As an estimate for the in-situ spalling strength around massive underground excavations to moderately jointed brittle rocks,crack initiation stress marks the initiation of rock micro fracturing.It is crucial to accurately identify crack initiation stress level by proper method.In this study,confined compression tests of sandstone samples are used to examine the validity/applicability of proposed axial strain stiffness method.The results show that by highlighting the minuscule changes in stress-strain curve,the axial strain stiffness curve provided further insight into rock failure process and revealed five stages:(a)irregular fluctuation,(b)nearly horizontal regular fluctuation,(c)irregular fluctuation gradually decreasing to zero,(d)extreme fluctuation,and(e)near zero,which mainly correspond to five stages of stress–strain curve.The ratio of crack-initiation stress to peak strength determined using this approach is 0.44–0.51,similar to the ranges previously reported by other researchers.In this method,the key is to accurately detect the end point of the stage(b),"nearly horizontal regular fluctuation"characterized by a sudden change in axial strain stiffness curve,and the sudden change signifies crack initiation in rock sample.Finally,the research indicates that the axial strain stiffness curve can provide a mean to identify the crack-initiation stress thresholds in brittle rocks.
High geothermal temperatures appear to be unfavorable for the construction of tunnels in slate rocks with high overburden. To investigate the mechanical characteristics of slates at various levels of geothermal temperature, conventional triaxial compression tests at different levels of confining stress were carried out at 4 different temperatures from 20℃ to 120℃. The obtained results show high confining pressures weaken the thermal effects on rock mechanical characteristics while higher temperatures enhance the effect of confining pressure.At higher levels of confining stress the thermal effects on the rock strength characteristics decrease. The higher the temperature, the larger is the effect of confining pressure on the mechanical characteristics of the slate. Increase of temperature leads to a decrease of the peak strength but increases the deformability and ductility of the slate, the thermo effect on the peak strength and Poisson's ratio is larger than on the elastic modulus. Higher temperatures reduce the shear strength of slate, the decrease is mainly caused by a decrease of the cohesion. In general, the slate samples fail in shear failure.
水压会刺激岩石裂纹的产生和加速岩石破裂,对岩石的变形破坏特征和破坏机制有重要影响。利用MTS815 Flex Test GT岩石力学试验系统和PCI-Ⅱ声发射仪开展了砂岩在不同围压下的水-力耦合试验。结果表明:在整个岩石破裂过程中,声发射活动随加载时间、应力变化表现出不同的特征;声发射活动在岩石的峰后阶段随着水压的增大更为集中,强度也更高,而随着围压的增大其集中程度和强度都有所降低;在相同围压下,声发射累计振铃计数和累计能量随着水压的增大而增大,在相同水压下,声发射累计振铃计数和累计能量则随着围压增大而有所减少;随着水压的增大,岩石最终失稳破坏时刻的声发射三维定位图中裂纹数量增多,裂纹的集中程度也更高,在宏观破坏形态上表现出破坏角减小。这些成果揭示了水-力耦合作用下岩石的破坏机制由压制剪切向压制张裂变化,岩石破裂的脆性破坏特征增强。
