Abstract：Research purposes: Large span tunnel is easy to collapse because of its complicated geological conditions at the tunnel opening and bad integration of wall rock which generate much influences on construction procedure and quality. In combination with the treatment of collapse of outlet of Heluo Mountain Tunnel on Dali-Lijiang Railway, the summary is made for construction of large span tunnel under complicated geological conditions.
Research methods: According to the landform and geological conditions and collapse situation of outlet of Heluo Mountain Tunnel , the economic and rational treatment measures were decided by adopting the methods of comparing similar constructions , data analyses and optimising programs , and survey and analyses were made for the wall rock and structure deformation, including doing necessary verifications for the main structures.
Research results: According to the landform and geological conditions and collapse situation of outlet of Heluo Mountain Tunnel , the effective measures were taken and good treatment result was obtained.
Research conclusions : For construction of large span tunnel with the conditions of complicated landform and geology and weak wall rock, effective prevention and treatment of collapse and smooth engineering at tunnel opening were the key points for successful tunnelling. The big pipe fence and small pilot pipe grouting were adopted along with remaining core soil for excavation in advance in this construction , so collapse was controlled and treated effectively , engineering succeeded in collapse section and the hard problem in tunnelling was solved.
2007, Vol. 24 
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