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Abstract Such discontinuities as weak interbeds，faults，joint distributed in surrounding rock mass play an important and decisive role in the stability of surrounding rocks of underground openings and the safety of supporting system in most practical engineering cases. Based on the equal deformation and strength for discontinuities，a method is presented，which can be applied to simulate the thickness of weak interbed in the numerical test with unique FEM mesh；and its validity，reasonableness and practicability are validated via some numerical tests. This method is composed by the COJO element，which is a perfect element formed in geotechnical FEM software FINAL to simulate the properties of contact，and influenced zone of interbed that is a fine mesh zone with constant thickness in FEM mesh. In this method，The mechanical parameters for influenced element are altered in order to reflect the various thicknesses of faults. Based on this method，systematic numerical tests have been conducted to study the stability of a tunnel surrounded by different thickness weak interbed buried in surrounding rock mass and the safety of supporting structures. These different thickness weak interbeds are with the thicknesses of 0D，0.005D，0.010D，0.020D，0.200D，respectively，where D is span of underground opening；and the surrounding rock masses are graded in classes II，III，IV. Some qualitative and quantitive laws on the influence of weak interbeds with different thicknesses are summarized，especially the influences on the deformation，stress of surrounding rock mass，as well as the inner force of shotcrete supporting structure. These results may have reference and guide significance to the selection of location and layout of underground opening，the design of supporting system and the construction of underground openings，especially these tunnels stroken to the faults.
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Received: 24 April 2009
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