During the excavation of the underground works for the execution of the new TERNA S.p.A. 380kv power line, that connects Sicily and Calabria, a 12% inclined and 2.8 km long tunnel was made. It finished in a vertical shaft that was 300 m deep and 7 m diameter.
This tunnel excavation, which was executed by means of a double-shield TBM (Tunnel Boring Machine) with 4.1 m diameter, passed through a large part of the Calabro Arch’s crystalline-metamorphic base. The scientific literature used to draft the preliminary project and the geological surveys done during the project phase confirmed that the geological properties led to Calabro Arch’s crystalline-metamorphic base.
However, the nature of some parts of the terrain in some excavation areas, showed how this type of rock mass can present extreme variations. It showed fault areas with extremely high grades of broken rock, with very low GSI (Geological Strength Index) values. It also showed a granulometry similar to a sand-consistency, sometimes similar to a clay-consistency, with a significant quantity of water.
These fault areas, with a massive quantity of water moving inside them, gave huge problems during the tunnel excavation. They created very low progress areas and three "geological events" which caused TBM interruption in the tunnel.
As shown in this paper, the three TBM interruptions required particular and delicate work, which could only be carried out after some interventions to condition the rock mass property by the use of mono and bi-component chemical grouting using resins. These interventions aimed to improve geo-mechanical properties and allow excavating small tunnels to “unlock” the TBM and then the crossing of fault areas.
Subjects: None provided
Author: Pasquale Petrocelli, Aldo Bellone, Francesca Rossano and Marco Aurelio Piangatelli
Publisher: DFI, EFFC