低应力区岩石变形记忆效应形成机制及失忆性

Mechanism of rock deformation memory effect in low stress region and its memory fading

地应力信息储存于开挖后的岩石中,变形率变化法(DRA)是基于岩石变形记忆效应测量地应力的方法,正确的机制是认识变形记忆效应与推广、改进DRA法的基础。传统推测的机制并不能解释低应力区存在变形记忆效应及失忆性等现象。首先进行火山沉积岩试样的物理试验,证明低应力区仍然存在变形记忆效应,记忆信息可同时采用侧向应变及轴向应变测量,两者精度一致。提出在低应力区,岩石变形记忆效应机制为微裂纹接触面及颗粒接触面的摩擦滑动。基于此机制,采用弹性元件、黏性元件和圣维南体构建相应的轴对称理论基本单元模型及多接触面理论模型。进行不同放置时间下的单轴循环压缩试验。结果表明,由轴对称理论模型可以得到低应力区岩石变形记忆效应、侧向DRA曲线与轴向DRA曲线准确度一致且在记忆信息处向上弯曲等、并对失忆性现象进行了初探。理论模型与物理试验结果一致,由此证明微裂纹接触面及颗粒接触面的摩擦滑动可以得到低应力区岩石变形记忆效应。同时,理论模型及物理试验结果为提高DRA法的测量准确度提供了依据。

The information of the in situ stress is stored in the rocks after excavation. The deformation rate analysis （DRA） method is used to determine the in situ stress based on the rock deformation memory effect（DME）. Cracks generation and propagation under the previous load was initially assumed to be the mechanism of the rock DME. However, this mechanism can not explain many phenomena in the rock DME. The lack of a theoretical model prevents the correct interpretation of experimental data and improvements of the DRA method. The physical experiment is firstly performed on a volcanic sediment sample. It is showed that the in situ stress in the low stress region can be determined by both the axial strain and the lateral strain, while the DRA curves by these two kinds of strain are same. Then, the frictional sliding over the crack interfaces and grain boundaries in rocks are proposed as the mechanism of the rock DME in the low stress region. Based on this mechanism, a theoretical model using elastic element viscous element and St. Venant bodies is constructed. Cyclic compressions with different peak stresses and delay time are performed on the theoretical model. Results show that, not only the formation of the rock DME is well explained but also many DME phenomena are observed, such as the memory fading and the accuracy of the DRA curve by the lateral strain as the same as the axial strain. All the results by the theoretical model are in accordance with the experimental results. By the theoretical model, the frictional sliding over the crack interfaces and grain boundaries is verified to be the mechanism for the rock DME in the low stress region. Moreover, the results by theoretical model provided a basis for the improvement of the DRA method in the determination ofthe in situ stress.