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Abstract The failure conditions of grades IV and V surrounding rocks in highway tunnel under different buried depths are studied through a number of model tests. The test simulation of failure process of non-supported tunnel under gravitational stress is carried out successfully；and two failure modes of arch collapse and caving collapse are obtained. The test results show that the failure of surrounding rock initiates from arch crown of tunnel and develops upward progressively. The smaller the buried depth is，the more likely the caving collapse occurs. When the buried depth is less than a certain value，the tunnel can maintain stability because the stress of surrounding rock induced by excavation does not exceed the strength of surrounding rock. For the caving collapse，the larger the buried depth is，the shorter the time of surrounding rock¢s stability is，the more serious the collapse degree is，and also the faster the collapse developing to ground surface is. For the same collapse type，the larger the buried depth is，the larger the changing amplitude of the stress of surrounding rock is.
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Received: 10 May 2010
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2010, Vol. 29 
