MC-Bauchemie and PORR Bau GmbH have jointly developed a new TBM backfill grout based on a geopolymer that the developers say is suitable for use in swelling rock formations. This cementless annular gap filler prevents swelling pressures, is sulphate-resistant and provides a permanent bedding for securing the tunnel tube.
A recent press release from MC-Bauchemie explains:
The two methods currently used for backfill grouting – 1C and 2C – have their advantages, but cannot be used for boring swellable rock, e.g. that containing anhydrite, because both are cement-based and release excess water to the surrounding substrate. If anhydrite comes into contact with water, gypsum is produced as the reaction product. This reaction is accompanied by a volume increase of around 60 per cent. The associated swelling pressures are so enormous that heave can occur at the surface and underground structures can be permanently damaged or even destroyed. When working in strata containing anhydrite, therefore, it is essential to prevent the release of surplus water from the grout to the surrounding rock. This is not possible with cement-bound materials.
MC-Bauchemie and PORR Bau have thus developed a completely new kind of annular gap filler based on a geopolymer. Geopolymers are inorganic binders that do not contain cement and whose structure can be traced back to aluminosilicates. The binder of the new backfill grout consists of granulated blast furnace slag and fly ash. Aggregates are used to stabilise the mix. An activator is added to the supply device via which the backfill grout is introduced into the annular space. This triggers the binder reaction. The grout also contains a variety of phosphates that suppress anhydrite swelling. The concrete-like properties of this TBM backfill grout enable production in a ready-mix concrete plant located at the construction site. The long processing time of the material gives tunnel builders more flexibility, as without addition of the activator the grout can be stored for long periods without deterioration. This novel grout material is currently being successfully used in the anhydrite bearing unleached gypsum keuper of the “Stuttgart 21” project.