Cracking of segmental lining under investigation 19 Sep 2019

Cracks have appeared in a section of segmental lining that had been reinforced with synthetic fibres on an extension of the Izmir Metro in Turkey. According to local news reports, the segments were reinforced with a combination of glass-fibre reinforced polymer (GFRP) and macro-synthetic fibres (MSF). The exact cause of the cracking is not yet known and is subject of an ongoing an investigation.

Although local news reports focused on the use of synthetic fibres, industry experts told TunnelTalk that cracking could have been caused by a range of factors. “It is likely that the segment was not properly designed,” said Ralf Winterberg, Group Chief Engineer of synthetic fibre manufacturer BarChip. “It is not the type of reinforcement but the design that is inadequate.”

Cracks developed in the synthetic fibre reinforced segments

“It could indicate a misalignment of the tunnel segments or an area of higher pressure,” added Jeroen Smet, Technical Sales Director of Adfil. “It could be something to do with the concrete itself. You should also consider that a TBM pushes on these segments with a force far greater than anything induced by the hydraulic or ground pressure. The moment a TBM is slightly out of alignment, there is a huge peak force on these segments that could cause cracking.”

Neither BarChip or Adfil are involved in the Izmir project. The supplier of the fibre reinforcement has not been disclosed.

Smet also warned against overreacting to the situation. “If one segment cracks, if does not mean the safety of the tunnel cannot be guaranteed anymore.” There is no indication in local reports that the cracked segments on the Izmir Metro resulted in any risk to safety or to the successful completion of the project. Once the cracking was observed, the reinforcement was changed to steel but it is not known if that is steel fibre or steel rebar cages.

Interest in the application of synthetic fibre reinforced concrete segments is high in the industry with some yet to be convinced of its use, an attitude that Winterberg was quick to counter. “If you look at what synthetic fibre can do, if properly designed, it will work. For example, you can find GFRP being used successfully for permeant reinforcement on the Thames Tideway project [in London], while MSF was successfully used recently in Missouri on the Blacksnake Creek stormwater runoff tunnel, as well as in other locations.”

The one-pass segmental lining on the Blacksnake Creek project is reinforced with BarChip 54 MSF as an alternative to steel fibre or traditional rebar reinforcement. The decision by project designer Black & Veatch to use MSF reinforcement came after successful laboratory tests and field trials.

It is not clear when the results of the investigation into the causes of the Izmir Metro lining cracking will be released. In the meantime, Smet cautioned against drawing conclusions too quickly without knowing the full facts. “It is always dangerous to judge from a distance. In the end, there can always be unforeseen things that cause segments to crack.”

To support the development of the technology, the ITA, through its Working Group 2 and ITAtech Committee, has published best practice guidelines and reports for the design and production of segmental linings. ITAtech has produced two fibre reinforced segments best practice guidelines, one on design considerations and the other on production aspects . A report about all types of segmental lining design is published by the ITA WG2. Reports by the International Association on a wide range of industry related topics are available for free download from its Publications library.