Energy Release Analysis of a Severe Rockburst in a Headrace Tunnel Crossing a Tectonic Stress Zone

Table 5

Commonly used strength-stress ratio criteria.

Criteria	Proposer	Formula	Rockburst grade
			No	Light	Moderate	Severe
1	[16]		>7.0	4.0∼7.0	2.0∼4.0	<2.0
2	[8]		>10.0	10.0∼5.0	5.0∼2.5	<2.5
3	[11]		0.34	0.42	0.56	0.70
4	[9]		≤0.20	0.20∼0.30	0.30∼0.55	≥0.55

is the compressive strength of rock obtained from laboratory uniaxial tests; is the maximum principle stress of rock mass after excavation; is the maximum principal stress of in situ stress; is the horizontal stress component of in situ stress perpendicular to the tunnel alignment.