Simon Fraser University Engineering Geology and

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Simon Fraser UniversityEngineering Geology and
Resource GeotechnicsResearch Group
Numerical analysis of the influence of geological
structures on the development of surface
subsidence associated with block caving mining
Alex Vyazmensky Davide Elmo Doug Stead Jon Rance
Department of Earth Sciences Simon Fraser
University
Rockfield Technology Ltd.
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Block Caving and Associated Subsidence
Block cave mining is characterized by caving and
extraction of a massive volume of ore which
translates into a formation of major surface
depression or subsidence zone directly above and
in the vicinity of the mining operations.
The ability to predict surface subsidence
associated with block caving mining is important
for mine planning, operational hazard assessment
and evaluation of environmental and
socio-economic impacts.
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New Numerical Modelling Approach
Most natural rocks subjected to engineering
analysis are brittle failure in such rocks is a
result of brittle fracture initiation and
propagation.
Continuum and discontinuum modelling approaches
provide approximations of brittle fracturing to
some degree, none of them however offer realistic
representation of the actual brittle fracturing
phenomena which involves fracture growth,
propagation and material fragmentation.
A state-of-the-art FEM/DEM code ELFEN is employed
as the principal modeling tool. The code allows
physically realistic simulation of the caving
process as a brittle fracture driven
continuum-discontinuum transition with the
development of new fractures and discrete blocks.
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FEM/DEM Modelling Examples
Rock bridge failure
Step-path drive open pit wall failure
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Conceptual Study Strategy
Influence Matrix
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Modelling Methodology - Typical Model Setup
FracMan DFN model
Constraint
3D model
2D trace plane
fractures exported into ELFEN
2D ELFEN model
ore block
model response evaluation
ore block is undercut and fully extracted
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Subsidence Simulation Example
Caving
Initiation
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Subsidence Simulation Example
Crater
Formation
Evolution of vertical displacements (0.1 1m)
50m
20
70
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Conceptual Study Examples
Effect of Joint Orientation
5 ore extraction
100 ore extraction
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80
20
70
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Conceptual Study Example
Effect of Joint Orientation
Evolution of zone of major (10cm) vertical (YY)
and horizontal (XX) surface deformations with
continuous ore extraction

• Major subsidence deformations develop in a
  relatively rapid manner related to a quick
  mobilization of massive rock mass segments

• About 90 of maximum surface displacements are
  achieved by 50 ore extraction

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Conceptual Study Example
Effect of Joint Orientation
Extent of of major vertical (10 cm) surface
displacements
Change in joint orientation causes an increase in
the total major surface deformations extent of
up to 30
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Conceptual Study Example
Effect of Joint Orientation
Resultant surface profiles

• Rotation of the joint
• pattern shifts centre of
• surface depression
• Depth of the subsidence
• crater is related to the
• extent of the rock mass
• mobilized by the failure,
• - larger extent of
• rock mass mobilization
• results in shallower crater

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Conceptual Study Example
Effect of Joint Orientation - Conclusions

• Well defined, persistent, vertical to steeply
  dipping joints govern the direction of cave
  propagation and the mechanism of near surface
  rock mass mobilization.
• The shallower the dip of these joints the more
  inclined from vertical is the cave propagation
  direction and the more asymmetrical are the
  surface deformations.
• In cases where multiple well defined and
  persistent steeply dipping sets are present the
  steepest set will generally have the predominant
  influence.
• Major subsidence asymmetry
• is observed in the dip direction
• of the sub-vertical/steeply dipping set,
• where joints are inclined towards
• the cave, the rock mass fails
• through block-flexural and
• block toppling and detachment and sliding of
  major rock segments.
• Depending on joint inclination the joint
  persistence may have a very significant effect on
  surface subsidence induced by block caving.

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Subsidence Simulation Example
Influence of fault
Evolution of vertical displacements (0.1 1m)
Geometry
Subsidence crater development
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Effect of Fault Location
former fault position
60

60

60

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Conceptual Study Example
Effect of Fault Location and Inclination

• Steeply dipping faults, daylighting into the
  cave and located within an area of imminent
  caving are likely to be caved and are unlikely to
  play any major role in the resultant subsidence.
• Faults partially intersecting the caving area
  may create favourable conditions for failure of
  the entire hanging wall.
• Depending on rock mass fabric faults located in
  the vicinity of the caving zone may have minimal
  influence or decrease the extent of the area of
  subsidence deformations.
• A topographical step in the surface profile is
  formed where the fault daylights at the surface.
• Inclination of the fault partially intersecting
  the caving area controls the extent of surface
  subsidence deformations. Low dipping faults will
  extend and steeply dipping fault will decrease
  the area of surface subsidence.

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Conclusions

• A new approach to block caving subsidence
  analysis based on fracture mechanics principles
  and FEM/DEM modelling technique allows
  physically realistic simulations.
• Significant insights were gained into complex
  block caving subsidence phenomena.
• Future developments in parallel processing
  capabilities and improvements in 3D fracturing
  algorithms will allow realistic FEM/DEM modelling
  of block caving in 3D.

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Subsidence modelling example
Crown Pillar Collapse
QUESTIONS?