Assessing rock mass properties for tunnelling in a challenging environment. The case of pefka tunnel in northern Greece

Abstract

The investigation of alternative solutions for the Thesaloniki Ring Road has been one of the major project design challenges in the recent years in Greece. Pefka tunnel was included in one of the alternatives that have been proposed and thoroughly examined. The total length of the tunnel was *1450 m and it had two branches, with three lanes per branch. According to the geological design, the tunnel was to be excavated through a great variety of formations, such as travertine, shales and graphitic shales, alternations of meta-siltstones and graphitic shales, schistosed and intensively folded meta-siltstones, limestones, gabbros and peridotites. The large number of in situ and laboratory tests allowed (a) the reliable estimation of the intact rock properties and (b) the development of new relationships between the uniaxial compressive strength (σci) and the point load test index (Is50). These values were used for the estimation of the rock mass properties employing different methodologies. Finally, based on all available data, the anticipated rock mass behaviour in tunnel excavation is described. The key issues of this procedure are illustrated in the present paper.