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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). Introduction. Simrac Project SIM 040204 Numerical modelling to investigate: Shotcrete/rockmass interaction

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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)

introduction
Introduction
  • Simrac Project SIM 040204

Numerical modelling to investigate:

  • Shotcrete/rockmass interaction
  • Load/deformation performance requirements of shotcrete under a range of geotechnical conditions
modelling requirements
Modellingrequirements
  • Modelled rock mass needs to fragment
  • Effect of discontinuities on lining – local loading
  • Identify deformations under various geotechnical conditions – rock type, GSI, field stress
model design laboratories
Model design – ‘laboratories’

Various experiments to examine shotcrete loading due to discontinuities using UDEC

Generic tunnel (voronoitesselation)

Realistic tunnel in bedded strata

Wedge Ejection

objectives
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.
generic tunnel model
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 Rocscience’s Rocklab program.
  • 3.5 x 3.5 tunnel
limitations of including support
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
what is a ground reaction curve
What is a Ground Reaction Curve?

Support Pressure

Elastic response

Rock failure initiated

Unravelling

Tunnel wall deformation

grc model methodology
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
effect of stress change
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?
deformation in 2d and 3d
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
conclusions
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