The manufacture, storage, transport and installation of precast concrete lining has not always received the care and attention that is required for efficient tunnelling. The consequences are usually damage to segments, necessitating their replacement, or mix-ups in delivery causing tunneling delays. Added to these are client and engineer demands for detailed reporting and records so that any mishaps or faults in installation can be traced back to their cause, as far as is possible.

Such manufacture and supply cannot just be regarded as filling a mould with concrete and waiting for it to cure, so recent developments have been aimed at improving on any such tendency.

Materials
Fibre manufacturers of all types have had a long campaign to replace or add to traditional steel reinforcement in precast lining, and with increasing success. ‘Synthetic’ polymer fibres also have a role, now well understood, in preventing damage in the case of fires in tunnel use by melting and providing passages for pressure relief and so detering spalling damage.

A drawback of traditional reinforcement, apart from the price of steel, is the depth of unreinforced concrete needed to cover a rebar cage. This is susceptible to spalling damage during handling. Fibre manufacturers have shown that inclusion of their product in the mix toughens the outer layers of a segment, greatly reducing significant damage during handling, as has been shown by records of a few segment replacements necessary on the Channel Tunnel Rail Link tunnels (now HS1). Both steel and synthetic fibres have now been used for complete replacement of the rebar cage as structural reinforcement. In the case of ‘synthetic’ fibres this is not the same as the microfibers normally used for fire protection.

The precast market now provides another battleground, in addition to sprayed concrete lining, for fibre suppliers such as Bekaert, Maccaferri, Propex and EPC that serve the tunnelling market.

Moulding and curing
Where the size of the project or continuous orders justifies it, casting plants are organised using a carousel arrangement to pass moulds under the concrete mix feed and then, if ambient temperature variations justify it, through a curing oven.

The low dimensional tolerances often specified these days have been criticised, especially as actual measurements can be affected by conditions like temperature variations, concrete shrinkage and even damage in transit. However, it follows that the use of high accuracy moulds will give the project a head-start in quality control.

Herrenknecht Formwork is delivering static moulds to Roger Bullivant’s Buchan precast plant in the English Midlands for Costain’s eight-year National Grid contract for cable tunnels to improve power supplies to London. The order comprises two sets of 30 moulds for 4,000mm and 3,170mm i.d. tunnels. The moulds are fitted with vibration systems for concrete mix consolidation and air-bubble removal.

‘Combisegments’ were first designed by Herrenknecht Formwork for a sewer in Tsaritsyno to the south of Moscow, Russia. They have an integrated glass-fibrereinforced plastic (GRP) liner to protect against sewage corrosion with a single construction process, with the liner inserted at the moulding stage.

Plastic dowels are used to connect the segments. The thinner lining made possible by this construction makes a construction cost-saving contribution to the higher costs of the lining.

The Tsaritsyno project also used, for the first time, seal profiles anchored into the concrete on moulding. A grove in the mould houses the EPDM structured seal from PDT-Profiles (Phoemix Dichtungstechnik) that is then cast into the segment as the concrete is poured. In this case the GRP line is also laid into the mould for holding on the concrete segment. Seal casting (anchored gaskets) is also used by Morgan– Sindall at Ridham Dock for Lee Tunnel segments (see page 52) where CBE has supplied eight sets of moulds for an 8.5mdiameter ring. Other advantages of this procedure include the elimination of the possible environmental hazard of gasket glue solvent, greater production precision and fewer chances for seal assembly error.

Mould construction, together with the associated equipment, has become a very competitive business with specialist concrete equipment manufacturers such as CBE, Ceresola and CIFA all taking part in addition to Herrenknecht.

Handling
One means of gentler but secure handling is by the use of vacuum pads on whatever handling and manipulation device is being used. This eliminates the point-loading of hoist connections and clamps, thus reducing local stresses in the concrete, with the potential for reducing damage.

Vacuum pads were reportedly first developed by Kawasaki at the suggestion of TML contractors for the Channel Tunnel for more efficient segment placement through a rotary erector, although a successful prototype was produced by the contractor Thyssen in the mid-‘80s for the Three Valleys Water tunnel near London. There followed the award of a patent to the Thyssen developers and manufacture by Markham of Chesterfield. Since then the principle has been adopted by many manufacturers for all situations of segment handling including casting works manipulators and hoist attachments as well as erectors in TBMs. The concept has become so advanced that segment loads of up to at least 16t can now be handled.

Transport
The use of traditional flat-bed rail trucks or other vehicles for precast segment transport places a considerable onus on the local operative to ensure that the segment load is placed in the right position with the required packing to prevent undue stresses. Thus there is considerable potential for human error.

More dedicated vehicles have been introduced recently that generally comprise trackless tractors linked to trailers designed for various materials carrying duties, and even manriders. Two companies making such vehicles are Metalliance and TechniMetal Systemes.

Metalliance’s TSP multi-function vehicles on rubber-tyred chassis the powerful TSP 90 for hauling a train of segment trailers and the smaller TSP28M.

TechniMetal multi-purpose trackless or railbound vehicles come in three basic designs for TBM supply carrying segments and grout for annular gap filling. Vehicles with 12 wheels have a maximum payload of 18t, eight-wheel vehicles carry up to 50t and a double-structure 14-wheel chassis can have a maximum payload of 100t. The first is specially designed for metro tunnels while the second has a low lift distance for segment loading. In addition, as an option, the front cabin can be collapsed for easier loading and unloading. TechniMetal also makes special hoist clamps for segment handling.

Control and reporting
Most clients and supervising engineers are now demanding rigorous quality control procedures with full reporting. It follows that the procedures themselves should be as automatic and foolproof as possible to save time and maximise reliability.

Consequently precast segmental lining production and installation should be integrated into the computerised tunnel construction management systems now becoming widely available. Although some operations are still identifying segment rings with marker pens, the increased reliability of industrial barcode (or similar) identification tags makes such procedures a virtual necessity. The importance of correct segment delivery has already been mentioned. VMT’s Ring Management Program (RMP) carries out optimal planning of a ring sequence calculation faster and arguably more reliably than the most experienced TBM operator. The system can be set to calculate for only a few rings ahead or the complete tunnel. The system considers all parameters of influence for the best choice of ring including thrust jack pressure, tailskin clearance, any specific ring design criteria, cruciform joints incidence and the position of the TBM on the design axis. The procedure includes the calculation of any correction curve required through using the optimum ring sequence.

A recent addition from VMT is the Gaptrix instrument assembly to remove human error from checking the tailskin clearance. The semi-automatic system uses a Leica Disto instrument with Bluetooth data transmission technology to link with the TBM guidance system, eliminating tape measures and manual recording for later system input.

Bolting and fixing
Important considerations include the safe attachment of service lines and other items to the tunnel lining, without uncontrolled drilling through the concrete. Various fixture aids, such as metal channels, have been developed that are attached to the bolts used for segment ring assembly thus preventing hazardous point loading on the segments themselves.

Segment bolts and bolthole locators have been designed with the aim of easier segment ring assembly and, in the case of bolts, preventing corrosion for the design life of the tunnel.

Takenaka of Japan has developed a fluorocarbon polymer coating process that creates a thin, hard film on bolts etc that has been tested successfully in extremes of temperature and corrosive media. Applications have included transport and drainage tunnels for the Japan Ministry of Land, Infrastructure, Transport & Tourism, the Japan Railways Construction Public Corp, the former Metropolitan Expressway Public Corp, the Osaka Municipal Transportation Bureau, the Osaka Prefectural Government, the transportation bureaux of Nagoya, Kyoto, Kobe, etc, and other tunnel clients such as the Trans- Tokyo Bay Highway, Tokyo Gas, Kansai Electric Power, Hanshin Electric Railway and Kansai Rapid Railway.

If a high-temperature environment is anticipated, such as for fire resistance, Takecoat Ceramic is available.

As pre-cast segmental lining production emerges from a construction process to a manufacturing operation, so does the ‘tunnel factory’ approach reality through adopting industrial procedures for ‘best practice’ and quality control.