Characterization of Failure Modes

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Characterization of Failure Modes

• Prior to conducting any limit-equilibrium
  calculations for rock-slope stability
  evaluations, the engineer first must identify the
  geologic structural patterns and the subsequent
  geometrical interactions with the proposed
  slope-cut face.
•  
• Basic checklist for rock-slope stability
  analysis
•  
• 1. Do the orientations of rock mass
  discontinuities allow for the formation of any
  kinematically viable failure modes for the
  proposed slope cut (face)?
•  
• Look for plane shears, wedges, step-paths,
  toppling modes

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• Failure Modes
•  
• Plane-shear mode
•  
• A fracture set with nearly the same dip direction
  as
• the slope face (approx. 20o) and dipping more
• shallow (flatter) than the slope face. This set
  will
• have a cluster of poles displayed on a stereonet
  plot
• that coincides nearly with the pole of the slope
  face.  

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• Failure Modes
•  
• Step-path mode
•  
• A failure path comprised of the combination of
  two
• fracture sets, one flat and one steep, both of
  which
• strike nearly parallel to the slope face. The
  flatter set
• (known as the master joint set) must be
  daylighted in
• the face, while the steeper set (known as the
  cross
• joint set) provides step-ups to form a quasi-
• continuous step-path failure path.  

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• Failure Modes
•  
• Wedge mode (3-D analysis)
•  
• A tetrahedral wedge of rock formed by the
• intersection of two fracture planes so that
  potential
• sliding occurs along the daylighted intersection
  line.
• To be kinematically viable, such a wedge must
  have
• an intersection line that falls in between the
  dip
• directions of the two planes and parallel (or
  nearly
• parallel) to the dip direction of the slope face.
   

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• Failure Modes
•  
• Toppling
• A steeply dipping fracture set strikes nearly
  parallel to
• the slope face and forms tall slabs that peel
  away
• from the slope.
•  

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Characterization of Failure Modes

• Basic checklist for rock-slope stability
  analysis
• 1. Do the orientations of rock mass
  discontinuities allow for the formation of any
  kinematically viable failure modes for the
  proposed slope cut (face)?

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Characterization of Failure Modes

• Basic checklist for rock-slope stability
  analysis
• 1. Do the orientations of rock mass
  discontinuities allow for the formation of any
  kinematically viable failure modes for the
  proposed slope cut (face)?
• 2. For those failure modes considered
  kinematically viable, do the discontinuities that
  comprise them have sufficient length
  (persistence) to allow for significantly sized
  failures?
•  
• Depending on the project, significantly sized
  may indicate rock failure masses with a volume of
  at least 1 to 5 m3. Relatively short
  discontinuities form failures that break out only
  near the crest of the slope face, leaving a
  notched, ragged appearance at the top of the
  slope face but most likely not affecting the
  overall, large-scale stability of the rock slope.

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Characterization of Failure Modes

• Basic checklist for rock-slope stability
  analysis
• 1. Do the orientations of rock mass
  discontinuities allow for the formation of any
  kinematically viable failure modes for the
  proposed slope cut (face)?
• 2. For those failure modes considered
  kinematically viable, do the discontinuities that
  comprise them have sufficient length
  (persistence) to allow for significantly sized
  failures?
• 3. For those kinematically viable failure mode(s)
  with sufficient length, is there adequate shear
  strength along the sliding surface(s) to prevent
  instability?
•   (Driving forces are compared to resisting
  forces, and FS is computed.)

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• Plane-Shear Analysis
•  
• Required input for the 2-D analysis
• Slope geometry
• slope face (cut) angle (a)
• upslope angle above cut crest (q)
• Geometry of potential failure mass
• dip of plane-shear structure (b)
• vertical height from daylight point to
• slope-cut crest (h)
• horz. dist. from cut crest to tension crack
  (Dc)

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• Plane Shear (continued) 
• Rock properties
• rock-mass unit weight (g)
• shear strength (t) model for sliding plane
• average waviness angle (R)
• Ground water information
• depth to water level in tension crack (Dwc)
• depth to water table assumed parallel to the
• upslope (Dw)
• External loads (optional)
• surcharge load appl. to potential failure
  mass
• rock-bolt angle and loading information

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• 3-D Wedge Analysis
•  
• Required input
• Slope geometry
• dip and dip direction of the slope face (cut)
• dip and dip direction of the upslope
• Geometry of potential failure mass
• dip and dip direction of the two
  discontinuities
• forming the wedge
• dip direction of an assumed vertical t-crack
• vert. height from daylight point to cut crest
  

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• 3-D Wedge (continued) 
• Rock properties
• rock-mass unit weight
• shear strength model for each plane
• average waviness angle for each plane
• Ground water information
• depth to water table assumed parallel to the
• upslope and the same depth in t-crack
• External loads (optional)
• surcharge load appl. to potential failure mass
  rock-bolt direction and loading information

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• Options for Shear Strength Model
•  
• The plane shear and 3-D wedge stability analyses
  in program RkSlope allow the user to select
  several options for shear strength
• 1. General power curve model
• Includes linear model (atanf, b0,
  ccohesion)
• Includes power model through zero (c0 a b
  terms)
• Includes general nonlinear power model (a, b,
  c)
• 2. JRC model
• Input JRC (unitless), JCS (units of tsm), fb
  (deg.)

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• Options for Including Ground-Water Pressure
•  
• The plane shear and 3-D wedge stability analyses
  in program RkSlope allow the user to select
  several options for including ground-water
  pressure
• 1. Hydrostatic option
• Simultaneously computes FS for two cases
• Case 1 (FS) Boundary pressure distribution
  originates
• from water standing in the t-crack and
  results in two
• triangular pressure components (most
  commonly used).
• Case 2 (FSM) Failure mass treated as
  porous media
• with pore-pressure distribution equiv. to
  mirror-image
• of the saturated portion of the mass (rarely
  used).

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• Ground-Water Options (continued)
•  
• 2. Seepage option (Case 3)
• The fractured rock mass is assumed to have
  enough
• interconnected fractures to allow mass water
  flow, so
• that gradient and saturated volume can be
  used to
• compute seepage force.
• In most cases, this option provides FS values
  between
• those obtained from Case 1 (most optimistic)
  and from
• Case 2 (most conservative).