Challenging tunnel geometry

STB 450 offers flexible solution for narrow, inclined rail section

Data & Facts

  • Project
    • Four-tracking of Olten-Aarau rail section (Eppenberg Tunnel), Gretzenbach
  • Principal
    • Swiss Federal Railways (SBB)
  • Contractors
    • ARGE Marti Portal Gretzenbach:
    • Marti AG, Solothurn and STA Strassen- und Tiefbau AG, Olten
  • MEVA systems
  • Engineering and support
    • MEVA Schalungs-Systeme AG, Seon, Switzerland

STB 450 for height flexibility

To speed up rail traffic and boost the efficiency of the mainline rail network, the Swiss government recently gave the green light for a rail infrastructure development programme. One of the projects, in northern Switzerland, is the construction of the Eppenberg Tunnel, which will improve the east-west rail links in the Alpine republic. 

Near the municipality of Gretzenbach, the four-track route branches into two separate lines. Here, the two southern tracks gradually descend and pass below the cantonal road before entering the new Eppenberg Tunnel. For construction of the required approach tunnel, the top-down cut-and-cover method was adopted, with bored piles initially sunk into the ground and a concrete roof deck cast over them. 

Expertise as a crucial ingredient
“The site on one of Switzerland's most heavily trafficked railway lines requires in-depth know-how on the part of the whole team to ensure timely planning of the necessary work procedures,” explains Project Manager Michael Schüpbach. With regard to formwork, this prompted the Marti Group managers to seek MEVA's expert support in tackling some of the challenges posed by the tunnelling project.

Single-sided and space-efficient
Given the spatial constraints governing the works in the cut-and-cover tunnel, the assembly installed for casting the tunnel wall against the bored piling had to offer maximum space efficiency.  To cast the lining with single-sided formwork, the project team thus opted to use MEVA's proven STB 450 support frame. Only 2.45 m wide, the STB 450's robust, heavy-duty steel frame left ample space in the tunnel. This allowed simultaneous use of two mobile units on opposite sides and the unobstructed movement of plant and workers in the tunnel during construction, which helped to speed up the individual operations. 

Panel texture specified 
One outstanding feature of the Mammut 350 wall formwork adopted was its ability to accommodate fresh concrete pressures of 100 kN/m², making it ideal for large-scale applications subject to high pressures. Symmetrically arranged joints and ties, in conjunction with alkus all-plastic facings, which are fitted as standard, ensure uniform concreting results. “We are also glad to cater for any other types of fair-faced concrete finish that may be required for aesthetic reasons,” says formwork expert Raphael Klauser, who supervised the works on site for MEVA. “In order to deliver the specified formwork panel texture, a regular arrangement of flush-jointed panels was fitted over the form face. This completely transformed the visual impact on the finished surface.”

Moving forwards in 10 m steps
Use of the two wood-faced mobile units, each 10 m long, ensured that the works progressed rapidly. The concrete was uniformly placed via four filling nozzles. The operation for this length of pour lasted some six hours due to the hydrostatic pressure. The specified pumpable concrete was then compacted using some 40 pneumatic external vibrators.

STB 450 for height flexibility
After roughly half the tunnel ­section, the large mobile units comprising Mammut 350 wall formwork and STB 450 support frame had to overcome a small obstacle that would have posed major problems for less flexible assemblies. The STB trolley allows fast movement of the support frame units in situations where cranes cannot be used, but the additional challenge here was to climb an approx. 24 cm step. “By casting a small ramp to pull up the support frames, we were also able to use the trolley in the tunnel,” explains foreman Simon Wyss. Yet the change in levels automatically reduced the clear height of the cut-and-cover tunnel over the following metres, thereby necessitating adjustments to the single-sided formwork. “A downstand beam running along the edge of the roof deck gave us a bit of extra leeway in terms of height,” says Simon Wyss. “As a result, we only needed to dismantle a few panels in order to continue using the support frame.” The use of height extensions and differently sized formwork panels allows MEVA's STB 450 support frame to provide a height-adjustable solution. This lends the system ample flexibility in catering for constant or varying heights of up to 13.50 m.