Failure Criterion and Long-Term Strength of Rock

首先，对强度与加载速率相关性的研究进行了简要综述。众所周知，单轴(无侧限)抗压强度随着加载速率的增加而增加。虽然只有有限的数据可用，但一些作者报告说，约束强度随着加载速度的增加而增加。在总结试验结果的基础上，提出了考虑强度与加载速率相关性的破坏准则，即高加载速率破坏准则和低加载速率破坏准则。作为一种极端情况，可以得到在极低加载速率下的长期强度或破坏准则。长期强度的最简单形式是σ_1=aσ_3，其中σ_1和σ_3是主应力和小主应力，常数‘a’随岩石的不同而从2到6变化。为了验证所提出的破坏准则，在σ_1和σ_3图上绘制了岩石应力图。结果发现，除一个数据外，所有数据都绘制在直线σ_1=4_&lt；3σ&gt；下。合理地认为，在相对稳定的岩体中，岩石应力保持在低于长期强度的一定值或条件下。如果是这样的话，这一结果支持了所提出的破坏准则的有效性。

At first, study on loading-rate dependency of strength was briefly summarized. It is well known that uniaxial (unconfined) compression strength increases with loading rate. Though only limited data are available, some authors reported that confined strength increases with loading rate. Also, tensile strength obtained by uniaxial tension, indirect tension and bending tests increases with loading rate.Based on the summarized experimental results, a failure criterion in which loading-rate dependency of strength is considered was proposed ; high criterion for high loading rate and low criterion for low loading rate. As an extreme case, long-term strength or failure criterion at extremely low loading rate can be obtained. The simplest form of the long-term strength is σ_1=a σ_3 where σ_1, σ_3 are major and minor principal stresses, and constant 'a' varies from 2 to 6 depending on rock.For validation of the proposed failure criterion, rock stresses were plotted on σ_1 vs. σ_3, diagram. It was found that all data except one were plotted under the line σ_1=4_<3σ>. It is reasonable to consider that, in relatively stable rock mass, rock stress is maintained at a certain value or condition lower than a long-term strength. If so, this result supports the validity of the proposed failure criterion.