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  • Multi-scale simulation of ground support designs for extreme tunnel closure
    D. Beck, S. Kassbohm, G. Putzar, Beck Arndt Engineering

    The numerical simulation of ground support requires that the ground deformation and support response are both captured accurately. The mechanisms of rock mass damage, dilation and deterioration must first be simulated sufficiently to produce realistic tunnel deformation in 3-dimensions, then the physical response of the support elements must be realistic if the support and loading systems will come to a realistic equilibrium.

  • A numerical investigation of scale effects on the behavior of discontinuous rock
    (Request a copy of this paper)
    D. Beck, F. Reusch, S. Arndt, Beck Arndt Engineering

In numerical simulations of mining induced problems, it is not possible to represent discontinuities at all length scales. However, all length scales are coupled, so the homogenisation process should be very rigorous. To investigate some homogenisation concepts for mine problems, the load-deformation response of some discontinuous rock masses at an example mine have been simulated using explicit Finite Element models. The intent of the analysis was to investigate the effects of specimen size and confining stress on strength, dilation and comminution. The analysis is used as a basis for a discussion of some preliminary ideas and concepts for deciding what length scale of structures need to be included in numerical models of mining deformation.

  • Estimating rock mass properties and seismic response using higher order, discontinuous, Finite Element models
    D.A. Beck, M. J. Pfitzner, S.M. Arndt and B. Fillery
    Beck Arndt Engineering Pty Ltd, Sydney, Australia

    ABSTRACT: The load-deformation response ofdiscontinuous rock under static and dynamic loading con-ditions has been simulated using Explicit, Finite Element models. The intent of the analysis was to inves-tigate the effects of specimen size and confining stress on strength, dilation and comminution. The simulations allow the development of homogenised constitutive material properties for disconti-nuous rock masses using laboratory scale measurements and representative Discrete Fracture Networks (DFNs) A procedure for this is presented which includes a comparison of measured seismic response in a mine to the Dissipated Plastic Energy (DPE) that is released in the simulated rock masses. The models also show how confinement and scale affect the stress-strain and DPE response of the simulated rock specimens, reproducing a number of known rock phenomena that are often poorly captured in geo-technical modelling. A case study is presented showing a sufficient match between the model-derived, homogenised materi-al properties and values achieved by calibration of a mine-scale model where many thousands of seis-mic, displacement and damage measurements were available.
     
  • Quantitative forecasting of sidewall stability and dilution in Sub-level
    Caves
    F.Reuscha, D. Becka, D. Tylerb
    a Beck Arndt Engineering
    b BHP Billiton Diamonds and Specialty Products, Canada

At Panda Mine in Canada’s Northwest Territories, an innovative Sub-Level Caving technique has been employed that’s takes advantage of the ground conditions and the geometry and size of the Kimberlite Pipe. The technique has worked well but a residual exposure was to sloughing of waste material from the overlying completed open pit and the exposed walls of the SLC.
A process of detailed back analysis of stability and damage in the pit, forward analysis of damage and displacements due to caving and probabilistic analysis was used to simulate the likelihood of waste ingress on a range of scales. The use of the Alternate Point Estimate Method (APEM) on a mine scale to estimate the probability of instability, the likely volume of waste ingress and sloughing and induced surface deformation is described in detail and compared to measured results during mining.

Owing to software and hardware limitations, sub-modelling approaches are frequently employed to analyse complex problems; a larger donor model will be used to establish boundary conditions for a smaller more detailed model. This approach is a valid technique, but employed poorly can lead to very significant problems.
As computer power and software efficiency improve, it is becoming possible to represent a very wide range of length scales with adequate precision in a single model, simplifying the process of multiple length scale simulation. There are now examples of mine models with a precision adequate to represent material behavior from a drive to a mine scale in a single, multiple step simulation.
On each length scale, these simulations are not necessarily more complex than historic modelling, but there are fewer assumptions as there are now fewer boundaries between scales at which strain, stress and state data must be simplified. This opens up opportunities for better analysis, and it is possible that certain phenomena that were previously difficult to capture can now be simulated. With better representation of multiple length scales, it also appears that there is a justification for increased geometric and constitutive complexity, and this will impact on approaches to data collection and characterisation. The teachings from multiscale analysis will also aid better sub-modelling.
Opportunities arising from multi-scale non-linear modelling and improved sub modelling approaches are discussed and case studies for underground and open pit mines and some tests of sufficiency for multi-scale modelling are proposed.

A number of large and underground mines intend to commence a new level of underlying block caving toward the end of the existing operations. Some of these transition projects are amongst the highest value underground mining projects ever undertaken. The interaction between the developing cave and the existing operation during cave propagation, breakthrough and draw down need to be simulated so that the transition can be properly planned, and so that the risks and effects of the new block caves can be properly
appreciated.
At a number of mines, the interaction between new caves and overlying operations has been investigated using detailed three dimensional numerical models. A number of frequently observed phenomena have been recreated, and the causes and factors that influence them can be demonstrated. Some observations from these simulations of caving milestones are discussed, as well as some implications for the monitoring of caving operations.

Detailed, mine-scale non-linear numerical models have been calibrated using observations of displacement and damage at three deep mines. The calibrated models were then used to investigate the correlation between Dissipated Plastic Energy (DPE) and seismic event probability. A strong, non-linear relation between DPE and event probability was observed that describes the development, peak, and decline in seismicity as rock is deformed. The correlations are present for events at all magnitudes, and the where sufficient data was available for a comparison, a relation between DPE and event probability consistent with the relation between event magnitude and frequency described by the Gutenberg Richter equation was found.
The application of DPE analysis during the design and sequencing of mines will allow a more quantitative description of seismic hazards than is currently available.

Useful numerical simulation of shafts, crushers and other infrastructure excavations is amongst the most challenging of rock mechanics problems. The mechanical installations and concrete forms are usually designed with tolerances to deformation that are very low by mining standards and the effects of out of specification ground movements can be very costly. The high costs of problems in these excavation sets a very high standard of analysis for the rock mechanics engineer, yet modelling of these items is often treated
as an after thought. In this paper, fitness-for-purpose requirements for analysis of critical infrastructure excavations are discussed in terms of the capabilities of modern, commercially available analysis packages and
computational capacities. Probabilistic assessments of excavation performance and direct simulation of excavation deformation is discussed in detail.

  • Resilient Design of Large Open Pit Slopes
    D. Becka, S. Arndtb, F. Reusch b, D.Tyler c
    a Beck Arndt Engineering, Sydney, Australia.
    b Beck Arndt Engineering, Perth, Australia.
    c BHP Billiton Diamonds Inc, Canada.

Although significant advances have been made in understanding the
mechanics of pit slopes, numerical modelling hasn’t significantly
impacted on the reliability of the design process. It is well known that
legacy modelling approaches don’t simulate large slopes well.
In recent times, the emergence of very large finite element (FE)models
with more than ten million degrees of freedom and the ability to simulate
a large number of discrete structures, has allowed a step change in the
simulation precision for slopes. The model size allows global and local
effects to be realistically simulated in one model for the first time and the
process can be better integrated into normal open pit slope engineering
processes.
Some examples of simulation of very large open pits are shown that
demonstrate the challenges and some recent advances in modelling large
slope behaviour. Significant limitations in the current measurement and
observation regimes of open pits have been identified.

  • Deep and High Stress Mining – Deformation and Seismicity
    S. Arndta, D. Beckb, F. Reuscha, I. Thinc, C. Stonec, M. Heapd & D. Tylere
    a Beck Arndt Engineering, Perth, Australia
    b Beck Arndt Engineering, Sydney, Australia
    c BHP Billiton Nickel West, Perth, Australia
    d Argyle Diamonds, Perth, Australia
    e BHP Billiton Diamonds & Speciality Products, Yellowknife, Canada

Abstract: Recent advances in mine scale simulation allow direct calibration of modelled displacements using measurements of surface subsidence, tunnel wall closure, shaft deformation and observed damage. The ability to correctly simulate these phenomena on a mine scale, in a single model constitutes a significant step change. Legacy geotechnical modelling packages have been unable to simulate the whole of mine deformation.
The non-linear analysis, using the Mohr-Coulomb material model and user subroutines in ABAQUS/Standard, follows the stress path of the mining sequence in detail. Starting with the geostatic equilibrium, often itself the result of previous simulation of the geological history, the simulation includes methodical changes such as the transition from open pit to underground mining and often exceeds one hundred analysis steps in a 3D finite element model containing a mix of complicated underground geometry and geology. An excellent correlation of visible rock mass damage categories with plastic strain and the calibration of large numbers of seismic events with Dissipated Plastic Energy contribute to the relevance of forecasting capabilities. The implementation of performance indicators for underground support systems shows a typical application for using Finite Element simulations to improve the mine planning process.

Abstract: The Largest, Deep Open Pits are steadily approaching depths of a kilometre. The complexity of the mining process and damage phenomena for these pits are amongst the most challenging applications of finite element simulation. Although significant advances have been made in the many areas related to mechanics of pit slopes, computer resources have limited the use of numerical modelling for open pit problems to two-dimensions or overly simplified 3d models.
With safety and productivity of future mega-pits depending on new design rules for pit slope stability, better tools are required.  In recent times, the emergence of very large models with up to ten million degrees of freedom  and advances in parallel solvers have allowed a step change in the simulation precision for slope movements and rock mass damage. The model size allows global and local effects to be realistically simulated in the one model with small excavation steps, more correctly re-creating the stress-deformation history in the slope. Some examples of calibration and forecasting of very large open pit (LOP) models are shown that demonstrate the challenges of modelling large slope behaviour.
Keywords: Slope failure, Large Open Pit, Slope stability, Faults, Geology, Rock Mechanics, Geotechnical.

  
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