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Abstract The unloading failure mechanism of columnar jointed rock mass in tunnel was studied by using in-situ measuring method，laboratory and scanning electron microscopy(SEM) experiments method. The results indicated that：(1) The fractures contained in the columnar jointed rock mass mainly include three types of joints：joints between the columnar，implicit joints inside columnar and crosswise joints. The joints between the columnar formed in the cooling process of basaltic lava flows are rough. The implicit joints inside columnar，which would resolve into explicit joints after excavation，have plumose steep sill in their surfaces. The crosswise joints，whose direction almost horizontal，is a smooth surface. (2) Compared with SEM analysis under standard failure mode，the results of SEM in site showed that：implicit joints and the joints between the columnar are native tension joints，while the former is mainly in tension failure and the latter is composite tension-shear failure. The crystals is so well developed on the crosswise joints that no apparent breakage characteristic induced by excavation is found. (3) The unloading failure mechanism of columnar jointed rock mass is：The fractures are so well developed that the columnar joints would break from both exterior and interior which lead to the results that the collapse columnar are mainly small columnar and rarely have intact columnar. That is the columnar will first break between columnar and then break from inside when normal force is beyond the tension strength of joints between columnar and implicit joints，respectively. Then the friction between the columnar would come which formed shear failure. The small columnar would collapse at last under the effect of gravity and excavation- disturbance for the crosswise joints are so well developed and large scale collapse would happen in severe cases. This mechanism was a good explanation of three weak planes coexists at the rupture plane of sidewall in site.
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Received: 26 February 2013
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2013, Vol. 32 
