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Parameters Hoek - Brown

Parameter of rock breaking a

Parameter a is an exponent receiving values from 0.5 to 0.65 (for the original Hoek-Brown condition it is equal to 0.5) and depends on the degree of rock breaking.

Nonlinear parameters mb = m, s for a = 0.5

(index r denotes residual values)

Carbonate rocks with well-developed cleavage - dolomite, limestone, marble

Argillaceous rocks - mudstone, siltstone, shale, slate

Arenaceous rocks - sandstone, quartzite

Fine-grained igneous crystalline rocks - andesite, dolerite, basalt, rhyolite

Coarse metamorphic and igneous rocks - gabbro, gneiss, granite

Intact rock material,

Laboratory specimens have no discontinuities,

RMR = 100,

Q = 500

m = 7.00

s = 1.00

mr = 7.00

sr = 1.00

m = 10.00

s = 1.00

mr = 10.00

sr = 1.00

m = 15.00

s = 1.00

mr = 15.00

s = 1.00

m = 17.00

s = 1.00

mr = 17.00

s = 1.00

m = 25.00

s = 1.00

mr = 25.00

s = 1.00

Very good quality rock mass,

Rocks without isolated blocks with non-weathered discontinuities,

RMR = 85,

Q = 100

m = 2.40

s = 0.082

mr = 4.10

sr = 0.189

m = 3.43

s = 0.082

mr = 5.85

sr = 0.189

m = 5.14

s = 0.082

mr = 8.78

sr = 0.189

m = 5.82

s = 0.082

mr = 9.95

sr = 0.189

m = 8.56

s = 0.082

mr = 14.63

sr = 0.189

Good quality rock mass,

Slightly damaged rocks with non-weathered discontinuities spaced from 1 to 3 m,

RMR = 65,

Q = 10

m = 0.575

s = 0.00293

mr = 2.006

sr = 0.0205

m = 0.821

s = 0.00293

mr = 2.865

sr = 0.0205

m = 1.231

s = 0.00293

mr = 4.298

sr = 0.0205

m = 1.395

s = 0.00293

mr = 4.871

sr = 0.0205

m = 2.052

s = 0.00293

mr = 7.163

sr = 0.0205

Fair quality rock mass,

Partially weathered discontinuities spaced from 0.3 to 1 m,

RMR = 44,

Q = 1

m = 0.128

s = 0.00009

mr = 0.947

sr = 0.00198

m = 0.183

s = 0.00009

mr = 1.353

sr = 0.00198

m = 0.275

s = 0.00009

mr = 2.030

sr = 0.00198

m = 0.311

s = 0.00009

mr = 2.301

sr = 0.00198

m = 0.458

s = 0.00009

mr = 3.383

sr = 0.00198

Poor quality rock mass,

Numerous weathered discontinuities spaced from 30 to 500 mm,

RMR = 23,

Q = 0.1

m = 0.029

s = 0.000003

mr = 0.447

sr = 0.00019

m = 0.041

s = 0.000003

mr = 0.639

sr = 0.00019

m = 0.061

s = 0.000003

mr = 0.959

sr = 0.00019

m = 0.069

s = 0.000003

mr = 1.087

sr = 0.00019

m = 0.102

s = 0.000003

mr = 1.598

sr = 0.00019

Very poor quality rock mass,

Numerous extremely weathered discontinuities with filling spaced by less than 50 mm, fine-grained waste rock,

RMR = 3,

Q = 0.01

m = 0.007

s = 0.0000001

mr = 0.219

sr = 0.00002

m = 0.010

s = 0.0000001

mr = 0.313

sr = 0.00002

m = 0.015

s = 0.0000001

mr = 0.469

sr = 0.00002

m = 0.017

s = 0.0000001

mr = 0.532

sr = 0.00002

m = 0.025

s = 0.0000001

mr = 0.782

sr = 0.00002

Strength of rocks in simple compression σc, Poisson's ratio ν and unit weight of rock γ

Rock strength

Types of rock (examples)

Strength

σc

[MPa]

Poisson's ratio

ν

Bulk weight of rock γ

[kN/m3]

Solid rock

most hard solid rock, intact, compact and  dense quartz rock and basalt, other extraordinary hard rocks

>150

0.10

28.00 - 30.00

Highly hard rock

very hard granite rock, quartz porphyry, very hard granite, hard flinty shale, quartzite, very hard sand rock, and very hard calcite

100 - 150

0.15

26.00 - 27.00

Hard rock

granite, very hard sandstone and calcite, quartzite lode, hard conglomerate, very hard ore, hard limestone, marble, dolomite, pyrite

80 - 100

0.20

25.00 - 26.00

Rock

sandstone, ore, medium sandy shale, flagstone

50 - 80

0.25

24.00

Medium rock

hard mudstone, softer sand rock and calcite, chalky clay

20 - 50

0.25 - 0.30

23.00 - 24.00

Soft rock

shale, soft limestone, chalk, salt rock, frozen ground, anthracite, marl, remoulded sandstone, soft conglomerate, ground with fels

5 - 20

0.3 - 0.35

22.00 - 26.00

Weak soil

compact clay, soil eluvium, black coal

0.5 - 5

0.35 - 0.40

20.00 - 22.0

18.00 - 2

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