ABSTRACT. Flysch rock masses can be characterized as some of the weakest and most challenging geological formations that engineering geologists and geotechnical engineers face in the design and construction of major infrastructure projects. Flysch is composed of varying alternations of competent sandstones and weak, of low generally strength, pelitic sediments, and is associated with orogenesis, deformed from light fracturing and/or folding to severe thrusting and shearing. The flysch has thus suffered from intense compression, and very weak rock masses have been produced. In these cases, the original structure is no longer recognizable, and blockiness is often lost. Additionally, siltstone members (or clayey rocks as clayshales and claystones) are very vulnerable to weathering, and fissility may be developed parallel to the bedding when these rocks are exposed to the surface or are very close to it from wetting – drying processes. Therefore, the special weathering profiles and groundwater conditions, but also the paleogeographic evolution that can be met in a flysch environment, complete the possible geological model. Consequently, numerous lithotypes with undisturbed, fractured, heavily sheared or even chaotic structures can be produced in a flysch environment. Consequently, an engineering geological characterization of flysch rock mass types and its numerical values, must respect the various geological particularities and the engineering geological specific characteristic “keys” the geomaterial embodies.

Flysch formations exist in the margins of major mountain chains in the Mediterranean region. The experience gained by the design and construction of major infrastructure projects and more particular in tunnelling, surface excavations and dam foundation, across central and northern Greece, provided remarkable insight and plenty of data with respect to the engineering geological and geotechnical challenges in a flysch environment. In particular, the experience gained from the design and construction of the 62 tunnels along the Egnatia Highway in Northern Greece, in difficult and diverse geological conditions (Marinos, 2012), using the Hoek - Brown failure criterion and the GSI system led to one of the first geological additional versions of the GSI geotechnical classification approach to weaker, geologically more complex and heterogeneous rock masses like flysch (Marinos and Hoek 2000, Marinos, 2007; 2017). As a result, flysch formations are classified into 11 rock mass types (I to XI) according to the siltstone-sandstone participation and their tectonic disturbance. Use of the geological strength index (GSI) rock mass classification system and the associated m, s and a parameters transfer equations, linking GSI with the Hoek–Brown failure criterion (Hoek et al. 2002; Hoek and Brown, 2018), provides a demonstrated, effective and reliable approach for the prediction of rock mass strength for surface and underground excavation design and for rock support selection. Experiences from tunnelling, slope cuts and dam foundations in flysch environment are presented here.

 

Flysch and Landslides: A combination of weak rocks, steep slopes, rainfall and snowmelt, as well as strong expected ground accelerations from earthquakes, create a difficult environment, prone to landsliding. Therefore, the behaviour of flysch formations in slopes is often connected with severe deep-ground sliding, rotational and translational sliding, mudflows but also with systematic structural instabilities and rockfalls. Responsible for these variant sliding mechanisms are the different geological models that are built in the flysch series. Hence, slope failure mechanisms in flysch formations are different according to the siltstone-sandstone participation, the intensity of tectonic disturbance and the weathering degree. Several conceptual geological models, related to numerous landslide case studies, are illustrated and analyzed in accordance to the different sliding mechanisms.

To understand the ground behaviour that is responsible for the previously described significant slope instabilities, a standardisation of the qualitative engineering geological characteristics and the assessment of the slope stability behaviour for flysch rock masses have been performed. Flysch’s behaviour is controlled by the tectonic disturbance, the siltstone-sandstone heterogeneity, the weathering degree, the groundwater pressures and the geometry of the main structural features of the rocks. Therefore, the engineering geological evaluation includes the following criteria: i) qualitative characteristics of the rock mass quality (lithology, rock mass structure, weathering, strength, joint characteristics, water presence), ii) measurements of key structural elements, iii) geotechnical classification with the GSI system and, iv) estimation of the weathering profile.

 

Flysch and Tunnelling: The experience gained by the recent tunnelling projects in the Greek mountains with the creation and use of a geotechnical database named “Tunnel Information and Analysis System” (TIAS, Marinos 2012), comprising a great number of geological and geotechnical data from the design and the construction of 62 twin tunnels, excavated with conventional mechanical means, 12 of them in flysch, enabled the determination of the possible rock mass types of flysch and the engineering geological characterisation in terms of properties and the standardization of their behaviour in underground construction.

Flysch, depending on its type, can be stable, even under noticeable overburden, exhibit wedge sliding and wider chimney type failures, or cause serious deformation even under low cover. The magnitude of squeezing and tunnel support requirements for every flysch rock mass type, under different overburdens, are discussed.  A detailed presentation of the range of geotechnical behaviour in tunnelling for every flysch rock mass type (I–IX), which is based on engineering geological characteristics, is presented. Generally, the behaviour of the flysch formations during tunnelling depends on three major parameters: (i) the structure, (ii) the intact strength of dominant rock type, and (iii) the depth of the tunnel. The expected behaviour types (stable, wedge failure, chimney type failure, raveling ground, shear failures, squeezing ground) can be illustrated in a Tunnel Behaviour Chart (TBC) (Marinos, 2012).

Detailed principles and guidelines for the selection of the immediate support measures are proposed based on the principal tunnel behaviour mode and the experiences from numerous tunnels. A wide range of temporary support can be applied in flysch rock masses. They vary from very light to very rigid or yielding for severe squeezing conditions. Specific suggestions for the theory of temporary support in tunnel excavation through each flysch type are presented. These proposals take into account both the rock mass behaviour and the critical failure mechanism.

 

Flysch and Dam foundation: Dam foundation in flysch rock masses often includes challenges like compressible rock masses, presence of incompetent members and fractured and sheared zones of low to very low strength and diverse heterogeneity. These conditions often lead to a single choice selection of the construction of an embankment, earth/rock fill dam and/or, seldom, the strengthening of the foundation zone or the selection of an alternate dam location.

Flysch, a typically impermeable formation, has the particularity of presenting alternations of strong brittleness with weak rocks. Low permeability formations may not be of interest in terms of water resources, but they may develop a permeable zone close to the surface where they exhibit a loose and open structure due, mainly, to weathering. The question that arises relates to the depth of this permeable zone and the mean value of the corresponding permeability. This is particularly important in the case of dam foundations and the design of the grout curtain beneath it. The analysis of a good number of in situ permeability packer tests in the various litho-types of flysch reveal the low permeability of its rock mass with very small differences among the types. role of the presence of siltstone interlayers is predominant in all types, even in those types where their participation is very low. Additionally, the history of compression tectonics, from which the formation suffered, led to a homogenization of all types in terms of mean permeability values. This value is of about 5*10-7m/sec for the first tens of meters below surface. The decrease in relation to depth is progressive but with significant scatter. Taking into account a limit of 3 to 5 Lugeon, the depth of the grout curtain necessary for high dams may be of some tens of meters in the case where the flysch is not particularly tectonized and can reach 100 or even more meters when it is heavily disturbed.