三轴压缩岩石应变软化及渗透率演化的试验和数值模拟

三轴压缩下岩石峰后应变软化行为及渗透率演化规律是岩石工程稳定性分析的基础。取新疆巴里坤砂岩样在室内开展了三轴压缩试验和三轴渗流试验，获得了不同围压下巴里坤砂岩的全程应力应变曲线、体积应变与渗透率关系曲线。试验结果表明：随着围压增加，岩石峰后残余强度增加，体积扩容和脆性减弱；随着轴向应变增加，岩石先发生弹性压缩，空隙空间减小，渗透率降低；当应力达到屈服强度，岩石内裂隙开始扩展，渗透率降低速率趋缓；在峰值应力后，岩样破坏，裂隙扩展加速，并伴有新裂隙的萌生，岩样渗透率开始快速增长，岩样的渗透率呈“V”型变化。提出了描述围压对岩石峰后脆性影响的新参数，即脆性模量系数，围压与脆性模量系数之间服从负指数关系。基于脆性模量系数、强度退化指数和扩容指数，建立了考虑围压影响的岩石应变软化模型。在分析体积应变与岩石渗透率之间关系基础上，建立了基于体积应变增透率的岩石渗透率演化模型。在FLAC下模拟了巴里坤砂岩不同围压下的应变软化行为和渗透率演化过程，结果表明：岩石应变软化模型能很好地模拟围压对岩石残余强度、体积扩容和峰后脆性的影响；所示模型能较好地模拟围压和剪胀对岩石渗透率的影响；岩样峰后内部出现了明显的剪切破坏带，剪切破坏带与大主应力的夹角随着围压的增加而增大。在剪切破坏带内单元的渗透率显著增长，最后形成了一个流动通道。

Test and numerical modeling on strain softening behavior and permeability evolution of rock under tri-axial compression

The strain softening behaviors and permeability evolution laws of post-peak rock are the basis of the stability analysis of rock engineering.The tri-axial compression tests and tri-axial compression-seepage tests for the sandstone samples from Balikun Mine，Xinjiang were carried out in this paper.The whole stress-strain curves under varied confinement and the curve of volumetric strain and permeability were obtained.The test results show that：with the increase of confining pressure，the dilatancy and brittleness of rock decreases and the residual strength increases.With the increase of axial strain，the rock takes firstly on elastic compression，and the void space reduces，then the permeability decreases.The fractures in rock begin to expand when the stress reaches yield strength stress.As a result，the reduction rate of permeability slows down.After peak stress，the rock samples fail，and the fractures expand quickly and the new cracks form.Then the permeability begins to increase rapidly，and the permeability takes on “V” type.A new parameter，namely brittle modulus index，which is used to describe the effects of confinement on brittleness after peak-post rock，is presented.The relationship between confining pressure and brittle modulus index takes on a negative exponential form.Based on brittle modulus index，the strength degradation index and dilatancy index，a new strain softening model is established.The relationship between volumetric strain and permeability is investigated，and a permeability evolution model is established based on the enhanced permeability of volumetric strain.In FLAC the strain softening behavior and permeability evolution of Balikun sandstone under tri-axial compression are numerically modeled.The results show that the model in this paper can better describe the effects of confining pressure on the post-peak rock’s brittleness，dilation and residual strength.The model in this paper can better describe the effects of confining pressure and dilatancy on the permeability evolution of rock.A distinct band of degraded and dilated elements forms in the post-peak region，and the angle between these bands and the major principal stress increases as confining pressure increases.Lastly the band of highly permeable elements becomes a flow channel.