Engineering Appraisal for the Construction of a road tunnel between the villages of Castel dei Britti and San Lazzaro di Savena (Bologna, Italy)

The analysis of the interaction of the host medium with an underground structure plays a prominent role in the engineering of a tunnel. This project was regarding an engineering appraisal for the construction of a road tunnel near Bologna, Italy. The main object of the project was a characterization of the rock mass into which the tunnel was to be driven, with particular attention to the following properties:

- Topography and accessibility of the area.

- Location of the tunnel with respect to the ground surface and rock formation boundaries.

- Rock types in the rock mass, presence of discontinuities and other defects.

- Rock mass strength, with reference to a rational failure criterion.

- In-situ stresses.

- Hydrogeological regime.


Great importance was given to the description of the geology of the area and to the engineering properties of the rock mass. A 21-days survey period enabled a geological and an engineering map to be created. More specifically:

- The geological mapping made it possible to appreciate how the Bologna foothills, although of modest altitude and ordinary aspect, were characterised by a complex structure, consisting of different sedimentary rocks, which is the result of a long and complex geological history.

- The engineering mapping was carried out with the intention to provide indications concerning the presence of both active slope movements and potentially unstable slope deposits.

In order to construct a model for the behaviour of the intact rock mass laboratories tests were carried out on rock specimens collected during the geological mapping, successively implemented with data published on some journals and papers. The followings were carried out:

- Uniaxial test

- Triaxial test

- Point load test

- X-Ray diffraction test

The Geomechanics Classification System (Bieniawski, 1989), Q index (Barton et al., 1974) and the Geological Strength Index (Hoek, 1995), the latter implemented with the Hoek and Brown criterion (1997), were used to determine guidelines for the rock mass behaviour and support systems. The DIPS® and UNWEDGE® software packages enabled the study of the kinematic possibility of wedge failure to be carried out, whilst a new methodology based on a probabilistic approach and the utilisation of the @RISK® computer program was used to determine the engineering properties of the rock mass for a wider range of conditions. Finally, elements concerning the analysis of the proposed excavation method were also considered.