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K. Mair am Tinkhof:
"Rock failure mechanisms and rock strength near excavations";
in: "Mitteilungen für Ingenieurgeologie und Geomechanik, 6th Colloquium "Rock Mechanics - Theory an Practice" with Vienna Leopold Müller Lecture", E.H. Tentschert, R. Poisel (ed.); issued by: Institut für Geotechnik, FB Ingenieurgeologie; Eigenverlag, Wien, 2013, ISBN: 978-3-9501738-0-2, 149 - 169.

Brittle failure around openings in rock is a widely discussed topic. Especially deep seated tunnels or excavations in weak or brittle rock are prone to spalling or rockburst in the region of maximum tangential stress around the boundary of the opening. These brittle failure mechanisms can cause a hazard to workers because of falling slabs and problems for tunnel boring machines as gripper pads cannot be seated on the sidewall. Better investigation methods and understanding of the failure mechanisms can increase the security during construction and reduce the cost of tunnelling projects.
This paper will attempt to give an overview of previous research and of the methods to predict such failure mechanisms. It ranges from laboratory tests (Poisel, 1979; Schubert, 1980; Kaiser, Guenot & Morgenstern, 1981-1985; Evy & Cook, 1990) to in Situ observations (Feder, 1977; Martin, 1993-1999) and to their combination for the modelling and prediction of the failure process.
Difficulties in designing underground excavations often result from the fact that constitutive laws in numerical models do not necessarily reflect the actual behaviour of the rock. E. g. the Mohr-Coulomb yield criterion is not able to predict the spalling behaviour near tunnel walls. Based on the fact that tensile fractures depend on the level of confinement, Kaiser et al. (2000) and Diederichs (2003) developed an s-shaped failure criterion to properly describe the entire failure envelope from low confinement spalling failure to high confinement shear failure. Hajiabdolmajid, Kaiser and Martin (2002) suggested using a strain-dependent cohesional weakening, frictional strengthening model.
In order to predict the spalling failure around circular openings uniaxial compression tests on sandstone and marble samples with circular openings were carried out (Nussbaumer, 2013). Only in a few tests, especially on marble samples, brittle failure at the sidewalls could be observed. Thus the onset of plastic deformations was determined using the optical measurement system ARAMIS 3D (GOM, 2007). It became apparent that the maximal tangential stresses in the sidewalls highly depend on the diameter of the opening. The tests revealed that at diameters larger than 20 cm the tangential stresses, both for marble and sandstone, are significantly lower than the uniaxial compressive strength.

Tunneling, Failure Mechnisms, Brittle Failure