ROCK MECHANICS: L4

1
ROCK MECHANICS L4

• Rock and Soils 2006
• Don Cameron

2
LECTURE OUTLINE- Laboratory Testing

• UCS
• Triaxial
• Point load Index
• Brazil test
• Schmidt hammer

• Degradation testing

3
Field Tests

• Involve subjecting a large volume of rock to load
  monitoring the deformation
• more representative results
• the volume of rock still not big enough?
• more expensive
• test location/development changes?

4
LAB TESTS

• On discontinuities AND cores of intact rock
• Empirical models then can provide an estimate of
  the rock mass characteristics

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Range of Lab Tests

• Uniaxial Compressive Strength (UCS)
• Triaxial strength test
• Tensile strength test (Brazil test)
• Density moisture content
• Shear strength test on discontinuities
• Various index tests
• International Society for Rock Mechanics (ISRM)
  - Standardisation of Laboratory and Field Tests
• Australian Standards?

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Standards

• ISRM - Minimum number of tests that should be
  carried out on a particular rock type?
• AS 4133 Methods of Testing Rocks for Engineering
  Purposes

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(1) UNIAXIAL COMPRESSION

• The diameter of the specimen should be gt 50 mm
• practical maximum 63 mm? 10x rule?
• The l/d ratio should be 2.0 - 3.0
• The ends of the specimen are trimmed flat
• capping of weak material
• The ends of the specimen have to be perpendicular
  
• Lightly lubricated spherical seat on the upper cap

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UCS set-up
0.5 -1.0 MPa/sec 5 to 15 mins/test
Spherical seat important to keep load aligned
with sample axis for samples with
dsicontinuities
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Strain Gauges for axial/radial strain?

• Not always possible
• Rough surfaces
• Moist surfaces
• Oil or water
• Discontinuous samples?
• Inaccurate if near stress concentrations which
  cause micro-cracking

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NOTES

• compressive strength generally decreases as d
  increases
• especially for coarse grained fissured rocks
• strength of a core decreases as length, l,
  increases
• more flaws?
• greater chance of unfavourable orientation of
  flaws?

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Typical UCS values (?c)
Refer Table 1 for comprehensive listing
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Moisture

• Samples must be stored tested at a mc
  appropriate to the purpose
• Moisture can have a significant effect on the
  strength and deformability of rock
• Samples retrieved from below the water table may
  dry out if not carefully protected
• WARNING Samples retrieved above the WT may be
  too wet - drilling fluid?

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TRIAXIAL TESTING
Spherical top cap Strain gauged specimen -
axial and radial strain Stiff rubber jacket Solid
steel cell Oil instead of water to provide ?3
- (values in MPa) - (?1 ? ?3) Need to
apply ?1 through ram as oil loaded Usually no pwp
measurement
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Fine-grained mudstone - Johnston
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10
8
??3 max 3.56 MPa
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(?1 - ?3)max (MPa)
Excess pwp (MPa)
4
2
0
-2
10-4
10-5
10-3
10-2
Strain Rate (strain/minute)
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Influence of Discontinuities on Strength

• Orientation number of discontinuities affect
  strength CASE 1 a single discontinuity

?
?
Apparent angle of friction of joint ?w
0
90
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Intersecting Joints?
?
0
90
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Point Load Testing
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Point Load Testing

• Usually a core, diameter, D 50 mm
• BUT rock sample does not have to be cylindrical
  (Figure 7)
• effective diameter, De

Corrections to equivalent core diameter of 50 mm
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Point Load Test and UCS

• Is50 of VALID test correlated against UCS
• NOTE Valid tests if splitting rather than
  crushing
• ?c ? 20 Is50

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BRAZIL Test

• Testing of discs of rock
• Splitting test

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Shear Testing of Discontinuities
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Shear Testing of Discontinuities
Hoek shear cell
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Shear Results
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Portable shear device failings

• Direction of the normal load shifts with shear
  displacement corrections
• Difficult to maintain pressures with hydraulic
  jacks
• also pumping creates jerking disturbance
• Dilating shear planes create enormous normal
  forces through the cable ties, which can be
  unrealistic

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Slake Durability Test

• Resistance to two standard cycles of drying and
  wetting
• For exposed rock
• Rotating drum (20 rpm) rotates in water
• 10 small dry lumps rotated for 10 mins
• Dried out and repeated
• Mass loss recorded

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Slake Durability Test
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KEY POINTS?