test 3

• Primary structures


  Layers formed by the accumulation of material at the surface of the earth
• Sedimentary and volcanic rocks form in layers
• Large intrusive granitic bodies form homogeneous masses
• Layering


  Result of variations in the deposition rate with time
• In a normal sequence of layers the oldest ones are at the bottom
• Law of superposition


  Oldest layers are at the bottom
• Stress in the earth's crust causes deformation of the horizontal layering, giving rise to geological structures

  
• Brittle deformation


  • Rocks deform within their elastic field
  • Rocks have coefficients of elasticity in excess of 20GPa

  
• Ductile deformation


  • Rocks behave in a ductile manner due to the pressures and temperatures at depth
• Fault


  A shear plane across which the rocks move in opposite directions

  
• Types of faults


  • Normal faults
  • Reverse faults
  • Strike-slip faults

  
• Dip


  The angle of inclination of a non-vertical fault plane

  
• Horizontal compressional forces


  Produce reverse faults
• Horizontal extension


  Results in normal faults
• Compression occurs at convergent tectonic plate margins, characterised by thrust faulting

  
• Divergent tectonic plate margins are characterised by conjugate sets of normal faults that form grabens
• Joints


  More or less planar fractures in rocks with no or very little movement
• Types of joints


  • Unloading joints
  • Cooling joints
• Joints reduce the strength of rocks because failure along joint planes is much easier than breaking the intact rock
• Planes of weakness in rock masses


  • Bedding planes
  • Unconformities
  • Sedimentary structures
  • Fault planes
  • Joints
  • Foliation
  • Cleavage
  • Lithological contacts

  
• Rock Quality Designation (RQD)


  Percentage of core recovered in segments >10cm long
• RQD classification


  • 0-25% = Very poor
  • 25-50% = Poor
  • 50-75% = Fair
  • 75-90% = Good
  • 90-100% = Excellent
• Fold


  Deformation of rock layers under ductile conditions without fracturing
• Fold elements


  • Hinge point
  • Limbs
  • Hinge line
  • Fold axis
  • Axial surface

  
• Syncline


  Fold with younger rocks in its core
• Anticline


  Fold with older rocks in its core
• Shear zone


  Zone of ductile deformation between two undeformed blocks of rock

  
• Lithostatic stress


  Stress at depth due to the weight of the overlying rock material
• Calculating lithostatic stress


  1. Stress = density of rock x acceleration due to gravity x depth

  2. For layered rocks: sum of (density of layer x thickness of layer)

  
• Unfractured rock masses with very high strength are the best for tunnelling
• Fractured rock masses have low strength and require support
• Factors affecting tunnelling


  • Spacing and orientation of fractures
  • Shear strength along fractures
  • Orientation of tunnel relative to rock strata
  • Permeability of rock layers and water table
• Tunnel orientation effects


  • Horizontal strata - consistent hanging wall pressure
  • Parallel to strike of inclined strata - different side wall pressures
  • Parallel to strike of vertical strata - significant shear stress in hanging wall
  • Perpendicular to strike - more uniform stress distribution
  • Parallel to fold axis of anticline - less stress on walls
  • Parallel to fold axis of syncline - more stress on walls
• Fault planes intersecting a tunnel have the same effect as bedding planes, but may have much poorer shear strengths

  
• Stratigraphy


  The division of geology that attempts to produce a logical arrangement to the layers of rock strata
• All types of rocks (sedimentary, igneous and metamorphic) fall within the scope of stratigraphy
• Law of superposition


  Within an undisturbed sedimentary succession any given layer is younger than the layer upon which it rests
• Facies change


  The lateral change in the lithological and/or faunal characteristics of a rock
• Establishing stratigraphy of an area


  1. Determine rock sequence within a single exposure

  2. Trace components laterally

  3. Encounter new units

  4. Determine relative ages

  5. Create composite section
• Dip of beds


  Can be used to establish stratigraphic succession, as beds in the dip direction are normally the youngest (but not always)