sensitivities in rock mass properties a dem insight
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
Sensitivities in rock mass propertiesA 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 - DEMJoint 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
6m*6m*6m
Basalt
Mafic volcanicsMafic intrusives
6m*6m*6m
Basalt
Mafic volcanicsMafic intrusives
6m*6m*6m
Basalt
Mafic volcanicsMafic intrusives
6m*6m*6m
Basalt
Mafic volcanicsMafic intrusives
UCS [MPa] E [GPa]Lab. PFC3D Lab. PFC3D
Mafic intrusive 113.7 107.9 96.6 98.0Mafic volcanic 74.3 70 28.8 29.5Basalt 170 166.3 30 30Multi rock 112.6 70.9
• 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 strength Average 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 (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
Sensitivity to joint size
s=0.5 s=0.7 s=0.9 s=1.0
s=1.5s=1.1 s=1.2
• Unconfined compression test in N-S direction
Sensitivity to joint size
Intact rock UCS112.6 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
2
1
2
• 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)
fcicj Jexp
n) (r / J J nf
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 : [email protected]
Acknowledgement to the sponsors of the Large Open Pit (LOP) project (www.lop.csiro.au)