Integrated Modelling of Subsidence Mechanisms and Impacts Due to Mine Caving

Davide Elmo

1

O’Connor

3

C., Vyazmensky Eberhardt

2 1

E., Scoble

2

A., Stead

1

D., Dunbar M., and Moss

4

, A.

2

S.,

1

Simon Fraser University, Vancouver.

2

University of British Columbia, Vancouver.

3

Itasca Consulting Canada Inc.

4

Rio Tinto plc

Project framework

• Comprehensive numerical modelling study focused on block caving related surface subsidence - Large collaborative research initiative between the University of British Columbia (UBC) and Simon Fraser University (SFU), funded by Diavik Diamond Mines, Rio Tinto and NSERC.

• Numerical modelling of brittle fracture behaviour above the caved zone to further investigate the linkages between the processes involved in cave initiation, propagation and induced subsidence.

• Integrated approach: rock mass characterisation, surface and subsurface monitoring will be used as a fundamental constraint for numerical modelling.

• Numerical modelling will be used to analyse important issues related to crown pillar/pit walls stability where a transition from open-pit to underground operations is undertaken.

• State-of-the-art continuum and discontinuum numerical codes will be used to simulate these processes.

Project workflow

Block cave mining

Block cave mining is characterized by caving and extraction of a massive volume of rock which potentially translates into the formation of a surface depression whose morphology depends on mining draw rates, rock mass structure and properties, and surface topography.

Typical Block Caving illustration (Brady and Brown, 2004) Conceptual representation of surface subsidence associated with block caving mining and related subsidence characterization terminology (after van As, 2003).

Analysis of subsidence associated with block caving

Current approaches to assessing surface subsidence associated with block caving mining includes empirical, analytical and numerical methods: • Laubscher’s method (Laubscher, 2000). This empirical approach is based on a design chart that relates the predicted cave angle to the MRMR (Mining Rock Mass Rating), density of the caved rock, height of the caved rock and mine geometry (minimum and maximum span of a footprint).

• Analytical methods include limit equilibrium solutions for specific failure mechanisms (e.g. progressive sub-level caving of an inclined orebody).

• Different modelling approaches exist, based on the concept that the deformation of a rock mass subjected to applied external loads can be considered as being either continuous or discontinuous. The main differences between the various analysis techniques lie in the modelling of the fractured rock mass and its subsequent deformation.

Modelling of subsidence associated with block caving

Modelling strategy: • Importance of rock structures (e.g. joints, faults, etc).

• Representation of rock structures within conceptual and case-study models.

• Importance of intact rock bridges and step-path fracturing in the context of rock fragmentation and caving development.

• Definition of rock mass discontinuum approaches.

and properties continuum for modelling Rock Structure Intact Rock Bridges Rock Mass Properties Modelling Level of Detail “

we build models because the real world is too complex for our understanding; it does not help if we build models that are also too complex

” (Starfield and Cundall, 1988).

Run Times

Characterisation of fracture networks using a DFN approach

Discrete Fracture Network (DFN) approach is an ideal numerical tool with which to synthesise realistic fracture network models from digitally and conventionally mapped data.

• Thanks to the collaboration of Golder Associates, the DFN code FracMan will be used as part of the current project to quantify the effects of discontinuity persistence and intact rock bridges on caving mechanisms.

• In addition to using block theory analysis, the code FracMan will be also used as part of a study on primary fragmentation and characterisation of block shape/volume above the undercut level.

• Rogers et al.

(2007) recent work on characterisation of in-situ fragmentation for block and panel caving using a DFN approach.

(Elmo, 2007)

Rock mass properties

Synthetic Rock Mass Properties

, combining the response of the intact rock and the existing fractures (Discontinuum modelling approach).

What material properties should be used for the characterisation of the rock bridges in between the pre-defined fractures in a discrete model?

• In an idealized model, where all fractures can be simulated, these properties are equivalent to the intact rock material properties. The mesh refinement needed to represent such geometrical domain requires extensive CPU times.

• Since not all fractures can be directly represented in the model, an equivalent continuum approach can be used to account for the rock mass strength reducing effect of those fractures not explicitly included.

• The scope of the modelling approach is ultimately to define which processes can be modelled explicitly and which can be represented in an “averaged” way.

Rock mass properties

Larger Problem Discretisation / Less Details Idealized Discrete Model Smaller Problem Discretisation / More Details Simplified Discrete Model Continuum Model Simulation of step-path and intact rock bridge failure Randomly oriented ubiquitous joint planes could be distributed in the model to account for the effects of jointing Problem Scale

Rock mass properties

A combined Discrete/DFN modelling approach can be used to derive equivalent continuum properties.

Limitations of 2D approach. Ideally the analysis should be carried out in 3D.

! Structurally Controlled Response !

Elmo, PhD Thesis (2006)

Numerical examples of block caving subsidence using

FLAC 3D

The caving process, and the subsequent subsidence that can result, is in part controlled by the jointing in the rock mass. Although the rock mass may behave as a continuum at a large scale, the behaviour of that continuum is controlled by discontinuities in the rock mass at smaller scales. FLAC3D, being a continuum code, cannot explicitly model discontinuous behaviour. However, the use of an Equivalent Rock Mass Model (ERM) allows the incorporation of the effects of jointing.

3D reconstruction of the San Manuel mine and subsidence crater up to 1972 (Left). Displacement contours on a long section through the orebody after mining of the first 9 panels along with a 3D iso-surface of displacement magnitude highlighting the location of initial breakthrough (Right).

C. O’Connor, Itasca Canada

Modelling of block cave mining using a hybrid finite/discrete approach

Preliminary models of block caving have been undertaken at SFU using a hybrid finite/discrete numerical approach (ELFEN code). Failure of the intact rock material can be modelled through fracturing of the initial continuum meshed geometry. Accordingly, caving propagation can be simulated as a result of both failure of the intact rock material and displacement/rotation along pre-defined fracture planes.

• Currently, only 2D models have been tested.

• The DFN code FracMan is used to generate an initial fracture network.

• Effects of varying fracture parameters (e.g. orientation, spacing and persistence).

• Max. number of finite elements controls the discretisation process (mesh size), hence for large scale problems it requires the use of Equivalent

Fractured

Rock Properties.

Preliminary results of block caving subsidence using a hybrid finite/discrete approach

Effects of varying fracture orientation Displacement (m)

Transition from open pit to underground operations by block caving

With many large open-pit mines planning to extend their operations underground, it is increasingly important to understand the rock deformation mechanisms associated with the transition from surface to underground and the potential interaction between open-pit and underground mine operations.

Caving propagation will lead to failure of the surface crown pillar. The question to be answered is whether the connection to surface of the cavity generated by the caving will trigger slope instabilities on the pit walls due to induced deformation and/or failure of the crown pillar between the pit floor and the cave back.

Modelling of Block Caving

Large Scale Problem

Modelling of Slope Stability

Preliminary numerical analysis of a transition from open pit to underground operations by block caving

Two joint sets inclined at 10 and 80 degrees respectively Displacement xx (m) No underground mining Block caving following the open pit excavation Two joint sets inclined at 30 and 60 degrees respectively No underground mining Block Caving following the open pit excavation

Conclusions

• Main objective is to explore and develop the ability of advanced numerical modelling methods to characterize complex rock mass behaviour above block cave mines. The use of state-of-the-art continuum and hybrid finite/discrete numerical techniques forms a fundamental part of the ongoing research.

• Early numerical tests have shown the effectiveness of an Equivalent Rock Mass Model (ERM) approach coupled with an advanced 3D continuum modelling in simulating block caving.

• Although currently limited to 2D analysis, preliminary results also suggest that the use of a hybrid finite/discrete approach represents a powerful tool for characterising caving initiation and the subsequent progressive fracturing of the rock mass above the undercut. The approach can be extended to simulate the interaction between surface and underground operations.

• Further studies will investigate the use of specific fracture intensity parameters as meaningful tools to derive rock mass properties for the corresponding numerical modelling of block caving and open pit/underground mining interactions.

Acknowledgments

• Allan Moss and Andre van As, Rio Tinto London • Steve Rogers, Golder Associates Vancouver • Rockfield Software, Swansea UK • NSERC, Natural Sciences and Engineering Research Council of Canada • Roderick Tollenaar, MSc candidate UBC