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Abstract The failure phenomena such as large deformation and collapse usually occur in weak surrounding rock of tunnels after excavation because of low strength，poor stability and long deformation period. The failure phenomena result in intense deformation and damage of the first stage support，which have a strong impact on construction and security of tunnels and become a major problem in the construction of tunnels. Mechanical characteristics of the support system are analyzed by monitoring axial force of rock bolts，pressure of surrounding rock and stress in the steel frames for Tianpingshan tunnel of Guizhou—Guangzhou Railway. It is concluded that，for the weak surrounding rock which has large deformation rate in the first stage of excavation，the axial force of rock bolts is often large in the middle and small in the ends；and its variation often lasts for a long period of time. The profile steel frame has larger surrounding rock pressure than the grid steel frame by 70%–90% under the same geologic conditions；the profile steel frame has more uniform stress distribution and better support effect than the grid steel frame.
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Received: 21 January 2011
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2011, Vol. 30 
