Anisotropic behaviour of stratified rock masses in tunnelling P. Fortsakis a , K. Nikas a , V. Marinos b , P. Marinos a, ⁎ a National Technical University of Athens, School of Civil Engineering, Department of Geotechnical Engineering, 9, Heroon Polytechneiou Str., 15780, Zografou, Athens, Greece b Aristotle University of Thessaloniki, 541 24, Thessaloniki, Greece abstract article info Article history: Received 28 April 2011 Received in revised form 3 April 2012 Accepted 2 May 2012 Available online 10 May 2012 Keywords: Tunnel Rock mass Stratification Anisotropy Tunnel behaviour Convergence This paper investigates tunnel excavation through stratified rock masses from the engineering geological be- haviour to the rock mass properties quantification and finally to the study of tunnel response, based on nu- merical analyses results. Initially the spectrum of the engineering geological behaviour of stratified rock masses in tunnelling is delimited and the critical failure mechanisms according to rock mass structure are de- scribed. Rock mass simulation as an equivalent isotropic geomaterial through the widely used characterisa- tion systems in most cases cannot lead to a realistic prediction of the distribution and the values of total displacements. In addition, the complete and accurate simulation of all discontinuities networks involves high uncertainty. Therefore in the numerical analyses carried out, based on an already applied approach, the stratification planes, which contain less uncertainty than the secondary discontinuities and affect signif- icantly the behaviour of tunnel, were simulated as separate elements and the rock mass parts between them as an isotropic material. Additionally, using simple rock mechanics principles, an approach for the quantifica- tion of the rock mass properties involved in the analyses is described, which tries to obtain the equivalence between the stratified rock mass and the sum of the distinct rock mass elements (stratification planes and internal rock mass). The numerical analyses depict the mechanism of convergence development in stratified rock masses and the differences between isotropic, anisotropic and transversally isotropic approaches are clearly demonstrated. Based on the results of the numerical analyses the incorporation of the stratification planes leads to an increase of the convergence mainly due to the bending of the rock mass strata where the stratification is tangential to the tunnel section. This increase depends on the GSI value of the reference rock mass and the discontinuities surface conditions. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Tunnel design through stratified rock masses requires the consid- eration of a variety of failure mechanisms since the rock mass exhibits a wide spectrum of behaviour, from stable to squeezing, depending on intact rock and rock mass properties, in situ stresses and the rela- tive direction of stratification with respect to the tunnel section. The appraisal of tunnel convergence in the case of stratified rock masses is much more complex since dominant discontinuities may often lead to a highly anisotropic behaviour of the rock mass. Although the rock mass is principally an anisotropic material, it is often considered as isotropic in tunnel design. The rock mass properties are quantified via classification systems, through which rock mass is con- sidered as an equivalent “mean isotropic geomaterial”. The inaccuracy of this assumption is usually acceptable in cases of uniformly jointed, high- ly tectonised or disintegrated rock mass with no family of persistent parallel discontinuities to control rock mass behaviour. In the case of stratified rock masses at a scale of the tunnel section, the engineering geological behaviour during tunnel construction is mainly controlled by the characteristics of the stratification planes. Therefore it is impor- tant to examine and simulate this anisotropic behaviour based on a dif- ferent procedure: a) Simulation of the whole discontinuities network (dominant and sec- ondary discontinuities). In this analysis discrete elements method, beyond the simplifications of other methods, leads to a realistic sim- ulation of rock mass behaviour. Yet, there is high sensitivity of the results to the discontinuities geometry, persistence and length and the shape of the intact rock parts, data which are characterised from a high level of uncertainty, especially in tunnelling, where ini- tial information comes from surface geological mapping and mea- sured geotechnical data from boreholes. b) Rock mass simulation as a transversally isotropic material. This ap- proach takes into account indirectly the influence of stratification, incorporating different deformability properties at directions parallel and perpendicular to the surface of dominant discontinuities. Engineering Geology 141–142 (2012) 74–83 ⁎ Corresponding author at: National Technical University of Athens, School of Civil Engineering, Geotechnical Department, 9, Iroon Polytechniou str., 157 80 Zografou, Athens, Greece. Tel.: +30 210 7723430 & 3490; fax: +30 210 7723770. E-mail addresses: fortsakis@gmail.com, pfortsa@central.ntua.gr (P. Fortsakis), konstantinos.nikas@gmail.com (K. Nikas), marinosv@geo.auth.gr (V. Marinos), marinos@central.ntua.gr (P. Marinos). URL: http://users.civil.ntua.gr/marinos/ (P. Marinos). 0013-7952/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2012.05.001 Contents lists available at SciVerse ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo