基于电镜扫描实验的柱状节理隧洞卸荷破坏机制研究

RESEARCH ON UNLOADING FAILURE MECHANISM OF COLUMNAR JOINTED ROCK MASS IN TUNNEL BASED ON SCANNING ELECTRON MICROSCOPY EXPERIMENTS

采用现场调查、室内力学实验、细观实验的方法,对导流洞内柱状节理玄武岩的卸荷破坏机制进行研究。研究结果表明:(1)柱状节理岩体内节理面主要包括:柱间节理面、柱内竖直隐节理面、柱内水平节理面,柱间节理面表面粗糙不平,为岩浆岩冷却后形成;柱内竖直隐节理面存在羽毛状陡坎,开挖卸荷后易松弛成为显节理;柱内水平节理面表面平整,方向近乎水平。(2)将现场破坏面与标准破坏模式下的电镜扫描结果对比表明:柱内竖直隐节理面为原生节理面,破坏形式主要为拉破坏;柱间节理面为原生张拉节理,破坏形式包括拉伸与剪切及其混合破坏;水平节理面为构造运动中形成,无明显开挖卸荷破裂特征。(3)柱状节理塌方机制为:受多组节理切割,柱体易沿节理面从柱体内外共同破裂,呈小柱体垮落,即硐室开挖后,当法向力超过柱间节理面抗拉强度时,柱状节理沿着柱间节理面开裂,并滑移;在滑移过程中,柱间节理面相互摩擦,形成剪破坏特征;当法向力超过柱内竖直隐节理抗拉强度时,柱体内部沿着柱内竖直隐节理破裂;节理面相互切割形成小柱体,从柱体内外共同破裂,并向临空面滑落;由于水平节理面非常发育,小柱体最终会在重力及开挖扰动作用下垮塌脱离母岩,严重时形成塌方。该机制可以很好地解释现场边墙柱体破裂面中三弱面共存的原因。

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 thatthe 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.