MRMR 2001

The “Mining Rock Mass Rating”, MRMR, system was first presented by Laubscher in 1977 [1], formalized in 1990 [2] and updated in 2001 [3].

The following makes reference to the MRMR 2001’s version.

The main aspect that make this version different is the consideration of veinlets into the calculations. This transforms the way the rock strength and the discontinuities frenquency are evaluated.

Figure 1: Structural terms.

Joint condition is calculated simillarly, without considering the groundwater presence and including and extra adjustment factor related to the joints infill hardness.

Adjustment factor are the same, even though a new one is included to include the groundwater.

Parameters

The following parameter descriptions are based on the 2001 version [3].

Rock Block Strength

The rating assigned to the calculated RBS is taken from the following figure:

Figure 3: Rock block strength rating as a function of rock block strength.

Which can roughly approximated to:

\[P(RBS) = 0.00001 \cdot RBS^3 - 0.0042 \cdot RBS^2 + 0.5056 \cdot RBS + 0.3147\]

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Structures Spacing

The IRMR system accounts for two types of discontinuity spacings:

Open Joint Spacing

Figure 4: Rating for open joint spacing.

The curves can be formulated as:

\[P_{0}(J_{s})_{1SET} = 8.3495 \cdot \ln (J_{s}) + 29.299\]

\[P_{0}(J_{s})_{2SET} = 8.5188 \cdot \ln (J_{s}) + 25.718\]

\[P_{0}(J_{s})_{3SET} = 8.1919 \cdot \ln (J_{s}) + 21.916\]

Cemented Joint Spacing

Figure 5: Adjustment factor for cemented joints where the strength of the cement is less than the strength of the host rock.

Usually, this curves are defined linearly piece-wise.

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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	Adjustment
Wavy multi directional	100
Wavy uni directional	95
Curved	90
Slight undulation	85
Straight	85

Small-scale Expression

200mm x 200mm

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

Figure 6: Joint roughness profiles.

This is the same as in MRMR 1990.

Small-Scale Joint Expression (Factor B)

Description	Adjustment
Rough stepped/irregular	95
Smooth stepped	90
Slickensided stepped	85
Rough undulating	80
Smooth undulating	75
Slickensided undulating	70
Rough planar	65
Smooth planar	60
Polished	55

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.

This is analogous to its implementation in MRMR 1990.

Wall Strength Contrast (Factor C)

Description	Adjustment
Joint wall alteration weaker than wall rock and only if it is weaker than the filling	75

Infill

Laubscher [4] reduced the available filling option to just two options.

Some practitioners use the MRMR 1990 version of infill adjustment factors. Some others just set this factor to 100.

Joint Infill (Factor D)

Description	Adjustment
Gouge thickness < amplitude of irregularities	60
Gouge thickness > amplitude of irregularities	30

Infill Hardness

Joint Infill Hardness (Factor E)

Hardness	Type	Adjustment
5	Apatite	95
4	Chalcopyrite/Fluorite	90
3	Calcite/Anhydrite	85
2	Gypsum/Chlorite	80
1	Talc/Molybdenum	75

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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]

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.

[3] (1,2)

D. H. Laubscher and J. Jakubec. The mrmr rock mass classification for jointed rock masses. In W. A. Hustrulid and R. L. Bullock, editors, Underground Mining Methods: Engineering Fundamentals and International Case Studies, pages 475–481. Society for Mining, Metallurgy, and Exploration (SME), Littleton, Colorado, 2001.

[4]

D. H. Laubscher. A practical manual on block caving. Unpublished manual, 2000. Prepared for the International Caving Study, JKMRC and Itasca Consulting Group, Brisbane, Australia.