ROCK MECHANICS

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ROCK MECHANICS

• Rock and Soils 2006
• Don Cameron

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COURSE OUTLINE

• Engineering classification of rock
• Field and laboratory testing
• strength and deformation
• rock mass v rock substance
• Stability of slopes and excavations
• Foundations in rock
• deep and shallow
• settlements, bearing capacity
• Anchorages in rock

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INHERENT COMPLEXITIES

• Rock fracture
• under compressive stresses
• Size effects
• response of rock to loading affected by the size
  of the loaded volume (joints fractures)
• Tensile strength
• is low (similar to concrete) HOWEVER a rock mass
  can have even less tensile strength

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COMPLEXITIES.

• Groundwater effects
• water in joints if under pressure, reduces
  normal effective stress (less resistance along
  joints)
• water in permeable rocks (e.g. sandstone) ?
  effective stress law
• softening of clay seams argillaceous rocks
  (e.g. shales)

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COMPLEXITIES.

• Weathering
• chemical/physical alteration, reduction of
  engineering properties
• limestone caverns, sinkholes Karst
• basic rocks with olivine (e.g. basalt) and
  pyroxene minerals are reduced to montmorillonite
  by hydrolysis

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STRUCTURAL FEATURES or DISCONTINUITIES

• Bedding planes
• Folds
• tension joints at the crest of a fold (strike,
  dip shear joints)
• folding may cause shear failure along bedding
  planes (axial plane
  or fracture cleavage)

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Folding
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DISCONTINUITIES

• Faults
• shear displacement zones - sliding
• Faults may contain
• Fault gouge (clay) weak!
• Fault breccia (re-cemented rock) weak!
• Rock flour weak!
• Angular fragments may be strong

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DISCONTINUITIES

• Shear zones
• bands of materials - local shear failure
• Dykes
• igneous intrusions (near vertical)
• weathered dykes, e.g. dolerite weathers to
  montmorillonite
• unweathered dykes attract high stresses
• Joints
• breaks with no visible displacement

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Joint Patterns

• sedimentary rocks usually contain 2 sets of
  joints, orthogonal to each other and the bedding
  plane

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JOINTS

• Open
• Filled
• Healed or closed
• Stepped
• Undulating
• Planar
• Rough
• Smooth
• Slickensided

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Order of Description of Rocks (AS 1726-1993)
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DESIGN IN ROCK (Lecture 1)

• Take into account
• Local geological structure
• Shear strength of the rock mass
• Impact of water on stability
• Rock anchoring?
• Drilling and blasting procedures
• Monitoring of stability
• the observational method

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Intact Rock

• Heterogeneous
• Anisotropic (soils less so)
• Spatial variability (soils the same)
• Yield mechanisms are non-linear depend on
  stress level and rock type
• Failures are often brittle
  (soils strain soften or harden past the peak
  strength)

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Rock Masses

• Contain discontinuities with little tensile
  strength
• Scale effect
• response is dependent on stressed volume
• Affected by groundwater weathering
• In-situ stresses difficult to estimate

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DEFINITIONS

• Dip angle, ?w
• the acute angle measured in a vertical plane
  between the line of maximum dip in a
  non-horizontal plane and the horizontal plane
  
• i.e. 0 ßw 90.
• Dip direction, ?w the geographical azimuth
  measured in a clockwise direction from north (0)
  of the vertical plane in which the dip angle is
  measured i.e. 0 ?w 360.

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Dip Angle
North
Horizontal
?w
Line of maximum dip
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Dip direction
Azimuth is the direction of an object, measured
clockwise around the observer's horizon from
North, i.e. an object due north has an azimuth of
0,
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Dip Direction
North
Horizontal
?w
Line of maximum dip
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SLOPE DESIGN IN ROCK
Terminology
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Empirical Design

• EXPERIMENTAL EVIDENCE (empirical laws)
• Surveys of heights (H) of stable and unstable
  slopes excavated in hard rock
• - plotted in terms of the overall slope angle
• FINDINGS
• there is a practical limit to how steep slopes
  can be (equation to the line has little use for
  serious slope design)
• some slopes remain stable near their practical
  limit, many slopes were unstable at angles far
  less than this angle

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Empirical design chart for slopes in hard
rockstable slopes small dots
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Slope Design in Rock Masses with Planar Defects

• A vertical slope is intersected by a planar weak
  seam dipping at an angle ßw
• The friction angle of the weak seam is 20
• The density of the rock overlying the seam is
  2500kg/m3
• The slope is dry

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Maximum height v Angle of defect
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The geometry
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SLOPE DESIGN IN ROCK

• Step 1 one batter (no berms)
• Determine a safe batter angle (?f)
• overall height (H) of the slope
• strength of the intact rock
• geometry strength of discontinuities
• size and direction of external loading
• groundwater conditions

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Design steps
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SLOPE DIMENSIONS

• Step 2 need benches to catch rock falls?
• Bench widths 1 to 3 m in width
• Every 10 m height in weathered rock
• 10 20 m height in good rock
• Step 3 safety berms? benches 3 to 5 m wide
• Designed to catch more rock
• Evenly spaced on the slope, e.g. every 3 - 5
  benches

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Overall slope angle (ßF)
From the geometry.
Note eqn. in text and Figs are in error
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SUMMARY

• Rock mechanics needs careful consideration of the
  influence of defects on rock mass behaviour
• Empirical methods for first assessment only

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Key Points

• Rock descriptions
• there is an Australian Standard
• Design
• control of risk
• Introduction to rock slopes