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Sensitivities in rock mass properties A DEM insight

Cédric Lambert (*) & John Read (**) (*) University of Canterbury, New Zealand (**) CSIRO - Earth Science and Resources Engineering , Australia. Sensitivities in rock mass properties A DEM insight. Sensitivities of the properties.

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Sensitivities in rock mass properties A DEM insight

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  1. Cédric Lambert(*) & John Read(**) (*) University of Canterbury, New Zealand (**)CSIRO - Earth Science and Resources Engineering, Australia Sensitivities in rock mass propertiesA DEM insight

  2. Sensitivities of the properties • Application to a particular geotechnical domain of an Australian mine • Synthetic rock mass approach, based on real mine data • Sensitivity of rock mass properties: • To lithology • To joint size • To fracture frequency

  3. The synthetic rock mass (SRM) framework Generate a discrete rock mass specimen representative of field conditions Based on measurable parameters Joint fabric – 3D Discrete Fracture Network (DFN) Intact rock - DEM SRM - Bonded particle assembly intersected with fractures

  4. Structural modelling & DFN generation • Stochastic generation of a DFN model based on structural information • Available information: window mapping and core logging • Persistence and density estimated following method proposed by Mauldon (1998) & extended by Lyman (2003) • Trace length distribution determines the persistence parameters (or diameter distribution) • Number of traces or spacing controls the joint density

  5. Intact rock representation • Intact rock modelled with PFC3D • Calibration of microproperties on 2m x 2m x 4m specimens (UCS & elastic properties) • Resolution of 4 particles across the specimen • Actual geology had to be simplified: • 3 dominant lithologies considered • Calibration for each lithology • Randomly distributed with same relative proportion • Multi rock model tested under unconfined compression conditions

  6. Synthetic rock mass specimen 24m rock mass specimen Intact rock DFN smooth-joint contact model ( PFC3D v4.0 manual)

  7. Unconfined compression test on 24m rock mass specimens UCS test performed in N-S direction • Characterisation of the stress strain behaviour of the rock mass • Extract rock mass mechanical properties (i.e. UCS, Young’s modulus)

  8. Sensitivities of the rock mass properties • Mine management wanted to know if the methodology could be applied to their mining environment • Simplified intact rock representation  Sensitivity to lithology ? • No censoring information available on trace length distribution  Accuracy of the size distribution ?  Sensitivity to joint size ?

  9. Sensitivity to lithology Average compression strength Average deformation modulus • Compression tests have been performed on specimens considering a homogeneous lithology • Same 24m DFNs have been used for each lithology

  10. Sensitivity to joint size • 24m rock mass specimen have been generated applying a multiplication factor s to the size of the joints of the reference sample (0.5 0.7 0.9 1.0 1.1 1.2 1.5) • Same orientation, same position and same spatial density • Cover a wide range of interlocking s=0.5 s=0.7 s=0.9 s=1.0 s=1.1 s=1.2 s=1.5

  11. Sensitivity to joint size • Unconfined compression test in N-S direction Intact rock UCS 112.6 MPa Bilinear relationship  two distinct mechanisms ? s ≤ 1.1 : more than 30% of the rock mass is continuous  failure involves brittle failure through intact rock s > 1.1: towards blocky rock mass  controlled by interlocking? 1 1 2 2

  12. from Ramamurthy et al. (2001) Sensitivity to fracture frequency • Fracture frequency (in loading direction) varies with joint size and joint spacing (or density) • Specimens have been tested varying the spacing keeping other properties of the DFN identical Fracture frequency is control parameter in the direction of loading  fracture intensity probably more suitable parameter

  13. Conclusion • Sensitivity analysis exhibited a linear relationship between intact rock properties and rock mass properties • Uncertainty could easily be extrapolated from intact rock to rock mass • Random distribution of lithologies is certainly not a realistic representation of the intact rock condition • Recent work shows that spatial variation of properties in slopes reduces factor of safety (Jefferies et al. 2008) • Length of spatial variation is critical • Enhanced the significant variation of strength with fracture intensity • Highlighted the importance of getting and collecting structural data right!

  14. Corresponding author : cedric.lambert@canterbury.ac.nz Acknowledgement to the sponsors of the Large Open Pit (LOP) project (www.lop.csiro.au) Thank you for your attention

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