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

核电站等许多重要建筑物的地基往往由软沉积岩组成，受到地震等周期性荷载的作用。因此，无论是在静荷载作用下，还是在循环荷载作用下，这些岩石的稳定性都需要这些地基的承载能力。本研究的目的是研究软沉积岩的静、动强度之间的关系、循环荷载作用下的破坏机理和粘弹性性质。首先，在固结和不排水条件下进行了三轴压缩试验和分步循环加载试验。通过对两种试验结果的比较，得出动静强度在总应力表达式中的比值为1.0~1.5，而在有效应力表达式中为1.0。这意味着只有孔压的行为有所不同。循环疲劳试验研究了每个应力循环下的孔压行为。结果表明，孔压行为发生了系统的变化，这种变化可以用两个量来描述，即应力周期引起的孔压最大值和最小值的差值和应力周期引起的孔压周期相位差的滞后。认为岩石的破坏是内部裂纹累积扩展的脆性过程的结果。我们解释说，这种行为是由裂纹的萌生和扩展造成的。事实上，从破坏后的岩石试件中可以观察到内部裂纹，其方向平行于最大压缩和最终断裂面。此外，应力松弛实验表明，弛豫谱的峰值分散分布，依赖于温度。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.