高围压高孔隙水压作用下岩石蠕变特性

Creep properties of rock under high confining pressure and high water pore pressure

对重庆某地深部砂岩进行高围压和高孔隙水压作用下分级加载蠕变试验。结果表明:加载初期的一较短时间内,孔隙水压力在一定程度上延缓轴向变形,随后整个加载过程表现为增强岩石轴向变形。相同围压下,有孔隙水压和无孔隙水压时岩石蠕变可能存在一个相同的应力强度比阀值,当应力强度比超过该阀值后便会出现加速蠕变破坏。选取三元件广义Kelvin模型描述高围压高孔隙水压作用下加载应力低于屈服应力σs时的岩石蠕变特性,效果较为理想。基于高围压高孔隙水压作用下砂岩加速蠕变试验曲线特性,采用拟合、类比的方法引入一个二元件黏塑性模型,将其与三元件广义Kelvin模型串联组成一个新的可以描述岩石加速蠕变全过程的非线性黏弹塑性蠕变模型。采用基于Quasi-Newton非线性优化算法(BFGS)的Matlab编程,实现对高围压高孔隙水作用下岩石加速蠕变全过程曲线进行辨识,对比试验曲线和模型拟合曲线,二者吻合较好,验证非线性黏弹塑性模型的正确性。

Step loading triaxial creep test was carried out for deep sandstone from somewhere in Chongqing under high confining pressure and high pore water pressure. The result shows that the pore water pressure delays the rock creep deformation to some extent at the initial stage, but then to enhance performance of rock creep deformation; with or without pore water pressure, there might be a definite threshold of stress strength ratio under which the tertiary stage of creeps in different loading conditions begin to occur. The three component modified Kelvin model is selected to describe the characteristics of rock creep when the loading stress is below the yield stress. Based on the action of sandstone accelerating creep curves under the loading condition of high confining pressure and high pore water pressure, a two-component viscoplastic model is acquired by the method of fitting and anology, which is connected with three- component modified Kelvin model in series, and then a new nonlinear viscoelastic-plastic constitutive model is obtained. The Matlab programming based on Quast-Newton nonlinear optimization algorithm （BFGS） is adopted to identify the model parameters. Comparing testing curves with fitting curves, it is shown that this new model is suitable to describe the three creep stages of rocks.