饱水裂隙砂岩力学及前兆异常特征量化分析

Quantitative analysis on mechanical and precursory anomaly properties of water-saturated sandstone with a flaw

为定量分析裂隙岩体的遇水软化特性,首先采用高速水刀切割技术在砂岩试件中预制裂隙,裂隙倾角分别设置为30°,45°和60°,代表缓倾、中缓倾以及陡倾结构面。然后,对自然和饱水状态下裂隙砂岩试件进行单轴压缩试验,利用数字图像相关方法对加载过程中试件进行非接触式全场变形观测。其次,引入标准差,描述应变场数据的离散程度。最后,对应变场标准差-轴向应变曲线进行有限差分求导,计算应变场分异速率,提出一种识别前兆异常的方法,定量描述饱水作用对裂隙砂岩前兆异常特征的影响规律。研究结果表明:自然和饱水状态下试件的应力-应变曲线趋势相似,可划分为原生缺陷压密、线弹性变形,裂纹萌生至稳定扩展,裂纹非稳定扩展以及峰后等5个阶段。饱水作用劣化岩体性能并加剧非均质性,裂隙倾角也会影响饱水试件的软化系数,而陡倾裂隙的劣化作用更为显著;起裂应力、抗压强度以及弹性模量软化系数分别在0.47~0.60,0.48~0.66以及0.78~0.85。数字图像相关方法可识别和定位裂纹扩展路径,表现为应变局部化;饱水试件的裂纹起裂、扩展及贯通先于自然试件。岩体最终失稳破坏前,自然和饱水状态下裂隙砂岩试件的应变场分异速率-轴向应变曲线先后出现明显增长点P1和尖点P2,分别对应张拉裂纹和剪切裂纹起裂点。饱水作用使得P1点出现变早;而P2点对应的应力与峰值应力更接近。

In order to quantitatively analyze the softening characteristics of flawed rock in contact with water,the flaws were prefabricated in the sandstone specimens by using the high-pressure water-jet cutting technology.The flaw inclination was set as 30°,45°and 60°,respectively,which represents gentle-inclined,moderate-inclined and steep-inclined structural plane.The uniaxial compression tests were performed on the sandstone specimens con-taining a flaw under natural and water-saturated states.The digital image correlation method was used to non-contact monitor the full-field deformation of the specimens during loading.The discreteness of strain field was described by using the standard deviation.The differentiation rate of strain field was calculated by the finite difference derivative of the strain field-axial strain curve.A method for recognizing precursory anomaly was pro-posed to quantitatively describe the influence of water saturation on the mechanical and precursory anomaly properties of fractured sandstone.The results show that the stress-strain curves of sandstone specimens under natural and water-saturated states have similar tendency,which can be divided into five stages:original crack closure,linear elastic deformation,crack initiation and stable extension,unstable extension and post-peak state.The mechanical properties of rock mass are weakened and its heterogeneity is aggravated due to the influence of water saturation.The flaw inclination affects the softening coefficient of water-saturated specimens,especially the steep-inclined flaw.The softening coefficients of crack initiation stress,compressive strength and elastic modulus are in the range of 0.47-0.60,0.48-0.66 and 0.78-0.85,respectively.The digital image correlation method can identify and locate the crack propagation paths,which present as strain localization.The crack initiation,propagation and coalescence of water-saturated specimens precede these of natural specimens.Obvious-growth point P1 and cusp P2 appear successively in the differentiation rate of strain field-axial strain curve of flawed sandstone specimens under natural and water-saturated states before failure,corresponding to tensile crack initiation point and shear crack initiation point.Due to the influence of water saturation,the precursor point P1 appears earlier,and the stress corresponding to P2 is much closer to the peak stress.