d.j. hutchison - 2000 rock and rockmass properties lecture 4 earth 691b: rock engineering materials...
TRANSCRIPT
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
D.J. Hutchison - 2000
Rockmass Strength
StrongMassive
FairDiscrete Joints
WeakLow Integrity
Relative Properties
Stope
Hutchinson and Diederichs, 1996
Hutchinson, 2000
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Hoek, 2000
s=1
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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)
D.J. Hutchison - 2000
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)
D.J. Hutchison - 2000
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
D.J. Hutchison - 2000
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
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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.
D.J. Hutchison - 2000
Hoek, 2000
Foliated rocks display an anisotropic response to triaxial testing
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Influence of Sample SizeHoek, 2000
18.0
5050
dccd
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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).
D.J. Hutchison - 2000
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
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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
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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
D.J. Hutchison - 2000
GSI
28
100exp
GSImm ib
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Mohr-Coulomb Parameters
Hoek, 2000
Hoek, 2000
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Cohesive and Frictional Strength
Hoek, 2000
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Deformation Modulus
For poor quality rockmasses, where ci < 100:
40
10
10100
GSI
cimE
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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.
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Post-failure Behaviour: Very Good Quality Hard
Rockmass
Hoek, 2000
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Post-failure Behaviour: Average Quality Rockmass
Hoek, 2000
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Post-failure Behaviour: Very Poor Quality
RockmassHoek, 2000
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Uncertainty in Rockmass Strength
Estimates: INPUT
Hoek, 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
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.
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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 under
High 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 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.