Bachiller

Tunnelling in overstressed rock
E. Hoek
Evert Hoek Consulting Engineer Inc., Vancouver, Canada

P.G. Marinos
National Technical University of Athens, Greece

Keynote address presented at EUROCK2009, Dubrovnik, Croatia, 29-31 October 2009. Published in Rock Engineering in Difficult Ground Conditions - Soft Rocks and Karst. Vrkljan, I (ed). London: Taylor and Francis Group, 49-60.COPYRIGHT NOTICE

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Tunnelling in overstressed rock
E. Hoek
Evert Hoek Consulting Engineer Inc., Vancouver, Canada

P.G. Marinos
National Technical University of Athens,Greece

ABSTRACT: Overstressing of the rock surrounding a tunnel can result in either brittle fracture of the intact rock or shear failure along pre-existing discontinuities such as joints or shear zones. These two types of failure can co-exist and the extent to which the failures propagate depends upon the characteristics of the rock mass, the magnitude and directions of the in situ stresses,the shape of the tunnel and the intensity and orientation of the discontinuities. Numerical analysis plays an increasingly important role in the assessment of tunnel stability and design of reinforcement and support. Rapid evolution of computer software and hardware offers the potential for the calibration or possible elimination of some of the empirical techniques upon which tunnel designers haveto rely.

1 INTRODUCTION The rock mass surrounding a tunnel can be overstressed when either the intact rock or the discontinuities fail as a result of the stresses induced by the excavation of the tunnel. There are a variety of conditions under which such overstressing can occur and three examples will be discussed in this paper. These are the failure of massive intact rock, sparsely jointedanisotropic rock masses and heavily jointed rock masses. These three cases serve to illustrate the basic principles of assessing the type and extent of failure and of designing reinforcement or support to stabilise the tunnel. When the conditions for overstressing exist it is seldom possible to prevent failure initiating. When such failure occurs the aim of the design of reinforcement or support isto control the propagation of the failure and to retain the profile of the tunnel. Stability becomes increasingly difficult to control if ravelling of the near surface rock pieces is allowed to occur and the interlocking or arching of the rock mass is destroyed by progressive deformation. Support must be chosen to match the deformation characteristics of the rock mass surrounding the tunnel. 2 INSITU STRESSES Of all of the quantities that the geotechnical engineer is required to estimate or to measure, the in situ

stress field in a rock mass is one of the most difficult. The vertical stress can be approximated, to an acceptable level of accuracy, by the product of the depth below surface and the unit weight of the rock mass. However, in complex tectonic environments the verticalstresses may be lower (Mayer and Fabbre, 1999) or higher (Stille and Palmström, 2008) than the overburden stress. Horizontal stresses of interest to civil and mining engineers are influenced by global factors such as plate tectonics and also by local topographic features. Zoback (1992) described the World Stress Map project that was designed to create a global database of contemporary tectonic stressdata. The data included in this map were derived mainly from geological observations on earthquake focal mechanisms, volcanic alignments and fault slip interpretations. The results included in this map, available at www.world-stress-map.org, are very interesting to geologists involved with regional or continental scale problems. However, other than providing a first estimate of stress directions,...