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

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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.