In awarding the main contract on Zurich’s four-track cut-and-cover Thunstetten tunnel – part of the Rail 2000 scheme to provide high speed rail links across Switzerland – client, Swiss Federal Railways made a series of stipulations.
Meier & Jäggi and Zschokke-Locher, with design and site supervision by Ernst Basler & Partner and Rothpletz Lienhard Engineering, was given a tight open construction time of no more than six weeks, with complete works of 1,840m of concrete arch to be finished in less than a year, regardless of prevailing weather conditions.
Significantly it was also asked to guarantee and maintain the watertightness of cracks of up to 1mm wide for up to 10 years, according to German STUVA Standards.
This task was passed to the Rascor group, which had previous successful experience on similar waterproofing projects. It was awarded the complete package for the detail design, supply and installation, as well as supervision of the concrete mix design and placing, and provision of the 10-year guarantee.
Rascor provided the detail waterproofing specifications, measures and control for the complete structure. These included the optimisation of the concrete design mix, limiting the water/cement ratio to 0.48, provision of crack inducers; supervision of concrete placing and curing; and injection of the joints.
System based on experience
The company, which has more than 35 years’ experience in sealing similar structures, opted for the use of its White Tank System. This has been developed through its long-term familiarity with repairing leaking tunnels using meta-acrylamide resins, and involves the use of Rascor’s patented re-injectable Rascotec crack inducers, and the optimisation of all construction processes, including the specification, control and supervision of concrete mix design and placing, all through a single point of responsibility.
As part of the design on the Thunstetten project, up to 800t of steel reinforcement was saved by reducing the amount of distribution steel needed through calculated spacing of the crack inducers. The use of re-injectable crack inducers allows the contractor to set whatever pour length is necessary to maximise productivity.
The crack inducers are effectively stiffened plastic boards with a continuous injection channel down the middle and foam strips either side that allow simultaneous grouting in both directions. These units allow “two-way” grouting in a single operation and can be supplied in widths corresponding to the thickness of the concrete.
With their use, according to Rascor, the only factors limiting the length of a pour is the level of deployment of the telescopic formwork and the restriction of the daylight hours available for concrete pouring.
During the project, which was configured as a 20m wide double arch cut and cover tunnel 920m long, a minimum pour length of 2 x 18m at a rate of one pour per week was achieved.
The sequence adopted was: day one – strip, set up shutter, commence steel fixing; day two – steel fixing and installation of crack inducers; day three – pour concrete and; days four to seven – concrete hardening and curing.
Rascor claimed this working cycle allowed it to set a world record for the 26,000m³ of concrete poured in this project.
A further saving was achieved by limiting the amount of excavation. This meant there was less excavated material to be transported. Excavated material was immediately backfilled four to six stages behind because the White Tank System does not require membranes.
Optic fibres have been installed to monitor the short- and long-term deflections and settlement (up to five years). The project design engineers provided the structural stability calculations on which all reinforcement and joint detailing were based.
Rascor Engineering provided laboratory testing and qualified engineers for installation and injection as well concrete placing and curing. All waterproofing was installed by trained, qualified and experienced in-house personnel. This enabled the main contractor to run the construction site with a team of just nine skilled formwork and concreting operatives.
Long-term maintenance
Rascor, in guaranteeing the long-term effectiveness of its waterproofing, cites its record of 20 years’ claims-free waterproofing. It says the possibility that it would have to return to make good defects is next to negligible.
Furthermore, it points out that the use of extremely low viscosity hydrophillic Rascoflex resins which can be hydrofractured under localised pressure as well as Rascotec crack inducers and channels means that access to joints under the track can be re-injected at any time in the future, should unexpected excessive settlement occur.
The company says that the system caters for temperature variations of between -40°C to +<= 100°C, and that Rascoflex is environmentally friendly and non-flammable.
Low risk of damage
The ability to seal the tunnel from the inside means that unexpected cracking or leakage can be dealt with easily, so that the risk of damaging the waterproofing is negligible.
Further factors enhancing the reliability of the system are that there are few parts which can be damaged during construction, and sealing can be successfully done at the point of leakage.
