Hoek & Brown [42]

Grimstad & Barton [44]

Palmstorm [45]

Ratio (σcΘ)

Description

Ratio (σcΘ)

Description

Competency Factor (Cg)

Failure Modes

σcΘ > 7

Stable

σcΘ > 100

Low stress, near surface, open joints

<2.5

No rock stress induced instability

σcΘ = 3.5

Minor sidewall spalling

σcΘ = 3 - 100

Medium stress, favorable stress conditions

2.5 - 1

High stress, slightly loosening

σcΘ = 2

Severe spalling

σcΘ = 2 - 3

High stress, usually favorable to stability, maybe unfavorable to wall stability

1 - 0.5

Light rock burst or spalling

σcΘ = 1.7

Heavy support required

σcΘ = 1.5 - 2

Moderate slabbing after one hour

σcΘ < 1.4

Severe rock burst problem

σcΘ = 1 - 1.5

Slabbing and rock burst after minutes in massive rocks

<0.5

Heavy rock burst

σcΘ < 1

Heavy rock burst and immediate rock deformation