キーワード：岩石，破壊基準，長期強度，岩盤内構造物，地圧

Failure Criterion and Long-Term Strength of Rock

Abstract

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}＝4σ_{３}. 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