Molasse consists of a series of tectonically undisturbed sediments of sandstones, conglomerates, siltstones and marls, produced by erosion of mountain ranges after the orogenesis. Molassic rock masses may have very different structure close to the surface as compared to those confined in depth, where bedding planes, do not appear as clearly defined discontinuity surfaces. The present paper focuses on the tunnelling experience through molassic formations, based on data from the design and construction of twelve tunnels in Egnatia Highway in northern Greece. The fundamental tool for this research was TIAS database, which has been used for the storage, retrieval and correlation of the available data. The rock mass behaviour that was anticipated during construction is described and the molasse specific GSI chart is validated. In conclusion, a standardization of the geotechnical behaviour of molassic rock mass types and the temporary support categories that have been successfully implemented is presented.

1 Introduction

The present paper addresses the issue of tunnelling through molassic rocks, based on the experience gained from the design and construction of tunnels in Egnatia Highway in northern Greece. The scope of this work is to set a framework concerning the different types of molassic rocks, the geotechnical behaviour of each type in tunnelling and the temporary support philosophy, both for underground construction and portal areas. The main characteristics of the geomaterial in question that cause its particular behaviour are (a) the lithological heterogeneity, as the formation consists of almost uninterrupted series of sandstones, siltstones, marls or claystones and conglomerates, with alternations of layers from just some centimeters to even few meters thick, (b) the low to moderate strength of the intact rock of these alternations, (c) the compact, almost intact structure in depth, evenwhen sandstone beds alternate with siltstones and (d) the problematic behaviour of the formation close to the surface, due to slaking and weathering, especially of the siltstone parts.

A fundamental tool of this research was a database specifically established, where all relevant to investigation, tunnel design and construction datawere stored and retrieved, the TIAS database (Tunnel Information andAnalysis System, Marinos et al. 2012).The 12 tunnels in the molassic formation of the Panagia - Grevena section of Egnatia Highway. are shown in Table 1.

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