孔隙压力对岩样全部变形特征的影响

Effect of Pore Pressure on Entire Deformational Characteristics of Rock Specimen

目的研究了孔隙压力对剪切带图案及岩样全部变形特征的影响.方法利用FLAC内嵌语言编制的FISH函数计算平面应变压缩岩样轴向、侧向、体积应变及泊松比.在峰前及峰后,岩石的本构模型分别取为线弹性模型及莫尔库仑剪破坏与拉破坏复合的应变软化模型.结果随着孔隙压力的增加,岩样的破坏区域越来越广泛;剪切带倾角都接近于Arthur倾角;峰值强度及所对应的轴向、体积应变及侧向应变的大小均降低.当孔隙压力较低时,峰后应力-轴向应变曲线及应力-侧向应变曲线软化段斜率基本保持不变,根据单轴压缩条件下的解析解,这是由于岩样的破坏模式不随孔隙压力的增加而改变.结论当孔隙压力较高时,大量的单元发生破坏将消耗较多的能量,这使应力-轴向应变及侧向应变曲线软化段变平缓;岩样在轴向应变较低时就可获得较高的侧向变形量及泊松比,甚至负的体积应变.岩样失稳破坏的前兆的明显程度不随孔隙压力的改变而改变.

FISH functions written and embedded in FLAC to calculate axial, lateral and volumetric strains as well as Poisson＇ s ratio were used to numerically investigate the influence of pore pressure on shear band＇s patterns and entire deformational characteristics of rock specimen in plane strain compression. In elastic stage, the adopted constitutive relation of rock was linear elastic; however, in strain-softening stage, a composite Mohr-Coulomb criterion with tension cut-off was used and the post-peak constitutive relation was also linear. Numerical results show that increase of pore pressure leads to 1）increase of area of failure zone; 2）increase of shear band＇s inclination angle closer to Arthur＇s predictions; 3）decrease of peak strength and corresponding absolute values of axial, lateral and volumetric strains. Pore pressure doesn＇t have an influence on the slopes of stress-axial strain curve and stress-lateral curve beyond the peak strength for lower pore pres- sure since the failure mode of specimen is not dependent on pore pressure based on the analytical solutions in uniaxial compression of rock specimen subjected to shear failure. However, for higher pore pressure, the number of failed elements is greatly increased, leading to gentle behaviors of the curves in strain-softening stage. Higher lateral deformation and Poisson＇s ratio as well as negative volumetric strain can be reached for higher pore pressure even though axial strain of specimen is lower. No apparent difference in precursor of instability failure of rock specimen is observed with the increase of pore pressure.