Use of numerical modelling to estimate shotcrete requirements using a Ground Reaction Curve approach

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Use of numerical modelling to estimate shotcrete
requirements using a Ground Reaction Curve
approach

• Kevin Le Bron (Golder)
• Tony Leach (Itasca Africa)
• William Joughin (SRK)

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Introduction

• Simrac Project SIM 040204
• Numerical modelling to investigate
• Shotcrete/rockmass interaction
• Load/deformation performance requirements of
  shotcrete under a range of geotechnical conditions

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Modelling requirements

• Modelled rock mass needs to fragment
• Effect of discontinuities on lining local
  loading
• Identify deformations under various geotechnical
  conditions rock type, GSI, field stress

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Model design laboratories
Generic tunnel (voronoi tesselation)
Realistic tunnel in bedded strata
Wedge Ejection

• Various experiments to examine shotcrete loading
  due to discontinuities using UDEC

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Objectives

• Derive magnitude of rock movements under a range
  of geotechnical conditions in SA mines
• Interpret movements applied to shotcrete
• Derive Ground reaction curves
• Assess effect of stress change on movement
• Assess effect of excavation size on movement
• Assess effect of bolting, shotcrete bond
  strength, etc.

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Generic Tunnel Model

• 2D model using UDEC
• Discontinuous rock mass created using a voronoi
  tesselation (0.2m block size)
• Simple properties based on UCS, GSI derived using
  Rocsciences Rocklab program.
• 3.5 x 3.5 tunnel

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Limitations of including support

• Need hundreds of models to cover support
  permutations!
• Generally in deep mines, support can supply
  sufficient pressure to prevent unravelling, but
  not to prevent failure or limit deformation prior
  to final unravelling
• Key factor is the deformation that shotcrete will
  undergo
• Adopt a Ground Reaction Curve approach

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What is a Ground Reaction Curve?
Support Pressure
Elastic response
Rock failure initiated
Unravelling
Tunnel wall deformation
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GRC model methodology

• Model tunnel excavated and initially internal
  rock is replaced with a high support pressure
• Pressure is incrementally reduced to zero
• Measure modelled wall deformation
• GRC is graph of pressure versus deformation

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Example of modelled GRC
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Range in rock mass cases
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Effect of excavation size
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Effect of support pressure on failure envelope
Excavation Size Depth of sidewall instability ( of width of excavation) Depth of sidewall instability ( of width of excavation) Depth of sidewall instability ( of width of excavation)
Excavation Size Support Pressure 1 kPa Support Pressure 10 kPa Support Pressure 100 kPa
3.5 m wide excavation 37 37 37
5 m wide excavation 32 32 32
7 m wide excavation 22 22 22
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Effect of stress change

• Stress change is the main inducer of deformation
  in mining
• How to account for stress change with GRC graphs?
• GRC graphs developed for static stress cases
• Is it reasonable to jump from one graph to the
  next?

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Effect of stress change
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GRC models versus explicit support
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Deformation in 2D and 3D

• How to relate GRCs from 2D models to point of
  installation of support relative to face?
• UDEC versus FLAC3D
• Simple tunnel model

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3D deformations (mm)
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3D deformations ()
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Conclusions

• Deformation applied to support is key
• GRC methodology adopted as best means to assess
  deformation applied to shotcrete
• Limited tendency for shotcrete to bulge between
  bolts
• Layer deflection smoothly distributed over tunnel
  height (except where slabs punch through)
• Consider bolt spacing as design slab size in
  assessing performance
• Consider total wall deflection/number of bolts as
  shotcrete panel deflection
• Permits design using yield line theory