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Rock Mass Rating (RMR)

For classification see Rock Mass Classes
For interpretation see Meaning of Rock Classes


Rock Mass Rating System*
A. Classification Parameters and Their Ratings
Parameter Range of Values
A1. Strength of Intact Rock Material ( see Field Estimates )
Point-Load
Strength Index
> 10 MPa 4 - 10 MPa 2 - 4 MPa 1 - 2 MPa For this low range -
uniaxial compressive
test is preferred
Uniaxial Compressive
Strength
> 250 MPa 100 - 250 MPa 50 - 100 MPa 25 - 50 MPa 5 - 25 MPa 1 - 5 MPa < 1 MPa
Rating JA1 15 12 7 4 2 1 0
A2. Drill Core
Quality - RQD

( see RQD )
90% - 100% 75% - 90% 50% - 75% 25% - 50% < 25%
Rating JA2 20 17 13 8 3
A3. Spacing of
Discontinuities
> 2 m 0.6 - 2m 200 - 600 mm 60 - 200mm < 60 mm
Rating JA3 20 15 10 8 5
A4. Condition of
Discontinuities

( see E )
Very rough surfaces
Not continuous
No separation
Unweathered
wall rock
Slightly rough surfaces
Separation < 1 mm
Slightly weathered
walls
Slightly rough surfaces
Separation < 1 mm
Highly weathered walls
Slickensided surfaces or
Gouge < 5 mm thick or
Separation 1 - 5 mm
Continuous
Soft gouge > 5 mm thick or
Separation > 5 mm
Continuous
Rating JA4 30 25 20 10 0
A5. Groundwater
Inflow per 10 m tunnel length (L/min) None < 10 10 - 25 25 - 125 > 125
Joint water pressure/
Major principal σ
0 < 0.1 0.1 - 0.2 0.2 - 0.5 > 0.5
General Conditions Completely dry Damp Wet Dripping Flowing
Rating JA5 15 10 7 4 0
B. Rating Adjustment for Discontinuity Orientations ( see Tunnelling )( see Dam Foundations )
Strike and Dip Orientations Very Favorable Favorable Fair Unfavorable Very Unfavorable
Rating JB Tunnels and Mines 0 - 2 - 5 - 10 - 12
Foundations 0 - 2 - 7 - 15 - 25
Slopes 0 - 5 - 25 - 50 - 60
C. Rock Mass Classes Determined from Total Ratings
Rating 100 - 81 80 - 61 60 - 41 40 - 21 < 21
Class No. I II III IV V
Description Very good rock Good rock Fair rock Poor rock Very poor rock
D. Meaning of Rock Classes
Class No. I II III IV V
Average stand-up time 20 yr for 15 m span 1 yr for 10 m span 1 wk for 5 m span 10 h for 2.5 m span 30 min for 1 m span
Cohesion of rock mass (kPa) > 400 300 - 400 200 - 300 100 - 200 < 100
Friction angle of rock mass (deg) > 45 35 - 45 25 - 35 15 - 25 < 15
E. Guidelines for Classification of Discontinuity Conditions**
Discontinuity Length (persistence) < 1 m 1 - 3 m 3 - 10 m 10 - 20 m > 20 m
Rating 6 4 2 1 0
Separation (aperture) None < 0.1 mm 0.1 - 1.0 mm 1 - 5 mm > 5 mm
Rating 6 5 4 1 0
Roughness Very rough Rough Slightly rough Smooth Slickensided
Rating 6 5 3 1 0
Infilling (gouge) None Hard Filling < 5 mm Hard Filling > 5 mm Soft Filling < 5 mm Soft Filling > 5 mm
Rating 6 4 2 2 0
Weathering Unweathered Slightly weathered Moderately weathered Highly weathered Decomposed
Rating 6 5 3 1 0
F. Effect of Discontinuity Strike and Dip Orientation in Tunnelling***
Strike perpendicular to tunnel axis Strike parallel to tunnel axis
Drive with dip - Dip 45 - 90° Drive with dip - Dip 20 - 45° Dip 45 - 90° Dip 20 - 45°
Very favourable Favourable Very unfavourable Fair
Drive against dip - Dip 45 - 90° Drive against dip - Dip 20 - 45° Dip 0 - 20° - Irrespective of strike
Fair Unfavourable Fair
*(after Bieniawski 1989)
**Some conditions are mutually exclusive. For example if infilling is present, the roughness of the surface will be overshadowed by the influence of the gouge. In such cases use A.4 directly.
***Modified after Wickham et al (1972)


Assessment of Joint Orientation Favorability
Upon Stability of Dam Foundations
Dip 0º - 10º Dip 10º - 30º Dip 30º - 60º Dip 60º - 90º
  Dip Direction  
Upstream Downstream
Very favorable Unfavorable Fair Favorable Very favorable
Note:  This table is based on experience and consideration of stress distribution in foundation rock masses. It assumes both the arch and the effects of gravity have an effect on a dam structure.

The initial in-situ state of stress is not considered here, as in dam foundations in-situ stresses are mainly important when considering grouting, drainage curtains and the excavation sequence of foundations. For this last point, recent evidence shows that high horizontal stresses may be expected in near-surface rock masses.


Rock Quality Designation*
RQD Rock Quality Classification
< 25% Very Poor
25 - 50% Poor
50 - 75% Fair
75 - 90% Good
90 - 100% Very Good

The RQD is defined as the cumulative length of core pieces longer than 10cm in a run divided by the total length of the core run.

*(Deere, 1989)

 


Field Estimates of Uniaxial Compressive Strength
Term Uniaxial
Compressive
Strength
(MPa)
Point
Load
Index
(MPa)
Schmidt
Hardness
(Type L -
hammer)
Field Estimate
of Strength
Examples*
R5
Extremely
Strong
> 250 > 10 50 - 60 Rock material only chipped under repeated hammer blows fresh basalt, chert, diabase, gneiss, granite, quatzite
R4
Very
Strong
100 - 250 4 - 10 40 - 50 Requires many blows of a geological hammer to break intact rock specimens Amphibolite, sandstone, basalt, gabbro, gneiss, granodiorite, limestone, marble rhyolite, tuff
R3 Strong 50 - 100 2 - 4 30 - 40 Hand held specimens broken by a single blow of a geological hammer Limestone, marble, phyllite, sandstone, schist, shale
R2
Medium
Strong
25 - 50 1 - 2 15 - 30 Firm blow with geological pick indents rock to 5mm, knife just scrapes surface Claystone, coal, concrete, schist. shale, siltstone
R1
Weak
5 - 25 ** < 15 Knife cuts material but too hard to shape into triaxial specimens chalk, rocksalt, potash
R0
Very
Weak
1-5 **   Material crumbles under firm blows of geological pick, can be scraped with knife highly weathered or altered rock
Extremely Weak 0.25 - 1 **   Indented by thumbnail clay gouge
*Well interlocked crystal fabric with few voids
**Rocks with a unixaial compressive strength below 25 Mpa are likely to yield highly ambiguous results under point load testing