rockmass strength properties

8/14/2019 Rockmass Strength Properties

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D.J. Hutchison - 2000

Rock and Rockmass Properties

Lecture 4

Earth 691B: Rock EngineeringMaterials used with kind permission

of Dr Jean Hutchison, Queens U


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Rockmass

Strength

Stope

Hutchinson and Diederichs, 1996

Hutchinson, 2000


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Hoek, 2000

s=1


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Hoek-Brown Failure Criterion

a

ci

bci sm

3

31

'''

Generalized Hoek-Brown failure criterion for jointed rock masses:

Where:

1and 3are maximum and minimum effective stresses at failure

mbis the value of the Hoek-Brown constant m for the rockmass

s and a are constants which depend upon the rockmass characteristicsciis the uniaxial compressive strength of the intact rock pieces

(11.1)


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Generation of Mohr-Coulomb parameters

from the Hoek-Brown failure criterion

Use Equation 11.1 to generate triaxial test results

Statistical curve fitting of data, using Equation 11.2:B

ci

tmn

ciA

'(11.2)

Where:

A andB are material constants

nis the normal effective stress

tmis the tensile strength of the rockmass (Equation 11.3), reflecting

the fact that the rock particles are interlocked and not free to dilate

smmbb

ci

tm 4

2

2 (11.3)


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Estimation of Rockmass Strength

Three rockmass properties are required:ci: uniaxial compressive strength of the

intact rock pieces

mi: value of Hoek-Brown constant mfor

these intact rock pieces

GSIfor the rockmass


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Intact Rock Strength

For intact rock, Equation 11.1 simplifies to:5.0

331 1

'''

ci

ici m

(11.4)

For tests conducted in the range of

0 < 3< 0.5ciand at least 5 tests

on each rock type

Hoek, 2000



Testing UCS for Weak Rock

Generally very difficult to do as sampleswill contain several discontinuities within

their volume.

Very high skill level and specializedequipment only available in a few places in

the world is required.

Use Point Load Test where load is appliednormal to the bedding plane orientations. If

the rock is very weak, and the platens indent

the rock, these tests are invalid.



Hoek, 2000

Foliated rocks displayan anisotropic response

to triaxial testing



Influence of Sample SizeHoek, 2000

18.0

50

50

dccd



Grade

*

Term UCS

(MPa)

Point

Load

Index

MPa

Field estimate of strength Examples

R6 Extremelystrong

> 250 > 10 Specimen can only be chipped with ageological hammer

Fresh basalt, chert, diabase, gneiss,granite, quartzite

R5 Very strong 100 to 250 4 to 10 Specimen requires many blows of a

geological hammer to fracture it

Amphibolite, sandstone, basalt,

gabbro, gneiss, granodiorite,

limestone, marble, rhyolite, tuff

R4 Strong 50 to 100 2 to 4 Specimen requires more than one

blow of a geological hammer to

fracture it

Limestone, marble, phyllite,

sandstone, schist, shale

R3 Medium

strong

25 to 50 1 to 2 Cannot be scraped with a pocket

knife, specimen can be fractured with

a single blow from a geological

hammer

Claystone, coal, concrete, schist,

shale, siltstone

R2 Weak 5 to 25 ** Can be peeled with a pocket knife

with difficulty, shallow indentation

made by firm blow with point of a

geological hammer

Chalk, rocksalt, potash

R1 Very weak 1 to 5 ** Crumbles under firm blows with point

of a geological hammer, can be

peeled by a pocket knife

Highly weathered or altered rock

R0 Extremely

weak

0.25 to 1 ** Indented by thumbnail Stiff fault gouge

** Point load tests on rocks with a uniaxial compressive strength < 25 MPa are likely to yield highly ambiguous results.

Table 11.2: Field estimates of uniaxial compressive strength

* Grade according to Brown (1981).

T bl 11 3 (H k 2000) V l f f i t t k b k V l i th i ti t



Coarse Very fine

Conglomerate Claystone

(22) 4

Breccia

(20)

Marble

9Migmatite

(30)

Gneiss Slate

33 9

Granite

33

Granodiorite

(30)Diorite

(28)

Gabbro

27

Norite

22

Agglomerate

(20)

Sandstone Siltstone

19 9

* These values are for intact rock specimens tested normal to bedding or foliation. The value of m iwill be significantly different if

failure occurs along a weakness plane.

Table 11.3 (Hoek, 2000): Values of m ifor intact rock, by rock group. Values in parenthesis are estimates.

Rock type Class GroupMedium Fine

Texture

Greywacke

Spartic

(10)

Gypstone

7

Chalk

(18)

Coal

(8 to 21)

16

Hornfels

(19)Amphibolite

25 to 31

Schist

4 to 8

Rhyolite

(16)

Dolerite

(19)

Breccia

(18)

Micritic

8

Anhydrite

13

Quartzite

24Mylonite

(6)

Phyllite

(10)

Obsidian

17

(19)

Dacite

(17)Andesite

Tuff

(15)

Clastic

Non-clastic

Organic

Carbonate

Chemical

19

Basalt

Sedimentary

Metamorphic

Non foliated

Slightly foliated

Foliated*

Igneous

Light

Dark

Extrusive pyroclastic type

Rock Texture



Very fineClaystone

4+/-2

Shale

(6+/-2)

Marl

(7+/-2)

Dolomite

(9+/-3)

Chalk

7+/-2

Slate

7+/-4

Peridotite

(25+/-5)

Siltstone

7+/-2

Gypsum

(29+/-3)

* Conglomerate and breccia may have a wide range of m ivalues, depending upon the nature of the cementing material, and

the degree of cementation. Hence their values may range from values similar to that of sandstone to those of fine grained

sediments (even < 10).

Rock

typeClass Group

Fine

Texture

Gabbro

Granite

32+/-3 25+/-5

(8+/-2)

Marble

9+/-3

Amphibolite

26+/-6

Migmatite

Hornfels

(19+/-4)

Metasandstone

(19+/-3)

Diorite

Schist

12+/-3

(29+/-3)

Granodiorite

Micritic Limestone

(9+/-2)Anhydrite

12+/-2

Quartzite

20+/-3

Gneiss

28+/-5Phyllite

(7+/-3)

Tuff

(13+/-5)

Clastic

Non-Clastic

Carbonate

Slightly foliated

Foliated**

Organic

Non foliated

Hypabyssal

Volcanic

Lava

Pyroclastic

Greywacke

(18+/-3)

Evaporite

Breccia

*

Crystalline

Limestone

(12+/-3)

Spartic Limestone

(10+/-2)

22

Dark27+/-3

Dolerite

(16+/-5)

Norite

Coarse MediumSandstone

17+/-4

Conglomerate

*

Porphyry

(20+/-5)

Diabase

(15+/-5)

Rhyolite

(25+/-5)

Andesite

25+/-5

Dacite

25+/-3

Basalt

(25+/-5)

** These values are for intact rock specimens tested normal to bedding or foliation. The value of m iwill be significantly

different if failure occurs along a weakness plane.

S

edimentary

Metamorphic

Light

Plutonic

Igne

ous

Agglomerate

(19+/-3)

Breccia

19+/-5

Hoek and Marinos, 2000



Geological Strength

Index: GSI

Hoek, 2000Strength of jointed rockmass

depends on:

properties of intact rock

pieces, and

upon the freedom of thesepieces to slide and rotate

under different stress

conditions,

controlled by the

geometrical shape of the

intact rock pieces as well as

the condition of the

discontinuities separating the

pieces


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Mohr-Coulomb Parameters

Hoek, 2000

Hoek, 2000



Cohesive and Frictional Strength

Hoek, 2000



Deformation Modulus

For poor quality rockmasses, where ci< 100:

4010

10

100

GSI

ci

mE



Effect of water on rockmass strength

Reduction in strength of rock, particularlyshale and siltstone.

Pressure: why?

This may not be much of a problem duringexcavation, because water pressures in the

surrounding rock are reduced to negligible

levels. If groundwater pressures are re-established after the completion of the final

lining, then consider in design.

Water handling.

H k 2000



Post-failure Behaviour:

Very Good Quality HardRockmass

Hoek, 2000

H k 2000




Average Quality Rockmass

Hoek, 2000


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Very Poor QualityRockmass

Hoek, 2000


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Uncertainty in

Rockmass

StrengthEstimates:

INPUTHoek, 2000


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Uncertainty in

Rockmass Strength

Estimates: OUTPUT

Hoek, 2000


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Practical Examples of Rockmass Property Estimates:

Massive Weak Rock, Braden Breccia, El Teniente Mine

Hoek, 2000

Hoek, 2000


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Massive Strong Rockmasses,

Rio Grande Pumped Storage Scheme

Hoek, 2000


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D.J. Hutchison - 2000 Hoek, 2000

Average Quality Rockmass, Nathpa Jhakri Hydroelectric

Partially completed 20 m

span, 42.5 m high

underground powerhouse

cavern of the Nathpa Jhakri

Hydroelectric Project inHimachel Pradesh, India.

The cavern is approximately

300 m below the surface.


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Average Quality Rockmass, Nathpa Jhakri Hydroelectric

Hoek, 2000


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Poor Quality Rockmass at Shallow Depth: Athens Metro

Hoek, 2000


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Poor Quality Rockmass at Shallow Depth: Athens Metro

Hoek, 2000

Hoek, 2000


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Poor Quality

Rockmass underHigh Stress


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Poor Quality Rockmass under High Stress

Hoek, 2000

Figure 11.28: Results of a numerical

analysis of the failure of the rock mass

surrounding the Yacambu-Quibor tunnel

when excavated in graphitic phyllite at ad h f b 600 b l f

Figure 11.29: Displacements in the rock

mass surrounding the Yacambu-Quibor

tunnel. The maximum calculated

displacement is 258 mm with no supportd 106 i h

rockmass strength properties

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