So, consistent, well-defined quality is decisive for a successful project. Recent developments in the fabrication process will help that. “Herrenknecht Formwork’s recent developments focus on improvement of the segment production itself,” says Medel. “For the first time we, together with Swiss construction company Marti, have designed, supplied and installed an unmanned segment production working line. The moulds open and close autonomously with the help of a hydraulic system. A robot cleans and greases the moulds at one workstation and inserts the injection sockets at another. This so-called ‘robot-supported moulding’ system significantly increases efficiency and safety in segment production. The next step will include further automation and digital networking of processes.”
Curing time is another area where improvements are coming. “Besides the often-used 28-day curing rule, more and more job sites are checking the strength development of the young concrete to determine the earliest time for installation,” says Medel. “This saves storage area and reduces costs.
It is important to consider segment manufacturing and storage as a single serial process. Serial productions are monitored much more closely now. It is important to ensure, for example, that the concrete recipe won’t be changed during the whole production period.”
Segments then present a combination of large size, heavy weight and awkward shape but need failure-proof logistics and precise and careful handling in confined spaces together with tolerances down to the sub-millimetre level. Even so, they enjoy widespread use. Segment engineers clearly enjoy a challenge.
Intelligent segments
Segments today are smart. Logistics and systems management are now being assisted by the segments themselves. Radio frequency identification (RFID) tags and bar codes are embedded into each one at the production stage. This uniquely identifies the segment not only during tunnel construction, but afterwards for maintenance and data gathering during the tunnel’s lifetime.
“The modular segment documentation system (SDS) developed by VMT is an example,” says Stefan Medel, managing director of Herrenknecht Formwork Technology. “The RFID implant monitors and documents all the key processes in the production and storage of segments, and records the position of the segment and installation data in the tunnel.” The individual processes are recorded and can be read by easy-to-use scanners or even smart phones, and their data can be evaluated in real time on a PC in the production management offices. The end result is plannable, efficient and transparent handling on site and afterwards when the tunnel is in use. “Such systems support production management and help minimise waiting times.
The goal is always the avoidance of errors and the optimisation of processes for cost reduction,” says Medel.
On some Grand Paris sites, space is so limited that they can only store enough segments for two days’ boring. Just-in-time delivery of the correct segments in the correct sequence is therefore vital, and RFID technology is key to it.
As part of that project, Bonna Sabla, a French subsidiary of Consolis, manufactures tunnel segments with RFID chip technology. Xavier Inda is innovations manager at Consolis. “The chips are embedded during casting,” he says, “and a database is updated in real time: place of production, date of production, type of reinforcement, mould number, concrete recipe, quality control and storage are all entered. In total, more than 100 pieces of data are collected per tunnel segment. Traceability of each element from the production line to delivery on site is ensured.”
Data can be digitally shared and the manufacturer is the starting point of a chain of data handover. The database that has been created during production, plus storage data, are transferred to the construction company after delivery. The construction company leverages the database to build the tunnel and enrich it with new data entries, such as installation date and the location within the tunnel.
Finally, the tunnel owner receives the complete database and continues to feed it with maintenance information. Each tunnel segment in effect holds its own readable life history. This can be updated – for example, with the location of any water leaks. The tunnel owner can then review the historical record, determine how many segments are experiencing the same problem, and whether it is occurring in other sections of the tunnel.
