A study of dynamic strength of rock masses related to microcrack propagation.

核电厂等许多重要建筑物的地基往往由软质沉积岩构成，并承受地震等引起的循环荷载。因此，这些地基的承载力是这些岩石在静荷载和循环荷载作用下的稳定性所必需的。本研究旨在探讨软质沉积岩的静动强度、循环荷载破坏机制与粘弹性特性之间的关系。首先，在固结不排水条件下进行了三轴压缩试验和逐步循环加载试验。对比两种试验结果可知，总应力表达式中动、静强度之比为1.0 ~ 1.5，而有效应力表达式中动、静强度之比为1.0。这意味着，不同的只是孔隙水压力的行为。通过循环疲劳试验，研究了不同应力循环下孔隙压力的变化规律。结果发现，孔隙压力的行为发生了系统的变化，并且这种变化可以用两个量来描述，即应力循环引起的孔隙压力最大值与最小值的差异以及应力循环引起的孔隙压力循环相位差的滞后。岩石的破坏被认为是内部裂纹累积增长的脆性过程的结果。我们认为这种行为是裂纹萌生和扩展的结果。实际上，岩石试样在破坏后，在显微镜下观察到内部裂缝，其方向平行于最大压缩面和最终破裂面。此外，应力松弛实验表明，弛豫谱峰分布分散，与温度有关。这些结果对建立考虑裂纹扩展影响的本构方程具有重要意义。

The foundations of many important structures such as nuclear power plants are often composed of soft sedimentary rocks and subjected to cyclic loading caused by earthquakes and so on. Consequently the capacity of these foundations is required for the stabibility of those rocks under not only static but also cyclic loading.The purpose of this research is to investigate the relation between static and dynamic strength, mechanism of failure under cyclic loading and viscoelastic properties of soft sedimentary rocks. First, the triaxial compression experiments and stepwise cyclic loading experiments were performed under consolidated and undrained conditions. Comparing the results of two kinds of experiments, we obtained that the ratio of dynamic strength to static strength in total stress expression is from 1.0 to 1.5 but 1.0 in effective stress expression. This means that it is only different in the behavior of pore water pressure.Cyclic fatigue experiments were performed to investigate the behavior of pore pressure by every stress cycle. As a result, it is found that the behavior of pore pressure is systematically changed and that this change is described by two quantities, that is, the difference between maximum and minimum values in pore pressure caused by a stress cycle and a lag of phase difference on pore pressure cycles for stress cycles. It is thought that failure of rocks results from the brittle process involving the cumulative growth of internal cracks. We interpret that this behavior results from crack initiation and propagation. In fact, internal cracks, with orientations parallel to a maximum compression and ultimate fracture plane, were microscopically observed from rock specimens after failure.In addition, stress relaxation experiments showed that the peaks of relaxation spectra dispersedly distributed, dependent on temperature. These results are very meaningful to establish the constitutive equations involving the effect of crack growth.