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MICROSTRUCTURE FRACTURE CHARACTERISTICS AND DILATANCY EFFECT OF ROCK BRIDGE UNDER DIRECT SHEAR TESTS
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摘要: 节理岩体的剪切贯通机制影响着边坡的稳定性。为揭示锁固段型非贯通节理岩体在不同连通率和法向应力下的破坏特征，在室内直剪试验中结合高速摄影与AE特征参数分析其剪切全过程及剪胀效应。结果表明：节理岩体直剪试验中，法向应力的增大及节理连通率的下降会致使峰值剪切应力及峰值剪切位移增大；节理连通率与法向应力对其破坏特征具显著影响，表现为节理连通率较高且法向应力较小时呈直接剪断的特性，节理连通率降低后呈拉剪复合破坏，出现剪胀现象，而法向应力的增大使得剪胀效应呈波动现象；AE特征与岩桥贯通过程一致，事件数峰值随节理连通率的降低及法向应力的增大而增大且位于峰后。试验得到的岩桥细观破坏特征与剪胀效应对研究锁固段型岩质边坡的贯通破坏机制具指导意义。Abstract: The jointed rock masses with incomplete end joints and coalescence mechanisms of rock slopes are often encountered in engineering construction. They are complex. In order to reveal the relationship between the failure characteristics and dilatancy effect of end rock bridge under different joint connectivity rates and normal stresses, we carry out the direct shear test of the rock bridge to explore the failure process of the front locked section slope. The whole process of shear stress change is analyzed by high-speed photography and AE characteristic parameters. After test, we find that the shear failure process of the end direct shear specimens can be divided into five stages: crack compaction, stable crack propagation, progressive propagation, strain softening and residual strength stage. The decrease of joint connectivity rate and the increase of normal stress lead to the increase of peak shear displacement and peak shear stress. The joint connectivity rate and normal stress have significant influence on the failure characteristics of rock mass. When the joint connectivity rate is high and the normal stress is small, the cracks propagate in a straight line. With the decrease of the length of joint connectivity rate, the phenomenon of shear dilatancy occurs, and the cracks become irregular curves. Meanwhile, the increase of normal stress causes the dilatancy phenomenon to fluctuate. The AE characteristics are consistent with the rock bridge crossing process. The peak number of AE events increases with the decrease of the joint connectivity rate and the increase of normal stress. The tests demonstrate that the microscopic failure characteristics and dilatancy effect of rock bridge are extremely significant for studying the failure mechanism of the locked section slope as a guide.
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Key words:
- Rock mechanics /
- Direct shear /
- Joint rock /
- Dilatancy effect
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图 2 加载系统、AE以及高速摄像机布置
Figure 2. Loading system, digital AE system and high speed camera layout
图 4 直剪全过程剪切应力-剪切位移曲线
a.法向应力0.70 MPa，节理连通率50%；b.节理连通率50%；c.节理连通率60%；d.节理连通率70%；e.节理连通率80%
Figure 4. Curve of shear stress-shear displacement in the whole process of direct shear
图 6 法向应力0.85 MPa剪切应力-剪位移曲线
Figure 6. Shear stress-shear displacement curve under normal stress 0.85 MPa
图 8 切向位移-法向位移曲线
a.节理连通率60%；b.节理连通率70%；c.节理连通率80%
Figure 8. Tangential displacement-normal displacement curve
图 9 节理岩体直剪试验加载及端部受力示意图
Figure 9. Diagram of loading and end force in direct shear test of jointed rock mass
表 1 剪胀阶段割线斜率
Table 1. Secant slope in dilatancy stage
节理连通率
/%法向应力/MPa 0.7 0.85 1 1.15 1.3 60 0.333 0.341 0.312 0.266 0.355 70 0.331 0.110 0.154 0.109 0.073 80 0 0 0.133 0 0 
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表 2 不同节理连通率下的3种剪切破坏模式
Table 2. Three shear failure modes with different joint connectivity
破坏方式 破坏过程及特征 示意图 破坏照片 直接剪断 试样破坏的初裂纹很小，且与剪切面小角度相交，甚至无初裂纹。岩桥直接剪断破坏，剪胀效应不明显，岩桥破坏面较平直，粗糙度较小 
压致拉裂
后剪断在未施加剪切位移之前，试样就出现有压制拉裂，裂纹由岩桥端部发育，之后岩桥剪断 
剪胀剪断 破坏面近于弧形，在剪切过程中拉裂纹的连通会导致掉快现象，剪胀效应也变得更为明显 
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表 3 特征曲线
Table 3. Acoustic emission characteristic curve
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