In mechanised tunneling, concrete segments can be positioned to create the inner lining of the tunnel. The action of the tunnel boring machine is toleranced to create a clearance between the existing rock and the segments. In order to enable transmission of the drilling forces and reduce surface subsidence, this annular gap has to be completely filled with grouting materials in a process generally referred to as backfilling, with the materials used falling into one of two categories – single-component (1C) and two-component (2C) grouts. The 1C grouts contain cement, additives, aggregates and water. Concrete admixtures are used to create specific application properties. 2C backfill grouts consist of cement, bentonite and water. Here stabilisers are used to provide for the necessary application properties while ensuring a working time of 48 to 72 hours. 2C grouts are suspensions made up with large amounts of surplus water, allowing them to be pumped over distances of several kilometres. They are thus comparatively easy to use. In Asia, 2C backfill grouts have been in use for many years. They were applied for the first time in Germany for the Emscher Project. And here too, MC Bauchemie worked closely with Porr Bau as a key player in developing the required formulation. MC supplied all the necessary additives and provided the application engineering support.
Limits of Conventional TBM Backfill Systems
Both methods – 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 huge volume increase of around 60 %. The associated swelling pressures are so enormous that elevations at the earth’s surface can occur and underground structures can be permanently damaged or even destroyed. When working in strata containing anhydrite, it is therefore essential to prevent the release of surplus water from the grout to the surrounding rock. This is impossible with cement-bound materials.
A Backfill Grout Suitable for Anhydrite Rock
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.