Failure Criterion and Long-Term Strength of Rock
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 σ１＝a・σ３ where σ１,σ３ 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 σ１ vs.σ３ diagram. It was found that all data except one were plotted under the line σ１＝4σ３. 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.
KEY WORDS: Rock, Failure Criterion, Long-Term Strength, Underground Structures, Rock Stress