Under high stresses

rock bursts

borehole breakouts and spalling are all common phenomena which occur around the underground openings. In substance

these three phenomena all belong to brittle failures of intact rock mass; but they reflect different failure extents of intact rock mass under the action of high stresses. After analyzing and comparing the stress-strength criteria for rock bursts

borehole breakouts and spalling put forward by other specialists

we find that these brittle failure phenomena share the same stress requirement. The stress requirements for brittle failures can be defined with the ratio of the maximum tangential stress around an underground opening to the uniaxial compressive strength ( / ) or the ratio of the maximum principal stress to the uniaxial compressive strength ( / ); actually

both indices reflect the same stress requirements. For /

the ratio of 0.4±0.1 should be the critical stress requirement

but for /

the ratio of 0.15±0.05 should be the critical stress requirement. Large amounts of engineering cases and the mechanical analysis based on the Hoek-Brown criterion also prove the validity of these two stress requirements. Here

the two stress requirements reflect a ratio range because the complicated rock mass ratings

lithology and geotechnical conditions shall have profound impacts on the stress requirement for brittle failures.