Demands on the performance of modern cast in-situ tunnel linings are more stringent than ever. This can be demonstrated alone by the fact that many of the underground structures currently being built around the world are so structurally complex and technically challenging that their realisation would have been inconceivable even just a few decades ago.

Regardless of the size of the project however, any durable cast in-situ final lining that is designed to minimise direct and indirect maintenance costs and handle all anticipated conditions throughout the design-life of the infrastructure, will need to take into consideration numerous factors.

These may include external or internal corrosion, static or dynamic ground loading, water control measures, internal impact and/or fire in transport tunnels, and internal pressures in hydropower, water supply, and sewers. Concrete mix design (including curing times, and a balance between early age and long-term strength), selection and design of the concrete forms, the logistical interaction of plant and equipment within the tunnel, pumping distances and pressures, as well as ever more demanding cost and time-schedule constraints, all also need to be resolved.

Leading tunnel formwork manufacturers and suppliers are doing their best to assist in meeting some of these challenges, working with tunnel designers and contractors to refine designs, increase standardisation and dimensional flexibility of formwork components, improve handling processes, and reduce both the capital and running costs of the equipment.

Minimising labour intensive operations involved in, for example, transportation phases, erection and disassembly, adjustments and placement, have been greatly reduced by the development of self-retracting and/or mobile forms. The need for highly-experienced operatives has also been greatly reduced with automated vibration and concrete pumping systems.

Complex cavern concreting

One of several recent ‘mega’ projects to require challenging and extensive concreting works was Austria’s Kops II project, which includes one of the world’s largest rock caverns, a 90m long, 30m wide and 60m deep powerhouse.

The Jäger Bau/Beton-und Monierbau/ Alpine Mayreder/Züblin JV, completed the excavation works on its Lot 3 contract – consisting of the powerhouse, a transformer cavern and associated tunnel systems – between November 2004 and December 2005. Following this, working to a fierce schedule and pushing site logistics to the limit, approximately 48,000m3 of concrete was cast within the caverns over a period of just 18 months.

This was concurrent with the numerous lining and backfilling works in the project’s tunnels and shafts, as well as hydro-mechanical equipment installation.

Forming concrete slabs of up to 8.5m in height, with thicknesses varying from 200mm to 4m, for the project’s caverns placed enormous demands on the flexibility of the formwork system used. Another vital aspect of the works was the intensity of logistics in the powerhouse, which required the stock of formwork components to be reduced to an absolute minimum.

Eventually a comprehensive concept for the erection of the massive reinforced concrete walls and slabs was provided by German formwork specialist PERI. Using standardised Peri TRIO formwork panels, which utilise a single connection for all element joints and use few components, adjustments and modifications for the different lining sections, with numerous box outs and reinforcement connections, were carried out quickly and efficiently.

Due to the large quantities of concrete required in the caverns and the expected high temperatures, the original intention at tender stage was that concrete pumping would not be used. However, the plan to place the concrete using crane and skip was soon ruled out. This was due to the fact that the commencement of the lining works coincided with the installation of the hydro-mechanical equipment, meaning very limited availability of the powerhouse’s auxiliary crane.

Instead, two Putzmeister concrete distribution booms were employed to reach all parts of the cavern and a pumpable concrete mix was developed, modified and adjusted several times, to keep the temperatures low and minimise shrinkage cracking. An in-situ concrete mixing plant was also installed on site to allow independence and flexibility of supply.

The construction sequence was arranged so that the assembly, installation and concreting works were carried out in a staggered approach between the generating unit locations. For single-face concreting, Peri’s TRIO panels were used together with brace frames in order to safely transfer the fresh concrete pressure into the sub-structure. The modular brace frame units were coupled for concreting heights up to 8.5m without requiring accessories.

The loads resulting from the 4m thick walls were correspondingly high, but during the concreting works were safely carried by Peri’s ST100 stacking towers and Multiprop aluminium slab props. These lightweight individual components, along with a simple assembly process, proved to be particularly advantageous in planning and reduced the need for time-consuming static calculations.

In areas of large load concentrations, Peri’s HD200 heavy-duty props supplemented the shoring concept. Also frequently used in bridge construction, the aluminium prop sections have an enormous load-bearing capacity (200kN) with low individual weights.

For scheduling reasons all other lining works on the contract also had to be completed at the same time as the lining works for the main powerhouse cavern. Works in the transformer cavern were scheduled to compensate for interruptions in the powerhouse, primarily during equipment installation, while for the reinforced concrete lining of the tunnels a Peri tunnel formwork carriage was used.

For some concreting sections of the surge tank a continuously adjustable Peri RUNDFLEX circular formwork was used without anchors. Instead, loads from the distribution walers were transferred to a central core through heavy-duty spindles.

Self-retracting in Spain

Overcoming the extremely complex logistics and tight space constraints at Kops II, by using simple and effective standardised components, was key to the successful completion of the demanding lining works, within an extremely tight project schedule.

However, the major trend seen in recent years has been the move away from labour intensive operations involved in the placement, erection and adjustment of formwork. Some of the largest self-retracting steel forms used to-date in Spain were recently employed by the UTE La Herradura JV, on three twin-tube highway tunnels, which form a major part of the Mediterranean Highway scheme between Cadiz and Barcelona.

Following drill and blast excavation of the 12.5m wide twin Cantalobos, Calaceite and Gato tunnels, which total 2.7km in length, the JV adopted twin 15m long CIFA forms to cast the final linings. The units came complete with an integrated wall vibration system and a CIFA DCL 750 automated concrete distributor.

Due to multiple curves and counter-curves in the alignments and a consistent variability in height, the use of self-retracting forms with adjustable articulated struts was selected over traditional anchored forms in order to reduce the time and labour that would have otherwise been required to deal with the variable geometry.

In particular, the need for continuous and precise overlap adjustment of the arch form on the kicker and time required for anchor installation would have had a big impact on schedule, “and could have lead to the risk of higher deformations on the longitudinal joint,” says Eugenio Bertino, head of CIFA’s tunnelling formwork division.

With a maximum casting rate of approximately 8 hours and a minimum curing time of 12 hours, concreting of each 15m long shutter section was performed in a prolonged single shift, with six 15m casts undertaken each week.

For the shorter Calaceite (278m) and Gato (220m) tunnels a single form was employed for each twin bore. On the longer Cantalobos tunnel (2170m) the two forms were used in parallel, achieving 4.3km of completed lining in just eight months. “These are considerable performances considering the complicated geometry and the long curves present,” states Bertino.

Once the main lining works were finished, one of the formwork units was modified in order undertake the lining of four 40m long widened tunnel sections that had been constructed as emergency parking niches. In order to configure the unit, elements from the original formwork structure were reused and a 2.2m wide ‘key’ added into the crown of the unit to create the wider profile.

For ease of transport and repositioning of the form between the parking niches, the entire unit was built onto a truck-mounted hydraulic “turret”. The use of this solution provided considerable reductions in overall capital equipment investment and job-site labour costs, as well as providing the contractor with the opportunity of future reuse, or even rental, of the “turret” transportation device.

Rentable solution in Trondheim

Along with the development of standardised formwork components, there has also been much growth in the rental market. One such example is a highway tunnel currently under construction in Norway. As part of the E6 northern relief-road project in Trondheim, a high-capacity bypass is being built to skirt the harbour area. Scheduled to open to traffic in 2010, the new bypass will alleviate traffic congestion in the city-centre. Part of this bypass will run through a tunnel 10m beneath sea level.

The 505m long tunnel is being built by Bilfinger Berger using two fully rentable travelling formwork units from Doka. The units are based on standardised rentable system components, and are fitted with Doka’s FF 20 wall formwork and 3-SO formwork sheeting.

The construction trench is inside a sheet-pile cofferdam with walls up to 16m high. Following completion of the in-situ casting of the concrete base slab, the tunnel walls and the cover-slab are poured in a single operation using the Doka travelling formwork units, in 12.5m long sections that are cast in a weekly cycle.

After each casting section has been completed, the entire formwork unit is transported via rail to the next casting section. Varying by up to 2m in both height and width, the tunnel cross-section has placed high demands on the formwork in terms of adaptability. Therefore interchangeable and compensating elements are incorporated to ensure precise, rapid alterations.

Casting collaborations

Although three very different projects, each demonstrates the importance of partnerships between project teams and manufacturers. Early involvement of suppliers in the planning process can often provide invaluable solutions to site specific problems, or simply assist in the reduction of project costs.

Extensive casting operations underway in Austria’s Kops II powerhouse cavern Mammoth concrete pours underway at Kops II, utilising Peri formwork units One of the two CIFA units used by the UTE La Herradura JV, in Spain Doka’s travelling formwork units at work in Trondheim, Norway