A 1 km long excavated section of the Semmering Base Tunnel has been analysed. With a few exceptions, two distinct and continuous features of the system behaviour can be observed: (1) the displacements of the sidewalls are larger than of the crown and of the shoulder points; (2) the displacements of the right sidewall are larger than those of the left sidewall. As the rock mass in this section comprises different types of rock and the rock mass structure varies too, the anisotropic displacement pattern must have its origin not only in the geology but also in the excavation geometry and in the boundary conditions. To identify the main reasons for the anisotropic behaviour, construction details, primary boundary conditions and the rock mass are analysed (from preliminary investigations / design, and as met during construction). Factors which do not cause such anisotropic behaviour are excluded. Results from numerical simulations validate hypotheses introduced to explain the anisotropic displacements. Including the geological and geotechnical site observations in the study of the displacement pattern, the cause for anisotropic displacements and for any deviation from the normal behaviour could be found. The anisotropy origins from the orientation of the foliation and from the primary stress state with the major horizontal stress pointing either to the left or the right side of the tunnel. Deviations are mostly caused by geological inhomogeneities and structural features.

1. Introduction

To evaluate observation results of accomplished projects is key for basic understanding in civil engineering. In tunnelling the engineers still have only an approximate idea about the conditions surrounding the opening even after the tunnel has been excavated. Thus, site observations can contain information helping in identifying the origin of events (i.e. rock falls) and to understand the ground- and system behaviour in general. As every tunnel with its unique distribution of boundary conditions is a prototype, from each case study valuable lessons can be learned for the safe construction of tunnels to be built. In this paper, the system behaviour (i.e. displacements of the shotcrete liner) recorded at the first kilometre of track 1 of the construction lot “SBT1.1”, part of the Semmering Base Tunnel in Austria, is evaluated. The Semmering Base Tunnel consists of two single track railway tunnels with a length of approx. 27.3 km. The construction of the most Eastern lot of which the tunnel section analysed in this study is part of, started in July 2015. With today, 10 August 2019, approx. 3.5 km of each of the 7 km long main tracks of lot “SBT1.1” are excavated. (ÖBB, 2019)

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