This paper analyses the data monitored during tunnel construction in the Giumello gneiss formation of the Ceneri Base Tunnel in order to find the factors that have a major influence on squeezing convergences. The observed deformations were distributed non-uniformly over the tunnel cross-section and varied along the tunnel. The Giumello gneiss formation is characterized by heterogeneity and anisotropy (faults, schistosity). Analysis of the data shows that the non-uniform distribution of the deformations in the tunnel cross-section is mainly due to the schistosity of the rock mass. The effect of rock anisotropy is, nevertheless, superposed by those of lithology and of fault zones, which also influence the distribution of the convergences.
In a recent paper the authors investigated the reasons for the variability of rock deformations observed during construction in the Sedrun section of the Gotthard Base Tunnel, where heavy squeezing conditions were encountered (Mezger et al. 2013). It was shown that the variability of the squeezing intensity along the tunnel could be traced back to a variable degree of tectonic shearing, variable schistosity orientation with respect to the tunnel axis and the effect of nearby weaker or stronger zones.
The present paper analyses data from the construction of the Ceneri Base Tunnel (a 15.4 km long twin tunnel, which also belongs to the Swiss AlpTransit project) in the Giumello Gneiss formation (hereafter referred to as "GGium"), which was crossed over a length of 1000 m in the eastern tube and 600 m in the western tube. Due to the poor rock quality and the high depth of cover (about 650 m), squeezing was expected from the planning phase. The deformations in the GGium were, however, higher than those predicted at the project stage, resulting in localised damage in the shotcrete shell (Pini & Skaanes 2006, Merlini & Falanesca 2013). After a brief overview of the available data on the geology (Section 2), excavation and support (Section 3) and rock response to the tunnelling (Section 4), an attempt is made to find empirical correlations between rock structure and observed deformations (Section 5). In contrast to the Sedrun section of the Gotthard Base Tunnel, where the alignment crossed the tectonic units almost perpendicularly, in the GGium formation the schistosity and fault zones lay parallel to the tunnel axis (a so-called "parallelismo zone"). Squeezing phenomena were therefore less variable along the tunnel. Nevertheless, squeezing was characterized by a very non-uniform deformation of the tunnel cross-section. As rock anisotropy is known to lead to potential asymmetric deformations of the profile (cf., e.g., Steiner 1996, Goricki et al. 2005, Leitner et al. 2006, Barla & Barla 2009), particular attention is paid here to the examination of the influence of schistosity on the convergences.