Estimation of geotechnical properties and classification of geotechnical behaviour in tunnelling for flysch rock masses

摘要

Flysch formations are generally characterised by diverse heterogeneity, presence of members with low strength and tectonically disturbed structures. The complexity of these geological materials demands a more specialized research and geological characterization, due to the special features of their rock mass types regarding both their structure and their lithological characteristics. The present paper proposes a standardization of the qualitative engineering geological characteristics, the geotechnical classification, the assessment of the behaviour in underground excavations and the qualitative instructions-guidelines for the primary support measures for flysch rock masses. In order to investigate flysch rock mass properties, 12 tunnels of Egnatia Highway, in Northern Greece, were examined taking into account data from their design and construction records. More specifically, flysch formations are classified here in 11 rock mass types (Ⅰ to Ⅺ), according to the siltstone-sandstone participation and their tectonic disturbance. A modified GSI chart for heterogeneous rock masses such as flysch is presented, where a certain range of GSI values is proposed for every rock mass type. The engineering geological behaviour of flysch in tunnel excavation lies within a large spectrum. A further classification of every flysch rock mass type based on their geotechnical behaviour during tunnel excavation, in relation to categorised failure mechanisms (deformation due to overstressing, overbreaks or wedge failure, "chimney" type failure, ravelling ground) and in situ stresses, is presented. Finally, general principles and guidelines for the selection of the immediate support measures are proposed.