D.J. Hutchison - 2000 Rock and Rockmass Properties Lecture 4 Earth 691B: Rock Engineering Materials...

D.J. Hutchison - 2000

Rock and Rockmass Properties

Lecture 4

Earth 691B: Rock Engineering

Materials used with kind permission of Dr Jean Hutchison, Queen’s U


Rockmass Strength

StrongMassive

FairDiscrete Joints

WeakLow Integrity

Relative Properties

Stope

Hutchinson and Diederichs, 1996

Hutchinson, 2000


Hoek, 2000

s=1


Hoek-Brown Failure Criterion

a

cibci sm

3

31'

''

Generalized Hoek-Brown failure criterion for jointed rock masses:

Where:’1 and ’3 are maximum and minimum effective stresses at failure

mb is the value of the Hoek-Brown constant m for the rockmass

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

(11.1)


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

tmnciA

' (11.2)

Where:A and B are material constants’n is the normal effective stress

tm is the ‘tensile’ strength of the rockmass (Equation 11.3), reflecting the fact that the rock particles are interlocked and not free to dilate

smm bb

citm 4

22 (11.3)


Estimation of Rockmass Strength

• Three rockmass properties are required:

ci: uniaxial compressive strength of the intact rock pieces

mi: value of Hoek-Brown constant m for these intact rock pieces

GSI for the rockmass


Intact Rock Strength

• For intact rock, Equation 11.1 simplifies to:5.0

331 1

'''

ciici m

(11.4)

For tests conducted in the range of 0 < ’3 < 0.5ci and at least 5 tests on each rock type

Hoek, 2000


Testing UCS for Weak Rock

• Generally very difficult to do as samples will contain several discontinuities within their volume.

• Very high skill level and specialized equipment only available in a few places in the world is required.

• Use Point Load Test where load is applied normal 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 display an anisotropic response to triaxial testing


Influence of Sample SizeHoek, 2000

18.0

5050

dccd


Grade *

Term UCS (MPa)

Point Load Index (MPa)

Field estimate of strength Examples

R6 Extremely strong

> 250 > 10 Specimen can only be chipped with a geological 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).


Coarse Very fineConglomerate Claystone

(22) 4Breccia

(20)

Marble 9

Migmatite(30)

Gneiss Slate33 9

Granite33

Granodiorite(30)

Diorite(28)

Gabbro 27

Norite22

Agglomerate(20)

Sandstone Siltstone19 9

* These values are for intact rock specimens tested normal to bedding or foliation. The value of m i will be significantly different if failure occurs along a weakness plane.

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

Rock type Class GroupMedium Fine

Texture

Greywacke

Spartic (10)

Gypstone

7Chalk(18)

Coal (8 to 21)

16Hornfels

(19)Amphibolite

25 to 31Schist4 to 8

Rhyolite(16)

Dolerite(19)

Breccia(18)

Micritic 8

Anhydrite13

Quartzite24

Mylonite(6)

Phyllite(10)

Obsidian

17

(19)Dacite(17)

Andesite

Tuff(15)

Clastic

Non-clastic

Organic

Carbonate

Chemical

19Basalt

Sedimentary

Metamorphic

Non foliated

Slightly foliated

Foliated*

Igneous

Light

Dark

Extrusive pyroclastic type


Very fineClaystone

4+/-2Shale(6+/-2)Marl

(7+/-2)

Dolomite

(9+/-3)

Chalk7+/-2

Slate7+/-4

Peridotite(25+/-5)

Siltstone7+/-2

Gypsum

(29+/-3)

* Conglomerate and breccia may have a wide range of m i values, 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 type

Class GroupFine

Texture

Gabbro

Granite32+/-3 25+/-5

(8+/-2)

Marble 9+/-3

Amphibolite26+/-6

Migmatite

Hornfels(19+/-4)

Metasandstone(19+/-3)

Diorite

Schist12+/-3

(29+/-3)Granodiorite

Micritic Limestone

(9+/-2)Anhydrite

12+/-2

Quartzite20+/-3

Gneiss28+/-5Phyllite(7+/-3)

Tuff(13+/-5)

Clastic

Non-Clastic

Carbonate

Slightly foliated

Foliated**

Organic

Non foliated

Hypabyssal

VolcanicLava

Pyroclastic

Greywacke(18+/-3)

Evaporite

Breccia*

Crystalline Limestone(12+/-3)

Spartic Limestone

(10+/-2)

22

Dark27+/-3

Dolerite(16+/-5)

Norite

Coarse MediumSandstone

17+/-4Conglomerate

*

Porphyry(20+/-5)

Diabase(15+/-5)

Rhyolite(25+/-5)Andesite25+/-5

Dacite25+/-3Basalt

(25+/-5)

** These values are for intact rock specimens tested normal to bedding or foliation. The value of m i will be significantly different if failure occurs along a weakness plane.

Sed

imen

tary

Met

amor

phic

Light

Plutonic

Igne

ous

Agglomerate(19+/-3)

Breccia19+/-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 these pieces 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


GSI

28

100exp

GSImm ib


Mohr-Coulomb Parameters

Hoek, 2000

Hoek, 2000


Cohesive and Frictional Strength

Hoek, 2000


Deformation Modulus

For poor quality rockmasses, where ci < 100:

40

10

10100

GSI

cimE


Effect of water on rockmass strength

• Reduction in strength of rock, particularly shale and siltstone.

• Pressure: why?

• This may not be much of a problem during excavation, 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.


Post-failure Behaviour: Very Good Quality Hard

Rockmass

Hoek, 2000


Post-failure Behaviour: Average Quality Rockmass

Hoek, 2000


Post-failure Behaviour: Very Poor Quality

RockmassHoek, 2000


Uncertainty in Rockmass Strength

Estimates: INPUT

Hoek, 2000


Uncertainty in Rockmass Strength

Estimates: OUTPUT

Hoek, 2000


Practical Examples of Rockmass Property Estimates: Massive Weak Rock, Braden Breccia, El Teniente Mine

Hoek, 2000

Hoek, 2000


Massive Strong Rockmasses, Rio Grande Pumped Storage Scheme

Hoek, 2000

D.J. Hutchison - 2000 Hoek, 2000

Average Quality Rockmass, Nathpa Jhakri Hydroelectric

Partially completed 20 mspan, 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.


Average Quality Rockmass, Nathpa Jhakri Hydroelectric

Hoek, 2000


Poor Quality Rockmass at Shallow Depth: Athens Metro

Hoek, 2000


Poor Quality Rockmass at Shallow Depth: Athens Metro

Hoek, 2000

Hoek, 2000


Poor Quality Rockmass under

High Stress


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 a depth of about 600 m below surface.

Figure 11.29: Displacements in the rock mass surrounding the Yacambu-Quibor tunnel. The maximum calculated displacement is 258 mm with no support and 106 mm with support.

D.J. Hutchison - 2000 Rock and Rockmass Properties Lecture 4 Earth 691B: Rock Engineering Materials...

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