Additional geotechnical tests on the breccia

• Technological improvements in tunnel boring machines would be desirable in order to cross the breccia at a reasonable rate of progress.

• The uncertainties, both about the geometrics of the palaeochannels and the geomechanical properties of the solution breccia, are too significant for such a major project.

Additional geotechnical tests on the breccia were performed by the engineering firms under an agreement with the Institute for Geotechnical Engineering, of Zurich, Switzerland.

The future alignment of the tunnel would therefore have to pass through the breccia of the palaeochannels, and the original drilling programme was resumed and adapted in the light of this new finding.

• The geological report, on the basis of an analysis of all the available data, proposed a longitudinal section, specifying the distribution of the various flysch deposits and breccia along the route.

This requires a deeper knowledge of the geomechanical nature of the breccia or, more generally, of the materials filling the troughs, in order to have a reliable basis for choosing an excavation method.

The aim of the PPP-07 study was both to update the results of the previous study (PPP-96), following the subsequent discovery of two palaeochannels (troughs) filled with Quaternary material (breccia) in the middle of the strait, and to study whether those deformable materials are suitable for tunnelling and look into appropriate drilling methods.

The technical, environmental, socio-economic and legal parts of the overall evaluation stressed the questions to be answered in the study of the geotechnical characteristics of undersea geological formations (breccias) and shortcomings in the development of certain technical and security aspects of the basic option.

Despite the well-known difficulties of inferring the geomechanical behaviour of deep undersea terranes on the basis of laboratory testing of samples that are inevitably very debased by comparison with on-site conditions, it may be indicated that, in principle, the flyschous breccia that constitutes the greater part of the fill material of the palaeochannels studied appears to be of an impermeability similar to that of the flysch formations in place, albeit with a resistance of about 70 per cent.