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

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.