砂质泥岩卸荷流变本构模型研究

Unloading rheological constitutive model for sandy mudstone

基于隧洞开挖过程中围岩应力的实际调整路径设计加轴压卸围压条件下的分级卸荷流变实验,对某水电站引水隧洞洞轴线主要穿越的砂质泥岩的卸荷流变力学特性进行研究。试验结果表明:1应力水平低于岩石破坏应力时,砂质泥岩的流变只表现出衰减流变阶段和稳态流变阶段,且随着围压的逐级卸除,应力水平逐渐超过岩石的长期强度,试样的稳态流变速率由最初接近于0的常数逐渐增大至一个大于0的常数;2应力水平高于岩石破坏应力时,砂质泥岩经过衰减流变阶段和稳态流变阶段之后进入非线性加速流变阶段直至发生破坏,通过对比发现,不同卸荷初始围压下试样发生流变破坏的总历时不同,且各试样发生非线性加速流变的启动时间也显著不同。对砂质泥岩卸荷流变试验结果分析表明,西元模型能够较好地描述应力水平低于破坏应力时砂质泥岩的流变特性,但却无法反映应力水平高于其破坏应力时的非线性加速流变特征,基于此,引入岩石非线性加速流变启动元件,通过将其与西元模型相结合建立了一个新的非线性黏弹塑性流变模型,对提出的模型进行参数辨识,并将流变模型拟合结果与试验结果进行对比,结果表明,新建立的卸荷流变本构模型能够较全面地描述砂质泥岩卸荷流变破坏的全过程,证明所建模型是正确合理的。

Based on the actual adjustment path of the surrounding rock stress in the process of tunnel excavation, the grading unloading rheological tests under unloading confining pressure with axial compression are designed, and the unloading rheological mechanical properties of sandy mudstone which is mainly threaded by the tunnel axis of diversion tunnel of a hydropower station are studied. The test results show that： （1） When the stress level is lower than the rock failure stress, the rheology of sandy mudstone only has the stages of attenuation creep and steady-state creep, and with the unloading of confining pressure step by step, the stress level is gradually over the long-term strength of rock, and the steady-state creep rate of sample gradually increases from being close to 0 originally to a constant greater than 0. （2） When the stress level is higher than the rock failure stress, the sandy mudstone goes into nonlinear acceleration creep stage until destruction after the stages of attenuation creep and steady-state creep. By comparison, the whole time of the sample for rheological damage under different initial unloading confining pressures is different, and the start time of the sample for accelerated nonlinear rheology is also significantly different. The analysis of the test results shows that Nishihara model can well describe the rheological properties of sandy mudstone when the stress level is lower than the failure stress, but it cannot reflect the nonlinear accelerating rheological characteristics when the stress level is higher than its failure stress. Based on this, by introducing a start element of nonlinear acceleration creep of rock and combining with Nishihara model, a new nonlinear visco-elastoplastic rheological model is established. The rheological parameters of the model are identified, and the fitting results of the rheological model are compared with the test results. It is concluded that the proposed unloading rheological constitutive model can fully describe the whole process of unloading creep damage of the sandy mudstone, and it is correct and reasonable.