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基于指数准则在莫尔空间对岩石剪切强度的研究

STUDY ON SHEAR STRENGTH OF ROCKS USING THE EXPONENTIAL CRITERION IN MOHR'S STRESS SPACE

  • 摘要: 岩石是多种矿物颗粒构成的天然材料,内部存在不同尺度的孔隙、裂隙等损伤;岩体工程设计及灾害防治所使用的强度准则仍在研究之中.材料的黏结和摩擦在局部不能同时存在,线性的Coulomb准则仅在小范围内近似描述圆柱试样的常规三轴强度,众多非线性强度准则只是经验公式而缺乏物理背景. 作者提出的指数准则可描述岩石剪切破坏时强度与围压的关系;基于对11组试验数据的拟合结果在莫尔应力空间分析黏结力和摩擦力的变化特征:岩石承载的剪切力存在上限即材料的真实黏结力c0;在试验范围内莫尔概念的内摩擦力达到约为0.38 c0的峰值,且黏结力在其附近相交. 材料真实黏结力与正应力无关,因而名义黏结力表征了完好材料剪切破裂的面积;基于裂隙面积计算的等效摩擦因子随正应力降低,意味着裂隙滑移的爬坡角减小,而后者取决于正应力与真实黏结力的比值. 等效摩擦因子与指数准则的材料参数具有确定关系,体现了岩石在压应力作用下剪切破裂的物理背景.

     

    Abstract: Rocks are natural materials composed of various mineral particles within fissures and pores in different sizes, those result in complicate mechanical properties. Strength criteria for rocks in engineering design and disaster prevention are still an open question. As cohesion and friction in rock do not work simultaneously, the linear Coulomb criterion proposed in 1773 is only reliable to describe pseudo-triaxial compression strength of cylindrical specimen in a small range of confining pressure. Many nonlinear criteria are merely empirical formulas but lack of physical background. The exponential criterion proposed by the author is applicable to fit the relationship between strength and confining pressure of rocks in shear failure; therefore, the cohesion and friction are analyzed in Mohr's stress space on the fitting solutions for eleven rocks. Shear stresses in rock have an upper limit, i.e. the genuine cohesion c0 of rock; and the internal friction has a peak of about 0.38 c0, by which intersection of the cohesion and internal friction is. The genuine cohesion is independent to normal stress, so the nominal cohesion of rock specimen represents the shear fracture area of intact material when rock specimen reaches its strength. The equivalent friction factor of slipping fissure decreases with the normal stress, so as the climbing angle that depends on ratio of normal stress to the genuine cohesion. Relationship between the equivalent friction factor and parameters in the exponential criterion reflects the physical background of shear fracture for rock under compressive stresses.

     

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