MRMR 1990

The “Mining Rock Mass Rating”, MRMR, system was first presented by Laubscher in 1977 [1], althought the commonly used first version is the one presented by himself in 1990 [2].

The following makes reference to the MRMR 1990’s version.

Parameters

The following parameter descriptions are based on the 1990 version [2].

Intact rock strength

IRS Rating

IRS [MPa] Range	Rating
> 185	20
165–185	18
145–164	16
125–144	14
105–124	12
85–104	10
65–84	8
45–64	6
35–44	5
25–34	4
12–24	3
5–11	2
1–4	1

RQD

RQD Rating

RQD [%]	Rating
97–100	15
84–96	14
71–83	12
56–70	10
44–55	8
31–43	6
17–30	4
4–16	2
0–3	0

Fracture Frequency

Fracture frequency rating (FF/m)

Average per metre	1 set	2 set	3 set
0,10	40	40	40
0,15	40	40	40
0,20	40	40	38
0,25	40	38	36
0,30	38	36	34
0,50	36	34	31
0,80	34	31	28
1,00	31	28	26
1,50	29	26	24
2,00	26	24	21
3,00	24	21	18
5,00	21	18	15
7,00	18	15	12
10,00	15	12	10
15,00	12	10	7
20,00	10	7	5
30,00	7	5	2
40,00	5	2	0

Joint Condition

Large-scale Expression

“Sub-section A caters for the large-scale expression of the feature, such as across a drift or in a pit face.”

Large-Scale Joint Expression (Factor A)

Description	Dry	Moist	Mod. pressure (25–125 lt/m)	High pressure (>125 lt/m)
Multi wavy directional	100	100	95	90
Uni wavy directional	95	90	85	80
Curved	85	80	75	70
Slight undulation	80	75	70	65
Straight	75	70	65	60

Small-scale Expression

“B assesses the small-scale expression and is based on the profiles shown…”

Figure 2: Joint roughness profiles.

Small-Scale Joint Expression (Factor B)

Description	Dry	Moist	Mod. pressure (25–125 lt/m)	High pressure (>125 lt/m)
Rough stepped/irregular	95	90	85	80
Smooth stepped	90	85	80	75
Slickensided stepped	85	80	75	70
Rough undulating	80	75	70	65
Smooth undulating	75	70	65	60
Slickensided undulating	70	65	60	55
Rough planar	65	60	55	50
Smooth planar	60	55	50	45
Polished	55	50	45	40

Wall Strength Contrast

“Section C is applied only when there is a distinct difference between the hardness of the host rock and that of the joint wall”

This means that Factor C is only activated or modified when there is a noticeable contrast in mechanical properties between:

• The intact rock mass (host rock), and

• The material or surface along the joints (joint walls), which might be altered, infilled, or weathered.

Wall Strength Contrast (Factor C)

Description	Dry	Moist	Mod. pressure (25–125 lt/m)	High pressure (>125 lt/m)
Joint wall alteration weaker than wall rock and only if it is weaker than the filling	75	70	65	60

Infill

Joint Infill (Factor D)

Description	Dry	Moist	Mod. pressure (25–125 lt/m)	High pressure (>125 lt/m)
Non-softening and sheared material – Coarse	90	85	80	75
Non-softening and sheared material – Medium	85	80	75	70
Non-softening and sheared material – Fine	80	75	70	65
Soft sheared material (e.g., talc) – Coarse	70	65	60	55
Soft sheared material (e.g., talc) – Medium	60	55	50	45
Soft sheared material (e.g., talc) – Fine	50	45	40	35
Gouge thickness < amplitude of irregularities	45	40	35	30
Gouge thickness > amplitude of irregularities	30	20	15	10

---

References

[1]

D. H. Laubscher. Geomechanics classification of jointed rock masses — mining applications. Transactions of the Institution of Mining and Metallurgy, Section A: Mining Industry, 86:A1–A8, 1977.

[2] (1,2)

D. H. Laubscher. A geomechanics classification system for the rating of rock mass in mine design. Journal of the South African Institute of Mining and Metallurgy, 90(10):257–273, 1990. doi:10.10520/AJA0038223X_1954.