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

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 Sensitivities of the properties

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

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 Structural modelling & DFN generation

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 Intact rock representation UCS [MPa]E [GPa] Lab.PFC3DLab.PFC3D Mafic intrusive Mafic volcanic Basalt Multi rock 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

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

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

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 ? Sensitivities of the rock mass properties

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

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

Unconfined compression test in N-S direction Sensitivity to joint size Intact rock UCS 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?

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 Sensitivity to fracture frequency from Ramamurthy et al. (2001) Fracture frequency is control parameter in the direction of loading  fracture intensity probably more suitable parameter

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! Conclusion

Thank you for your attention Corresponding author : Acknowledgement to the sponsors of the Large Open Pit (LOP) project (