The main purposes of a cast in situ concrete lining include improvement of safety from ground moments and falls, providing an acceptable interior finish, deterring water leakage (perhaps with other materials) and providing a secure base for fixing tunnel furnishing. Traditionally this has been achieved by largely empirical means with effectiveness often determined by thickness, with later steel reinforcement. However the economic pressures on underground construction, and the possibility of using new materials and ingredients, mean that accurate design and testing becomes much more important. Thus improved cost-efficiency and effectiveness should follow. Even so there have been some worrying incidents of spalling, sometimes fatal, in normal tunnel operation as well as emergency situations such as fire.

Depending on the designed structure, casting of the final lining generally takes place as a separate operation from tunnel excavation and immediate support. The main reasons for this will be logistical to reduce overall demands on the tunnelling transport system and to reduce the number of activities taking place in a small area. An important factor will be ground conditions and how well the temporary immediate support can cope with these until the permanent support of the cast concrete can ‘take the permanent load’. A concreting operation separate from other main tunnelling activity can usually ensure smooth progress, if managed well, whereas trying to integrate it with other activities can result in unacceptable delays.

In the mix

Many recent developments in cast in situ tunnel lining have been in the concrete mix and its reinforcement. The cured concrete has to meet strict specifications, and this involves much more than basic mix materials and thicknesses placed. These days important considerations include durability for the tunnel design life, the economic balance of lining strength and thickness especially considering reinforcement design, water permeability, corrosion resistance to chemicals likely to occur both within the tunnel and in the surrounding ground, and, in transport and cable tunnels, fire resistance.

Fire-resistance properties of the cast tunnel lining usually involves protective layers or the inclusion of polymer fibres in the mix. Similarly waterproofing, as covered elsewhere in T&TI, may require special measures, especially if moisture may be aggressive. Recent work by the Swiss Federal Laboratories for Materials Testing and Research (EMPA) has centred on the damage to tunnel concrete caused by high or low sulphate contents resulting in the formation of thaumasite (Romer et al, 2003). The motivation is that it appears natural groundwater conditions in the Swiss Alps can lead to sulphate attack forming thaumasite rather than ettringite

Fibres

One of the most important developments in underground concrete mix composition is the use of fibres, whether steel or polymer. It is also one of the most controversial. Fibres have found greatest application in sprayed concrete but can also provide benefits in cast concrete, depending on the design requirements. Depending on the fibres type used, these benefits include lessening of cracking and their deterioration, and improved resistance to fire damage.

There are many intense discussions about which types of fibre are suitable for desired properties and lining durability, and the arguments can only really be settled by independent testing, both in the laboratory and in site conditions. These recognised needs have resulted in the development of standards for the use of fibres so that design engineers can specify with confidence in materials’ performance. Even so, both designers and suppliers are reluctant to offer fibres for main reinforcement, using them only for secondary reinforcement for crack control and other required properties.

Although Bekaert steel fibres are widely used in reinforced sprayed concrete linings, and also in pre-cast concrete segmental linings, use in cast in situ concrete is less common, and found mainly in Asia. For example, the Kakegawa No. 1 twin road tunnel in Japan employed Bekaert Dramix FL-45/50 BN steel fibres at a dosage of 40kg/m3 in a 30MPa strength concrete mix. The final lining is approximately 500mm thick on top of a waterproofing membrane and reinforced sprayed concrete primary lining. The lining was formed at a rate of 160m3 placed each 8h day to cover 10.5m linear of the 15m diameter tunnel. 787 tonnes of steel fibres were used by contractor Maeda for the tunnel lengths of 541 and 656m. These were dosed accurately with an Incite dosing machine and counted in the wet concrete mixture with a Bekaert counting machine.

Also in Japan, Taisei used Dramix RC-65/60-BN fibres at a dosage of 40kg/m2 for the final lining concrete of the Shimuzu No.3 road tunnel, 1120m long. 679 tonnes of steel fibre were used. Other details were similar to the Kakegawa tunnels. These and similar tunnels form part of the second Tomei Highway Project. Other projects in which Bekaert steel fibres in the cast tunnel final lining mix have been the Quito road tunnel in Peru, the Beacon Hill light rail station, and renovation of Brunel’s sub-Thames tunnel.

Polypropylene

Unusually for fibre manufacturers, Propex Concrete Systems (formerly SI Concrete Systems) supplies both steel (Novocon) and polymer (Fibremesh polypropylene and synthetic macro-type) fibres for concrete mixes, and thus can be said to have an independent viewpoint. The recently developed high-performance polymer (HPP) Enduro macro-fibres are intended for use in both cast and sprayed concrete. Whilst they have been used mainly in sprayed concrete, they have been specified for some applications in cast concrete linings for high-speed rail tunnels, particularly in Spain, due to concerns about static charge build up associated with steel fibres and train movements.

Propex Novocon steel fibres are cold-drawn with high tensile strength, and engineered to provide uniform distribution throughout the concrete. Mixing and finishing attributes are said to be excellent. Flat-end fibres are know as Novocon FE whilst for higher performance Novocon HE fibres have hooked ends.

Propex Fibermesh polypropylene fibres are said to substantially reduce plastic settlement and shrinkage cracks by increasing the tensile capacity of plastic concrete. Fibermesh 150 fine monofilament fibre has been tested in the VSH Hagerbach galleries to prevent explosive spalling in fire conditions, but with minimal effect on the workability of concrete. Projects that have used this fibre include the Dublin Port Tunnel and London CTRL tunnels.

In comparing the mechanical performance of steel and polypropylene, Propex’s Trevor Atkinson commented that proponents of steel fibres raise the matter of ‘creep’ in the use of synthetic fibres since steel fibres can ‘hold’ a crack better. However macro synthetic fibres will still work for a considerable period over cracks if concrete starts moving. It all depends on the mode of concrete failure.

Newer materials such as glassfibre have also found their place as alternatives to steel in structural reinforcement, especially where the lining may need to be cut at a later stage in the construction. Glassfibre reinforcement cages (bars and stirrups) such as those produced by Sireg are typically used to provide ‘soft eyes’ for the passage of TBMs through reinforced concrete. These are employed on the Barcelona Metro in diaphragm walls for the Sagrena interchange station.

Transport

Depending largely on the distance from the batching plant to the point of placement, and the amounts required, methods of transport of concrete for tunnelling are dominated by rail (including remixers and silo cars), pumping (including placement) or trackless haulage (highway-type truck-mixers or specialist tunnelling/mining vehicles). The concrete mix ingredients (cement, aggregate, reinforcement and other additives) may also be transported near to the placement site separately for batching locally although this may be environmentally and logistically undesirable. Old methods of simply carrying the concrete mix in a standard wagon or tub can result in unacceptable separation requiring remixing or result in variations in placement quality.

Underground rail transport specialist Mühlhäuser, which is celebrating a century in business this year, supplies standard and custom designs in rolling stock including concrete remixer and silo cars, and special cars for aggregates and cement. A large fleet is also available for hire.

Pumping alone may be able to attain the whole transport requirement since, for example, the Putzmeister high-pressure concrete pump range have reached conveying distances of up to 2015m, and heights of up to 532m. Pump capacities range up to 187m3/h at a maximum concrete pressure of 260bar. Pumps can be stationary, trailer-mounted, truck-mounted or crawler chassis mounted.

Putzmeister’s M 24-4 truck-mounted concrete pump is specially designed for concrete placing in tunnels and other low-headroom locations. It is equipped with a multi-Z-folding boom, which has an unfolded height of less than 5m, but a horizontal reach of 19.5m. The versatile BSA 1002-D Multi concrete pump can be used for both mass concrete transport and shotcreting. It is pre-equipped with special mounts for an additive tank and metering pump.

More facilities are provided by the Puma placing boom, the last segment of which can be swivelled through 360° for placing flexibility in connecting to several placing tubes within the formwork. Putzmeister also supplies other heavy-duty concrete pipework including shut-off valves, directional valves and rotary distributors.

Where the project circumstances require it, Putzmeister will also develop customised equipment. For example, in the construction of a pedestrian escape passageway parallel to the Heslacher rail tunnel in Stuttgart, Germany, the small (10m2) cross-section required special concrete handling plant. A BSA 1005-D GRF concrete pump was mounted on a crawler track only 1.7m wide and to operate within a height of only 2.4m. The self-propelled crawler chassis can position the pump as required and also drives it outside for cleaning. During concreting the concrete was transported by a 2.5m3 grabbing dumper loading onto a 6m long belt conveyor to load the concrete pump hoppers.

Placing and forms

Formwork can, literally, take many forms depending on the type of structure and its section. The major trend over recent years has been to go away from labour-intensive operations involved in, for example, erection and disassembly, adjustments and placement. This has been achieved by means such as collapsible and/or mobile forms, integrated vibration systems and concrete pumping.

The major types of formwork used in tunnelling include:

• Full-round, crown (vault) or invert formwork for mined tunnels

• Rectangular and crown forms for cut-and-cover tunnels

• Specials for niches, crosscuts, suspended slabs (for soffits/multiple duct sections), caverns, and internal requirements such as shoulders

• Outside (counter) formwork for canopy tunnels

• Shafts systems including climbing and anchorless forms

• Travelling gantries associated with any of the above.

The firm of RSB in Fussach, Austria, designs, manufactures and supervise special circular formwork for many applications including tunnels. The company’s formwork supply schedule includes assembly and disassembly services, anchoring systems, concrete-placing aids and vibrating systems to ensure consolidation and air removal. This, claims RSB, ensures a smoother running project, as all concreting activities are the responsibility of one supplier.

Recent projects supplied include the Wienerwald Tunnel, Austria and the Ermenek hydropower project in Turkey. An example of RSB mining-type formwork is the 9.35m long transportable structure for connecting galleries in the Wienerwald high-speed rail tunnel being constructed by a consortium of Porr, Bilfinger & Berger, Zublin, Hochtief, Jägerbau and Swietelsky. The formwork can be split into two 4.674m pieces and is moved and positioned on the tunnel axis hydraulically, avoiding occupying normal tunnel transport. Hours of assembly and disassembly are also saved claim RSB. The transporter can be used with three different cross sections.

Another hydraulically driven formwork with controllable rubber wheels, and no rails, has been employed by Swietelsky Tunnelbau and Torno for the surge tank of the Part 2 of the Kops project in Austria. The tank consists of a sub-chamber, inclined chute and upper chamber. The RSB full-round formwork transporter is used for lining the two chambers in two diameters – 6.2m and 7m. Therefore the 6m long formwork transporter has to be broken down to pass through a 2.8m diameter passage forming the vertical flue. The formwork thickness is 8mm.

Other major suppliers of formwork for tunnels, including transporter systems and concrete distribution points include Ceresola, CIFA, PERI and Doka. Just a year ago the Cifa group was purchased by a new Italian private equity company, Magenta.

Testing & controls

The need for specific guidance in the use and purchasing of new materials for concrete mixes and structures has led to substantial research, testing and establishment of Standards. Part of this has been the publishing of European Strandards covering fibres. Standards EN 14889-1:2006 and EN 14889-2:2006 cover ‘Fibres for concrete. Definitions, specifications and conformity’ for steel and polymer fibres respectively. Testing methods to check whether fibres comply are covered by EN 14845-2:2006 titled ‘Test methods for fibres in concrete. Effect on concrete.’

An early development following the concern in Japan on transport tunnel concrete failure was the design of a ‘Drive-through Tunnel Concrete Inspection and Diagnostic System’, which carries out non-contact and non-destructive testing for defects in tunnel concrete. The joint developers were the Takenaka Corp., Kyoto University, the University of Tokyo and the Tokyo Institute of Technology. A rail-car, for example, carries the instrumentation and is driven through the tunnel at about 5km/h. It carries a high-definition camera to check for surface defects, a thermograph to check for defects in outer layers of the concrete, and ground-probing radar to check for deep defects such as cavities and big internal cracks. The collected data is fed into an evaluation and diagnostic program and the degree of danger (or repair urgency) for each square metre of concrete is displayed on a mesh for overall evaluation.

Work led by the independent public Works Research Institute and the Highway Research Institute has evaluated the load-carrying capacity of concrete linings under full-scale laboratory conditions. The results were compared with frame analysis and finite element methods for checking crack development. In comparison with plain concrete lining and fibre-reinforce material, the results showed that fibre-reinforcement prevented fall of debris after crack development, and also improved load-carrying capacity. The segments were used to simulate a semi-circular lining of 9.7m outer diameter, 300mm thickness and 1000mm high (wide). The Road Technology Research Group of the PWRI has its on tunnel research team conducting such work.

South Korean engineers with Chung-Ang University, Gyeong-Gi Do, have recently employed the resonance search (RS) technique to prove its use in testing the concrete lining in the ManDoek Tunnel.


Full-round steel rebar cage constructed inside a segmental lining for a protective finish in a collector sewer associated with the Mittlerer Ring Ost, Munich, Germany Full-round steel rebar cage Section through the Kakegawa road tunnel lining with cast in situ final lining with Bekaert Dramix fibres on a NATM primary lining Kakegawa road tunnel lining Propex Fibermesh 150 fine monofilament fibres provides resistance to explosive spalling of concrete during a fire Propex Fibermesh 150 This RSB rubber-wheeled, full-round transporter and formwork is used for lining the sub-chambers of the Kops section 2 surge tank in Austria RSB rubber-wheeled, full-round transporter and formwork Several Doka travelling formwork units were used on the Loetschberg Tunnel by the SATCO contractor consortium Doka travelling formwork unit The southern entrance of the Malmö City Tunnel is constructed by cut-and-cover with two Peri formwork carriages Malmö City Tunnel