中主应力对岩石材料影响的机理及理论模型研究

Strength model and mesoscopic mechanism of intermediate principal stress effect on rock strength

利用颗粒离散元法分析了中主应力对岩石在真三轴条件下峰值应力、微裂纹数目变化、颗粒摩擦耗散能、破坏方式、微裂纹分布的影响.数值模拟再现了随中主应力增大,岩石材料强度先增大后减小的变化规律.细观层面,中主应力增大使得微裂纹在加载平面上投影由均匀变得不均匀.中主应力比b大于0.5,岩样在屈服阶段产生微裂纹显著增多,颗粒摩擦耗能增大非常明显.中主应力一方面抑制了法向沿中主应力方向(或存在该方向分量)的微裂纹的扩展,这将导致岩样强度的增加.另一方面,法向沿小主应力(或存在该方向分量)的微裂纹的扩展得到了促进,这导致岩样强度的减小.这种对不同方向微裂纹扩展的抑制或者促进作用导致岩样宏观强度呈现先增大后减小的变化规律.基于Weibull统计理论,建立了能反映岩石强度中主应力效应的概率统计强度模型.采用剪应力来表示计算强度,推导出一个新的能反映中主应力和材料不均匀性综合影响的强度准则.该准则可以视为Mohr-Coulomb准则的修正,当材料绝对均匀时该准则可以退化为Mohr-Coulomb准则.通过和真三轴压缩试验结果、数值模拟结果以及常用强度准则对比发现本文强度准则能很好地反映中主应力对岩石强度的影响规律.

Intermediate principal stress effect on rock strength,strain,energy dissipation and micro crack distribution are studied by using particle discrete element numerical simulation software PFC3 D.Numerical simulation reproduces the typical phenomenon that rock strength has an initial increase and a subsequent decrease with the increase of intermediate principal stress.On micro level,the increase of intermediate principal stress makes the micro cracks distribution in horizontal plane from symmetrical to asymmetrical.When intermediate principal stress ratio b is large than 0.5,micro cracks have a substantial increase at yield stage and energy dissipated by frictional sliding also has a substantial increase.Intermediate principal stress restrains the expansion of micro cracks normal to intermediate principal stress direction（or with component in this direction）,which makes rock strength increases.On the other side,intermediate principal stress promotes the expansion of micro cracks normal to minimum principal stress direction（or with component in this direction）,which makes rock strength decreases.The restraint and promotion of micro cracks in different directions lead to the intermediate principal stress effect.Based on Weibull theory,a failure probability model is developed to describe the effect of the intermediate principal stress on rock strength.Using shear stress to express the computed strength of each micro-unit,a strength criterion is proposed to quantitatively reflect the intermediate principal stress effect on rock strength.New criterion can be regarded as a modified Mohr-Coulomb criterion with a uniformity coefficient and it becomes Mohr-Coulomb criterion when materials are absolutely homogeneous.New criterion matches previously published experimental results and numerical simulation results better than common strength criterion.