A large part of South Portugal (Fig. 1) consists of rocks of the Carboniferous age belonging to the large zone which is paleo geographically called the Carboniferous Basin of the iberic Peninsula (Oliveira et al.1979). Near S. Tiago do Cacem and Abe la the occurring formations are intensely folded shales and graywackes (dated from the Namurian) with frequent crushed zones and abundant faults, as well as acid and basic veins; metamorphism, however, is not very apparent. The Implantation in this zone of an over 13 km long tunnel (a part of the system which conveys water from the Sado river to the Morgavel dam for the Sines industrial complex) gave rise to a detailed geotechnical study of the involved rock mass, not only during the design stage, but also during the construction works (Mello Mendes, 1981).
Several in-situ and 1aboratory study techniques were tried in an attempt to obtain quantitative information for the purpose. The poor quality of a high percent age of the occurring rock led, however, to the conclusion that some of these techniniques, although usually employed for the geotechnical study of tunnels in rock, were in this case not applicable or only applicable with a restricted usefullness. In spite of the difficulties found, It was never theless possible to develop a convenient characterization methodology which, having already shown an unquestionable utility in the case of the aforesaid work, seems to be worth describing in order to allow future application to other works in the same or in similar rock masses(Ojima,1981).
(Figure in full paper)
Eomorphologically the region studied may be considered as a region with low relief, the maximum altitudes being near 200 m; for the above-mentioned tunnel the over burden varies between 40 and 120 m.
On a regional basis, the geostructural aspects are apparently simple; however, when analysed at the scale of the work, they be come quite complex, not only the already mentioned faults, veins and folds being of major importance but also the frequent changes of attitude of the folds and the intense jointing, which is due to various joint systems. The predominance of the shales over the graywackes also enhances the importance of the shale foliation in what concerns its contribution to the jointing and to the weathering penetration.
The surface topography and, even more, the photogeology clearly reveal the occurrence of zones of weak resistance, which result from faults or from crushings associated to folds.
The hydrogeologic conditions of the rock mass are regulated, on one hand, by the compartments built up by the argillaceous material of the faults and the crushed zones which form nearly impermeable screen, and, on the other hand, by the zones in which the very fractured quartz veins allow an easy percolation.
A first seismic refraction campaign, carried out during the stage of the preliminary studies for the tunnel implantation, gave for the considered rock mass a great variation of the wave propagation velocity, from less than 1600 m/s up
