Rock Load Estimation in Development Galleries and Junctions for Underground Coal Mines: A CMRI-ISM Rock Mass Rating Approach

Table 2

Adjustment factors for RMR.

Parameter	Adjustment of RMR	Adjusted RMR
(1) Depth
Less than 250 m	Nil	RMR × 1.0
250–400 m	10% reduction	RMR × 0.9
400–600 m	20% reduction	RMR × 0.8
More than 600 m	30% reduction	RMR × 0.7
(2) Lateral stresses
Small	10% reduction	RMR × 0.9
Moderate	20% reduction	RMR × 0.8
High	30% reduction	RMR × 0.7
(3) Induced stresses
No adjacent working; in the seam	Nil	RMR × 1.0
Extraction areas within; 20–40 m in the same seam	10% reduction	RMR × 0.9
Extraction areas within; 10–20 m in the same seam	Up to 30% reduction	RMR × (0.7 to 0.8)
Working with 10–20 parting	10% reduction	RMR × 0.9
Working with; 3–10 m parting	Up to 30% reduction	RMR × (0.7 to 0.8)
(4) Method of excavation
Continuous miner	10% increase	RMR × 1.1
Undercut and blasting	Nil	RMR × 1.0
Blasting off the solid	10% reduction	RMR × 0.9
(5) Gallery span
Less than 4.8 m	Nil	RMR × 1.0
4.8–6.0 m	10–20% reduction	RMR × (0.8 to 0.9)