___________________________________________________________________________
Utilization of Morrison Mine Experience in Management
and Mitigation of Micro-Seismic Risk at the Design
Stage of the Victoria Project
Mateusz Jakubowski1), Siavash Taghipoor2), Julian M. Watson3) 1)
KGHM International Ltd, Victoria Project, Sudbury, Canada, Mateusz.Jakubowski@kghm.com
2)
KGHM International Ltd, Morrison Mine/Craig Mine, Onaping, Canada, Siavash.Taghipoor@kghm.com
3)
KGHM International Ltd., Blaine, USA, Julian.Watson@kghm.com
Abstract
Owned by KGHM International Ltd (KGHM), the Victoria Project is an undeveloped copper-nickel resource located in the Sudbury Igneous Complex. This region has been historically mined for over 110 years for its sulphide nickel, copper, cobalt, and precious metal ores. Due to the anticipated high stress environment, stiff rock mass and significant depth of the resource (>1800 m below surface), severe mining-induced micro-seismicity, strain bursting and stress induced damage is expected. In order to manage hazard and mitigate the associated risk to personnel and production associated with this mining in this environment, appropriate geomechanics strategies will applied at the design stage of the Victoria Project (including micro-seismic monitoring, dynamic ground control systems, re-entry protocols, production extraction sequencing, and strategic location of LOM and development excavations). This paper describes the expected benefit to the Victoria Project by applying lessons learned from comparable mines currently operating within the Sudbury Basin, including KGHM’s Morrison Mine which seismically-active and developing strategies to manage micro-seismic risk.
Key words: micro-seismicity, ground control, mine design, hard rock environment, mining at
depth
Wykorzystanie doświadczeń Kopalni Morrison
w zarządzaniu i minimalizowaniu zagrożenia mikrosejsmicznego
na etapie projektowania Kopalni Victoria
Streszczenie
Projekt Victoria, będący własnością spółki KGHM, jest to aktywo górnicze rozwijane na bazie złoża miedziowo-niklowego wykształconego w obrębie kompleksu skał magmowych Sudbury. Region ten jest przedmiotem historycznej eksploatacji od ponad 110 lat z uwagi na bogate mineralizacje zawierające nikiel, miedź, kobalt oraz metale szlachetne. Z powodu oczekiwanych wysokich stanów naprężeniowych, mocnych skał oraz znacznych głębokości zalegania zasobów (przekraczającej 1800 m), w przypadku Projektu Victoria należy się spodziewać występowania indukowanych zjawisk sejsmicznych, odprężeń oraz zniszczenia górotworu. W celu zarządzania ryzykiem, a także minimalizacji jego wpływu na bezpieczeństwo pracującej załogi oraz osiągane wyniki produkcyjne, odpowiednie metody monitoringu i prewencji muszą być przewidziane już na etapie projektowania przyszłej kopalni (włączając w to monitoring aktywności sejsmicznej górotworu, obudowę podatną, protokoły dotyczące czasu wyczekiwania, właściwą sekwencję wybierania złoża oraz
właściwą lokalizację wyrobisk o znaczeniu kapitalnym). W niniejszym artykule opisano szereg spodziewanych korzyści płynących z wykorzystania doświadczenia innych kopalń prowadzących eksploatację w Zagłębiu Sudbury, włączając w to należącą do KGHM kopalnię Morrison.
Słowa kluczowe: mikrosejsmiczność, obudowa wyrobisk, projektowanie kopalń, skały trudnourabialne, głębokie kopalnie
Introduction
Owned by KGHM International Ltd (KGHM), the Victoria Project and Morrison Mine are located in the Sudbury Igneous Complex (see Fig. 1), a region which has been historically mined for over 110 years for its sulphide nickel, copper, cobalt, and precious metal ores. Most ore bodies are located at depth, and so are or have been extracted using underground mining methods.
Due to the steep in-site stress gradient, stiff rock mass and significant mining depths, mining-induced micro-seismicity is a common hazard faced at many operations located within the Sudbury Igneous Complex. Manifesting as strain/pillar bursting and stress induced damage, managing the risk associated with micro-seismicity is typically achieved through a combination of approaches.
Fig. 1. Location of the Victoria Project (40 km Southwest of Sudbury, Near the Denison Township) and Morrison Deposit (40 km West of Sudbury, Near the Onaping-Levack
Township) within The Sudbury Igneous Complex
The Morrison Mine is located on the western border of the richly-mineralised North Range of the Sudbury Igneous Complex. Extraction of the orebody uses selective mining techniques and two production shafts. The Levack mine, which mainly produced nickel, commenced its operations in 1915 and was closed in 1997. KGHM acquired the operation in March 2002 and renewed production five years later. Exploration was undertaken which resulted in the discovery of a high-grade copper, nickel and precious metals veins system.
KGHM acquired rights to the Victoria Project in 2002 and initiated an exploration program in this area. Approximately 14.5 million ore tonnes of inferred resources are documented, including high-grade copper, nickel and precious metals. The current scenario assumes that the project will be carried out in two stages; firstly sinking of an exploration shaft to enable advanced exploration to confirm and document the resource into the appropriate category; followed by design, construction, and operation of an underground mine.
1. Ground Conditions: Morrison Mine and The Victoria Project 1.1. Rock Mass Characterisation
The ground conditions at Morrison Mine are directly comparable to that found at the Victoria Project; characterised by a strong, stiff rock mass, which is typical of the Sudbury Igneous Complex. Strength and elastic property testing are shown in Table 1. Based on data collected from both sites, GSI estimates are typically >65 for all lithologies.
Table 1. Summary of Rock Property Test Results from Morrison Mine and the Victoria Project
Location Lithology Density (g/cm3) Young’s Modulus (GPa) Poisson Ratio UCS (MPa) Tensile Strength (MPa) Morrison Mine Granodiorite Gneiss 2.7 45.5 0.10 225.6 17.8 Mafic Gneiss 3.0 45.2 0.11 144.5 22.9 Meta Gabbro 3.0 47.9 0.09 182.0 28.6 Sudbury Breccia 2.8 44.1 0.12 294.0 18.5 Victoria Project Diabase Dyke 3.0 46.4 0.10 110.3 18.2 Quartz Diabase 3.0 54.9 0.10 239.3 26.3 Meta Basalt 2.7 48.3 0.10 123.5 7.8 Meta Gabbro 3.1 44.3 0.11 156.7 - Metcrystal Gabbro 3.1 56.6 0.10 191.6 18.5 Meta Sediments 2.7 33.1 0.12 112.6 12.0 Quartz 2.8 62.9 0.17 160.7 9.3 Rhyolite 2.5 55.2 0.13 165.5 12.7
1.2. In-Situ Stress Conditions
No in-situ stress tests have been completed for the Morrison Mine. However, field observations by KGHM’s geotechnical engineering group and the consultant [9] suggest that the major principal stress at Morrison is horizontal and locally trending parallel to a major geological structure, the F-Fault (striking NNW to SSE). These field observations had been verified through calibrated 3D inelastic numerical models run by KGHM’s geotechnical engineering group and the consultant (see Table 2). It should be noted that this compares well with other published regional stress state estimates [12, 2, 3].
While in-situ mini-frac stress estimates have been undertaken for the Victoria Project, the reliability of the results is unclear. Therefore, the far-field stress boundary conditions to date have been based on published data by Trifu & Suorineni [12], (Table 2). In order to confirm these assumptions, a suite of in-situ testing is planned when deeper positions in the rock mass can be accessed. As can be seen in Table 2 and Fig. 2, the estimated far-field stress conditions and gradients for the Morrison Mine and Victoria Project are comparable.
Table 2. Estimated Far-Field Stress Conditions Applied at the Morrison Mine and Victoria Project
Site Stress Magnitude (MPa)(1) Trend (°) Plunge (°) Comments Morrison Mine σ1 (0.026 1.8) Z 155 00
Derived from 3D numerical modeling back analysis
σ2 (0.026 1.4) Z 065 00
σ3 0.026 Z 090 90
Victoria Project
σ1 10.9 + 0.0407 Z 270 00
After Trifu & Suorineni [12]
σ2 8.7 + 0.0326 Z 000 00
σ3 0.029 Z 090 90
(1)
Where Z = meters below surface
2. Mining-Induced Micro-Seismicity
Mining-induced seismicity is directly associated with the interaction of mine excavations and the local rock mass (including geological structures) with regional and local stress fields. Seismic energy is released when the local stress conditions approach and / or exceed the shear strength or frictional capacity of the rock mass or pre-existing geological structures respectively. The resulting micro-seismic event manifests as dynamic displacement in the excavation boundary, including shaking / loosening of previously damaged rock or, in extreme circumstances, violent ejection of rock fragments into the excavation (i.e. rockburst). Both occurrences can adversely affect the safety of mine personnel and mine productivity.
As mining continues to greater depths in the future, the adverse effects of micro-seismic events on mining personnel and productivity is likely to have substantial economic implications. Therefore, the application and refinement of risk mitigation measures applied at other mining operations in the Sudbury Igneous Complex, such as KGHM’s Morison Mine will be critical when designing the Victoria Project.
2.1. Micro-Seismic Hazard: Risk Mitigation at Morrison Mine
Micro-seismic hazard and associated risk is managed at Morrison Mine through the application of several well-established mitigation techniques. These have been refined for adaption to local conditions, and include a combination of responsive and reactive approaches:
Extraction sequencing and mine design
Micro-seismic array and displacement instrumentation monitoring systems Dynamic ground reinforcement systems
Re-entry protocols (exclusion periods and zones).
2.2. Extraction Sequencing and Mine Design
The mine design process at Morison Mine utilises the application of 3D numerical modeling by the site-based rock mechanics engineer who has advanced skills and knowledge with this approach. This process enables the identification of areas in the mine design which may induce adverse localised stress conditions and micro-seismicity. In such areas, the mine design is modified and / or appropriate ground support is installed in these areas (discussed later) to control the hazard and minimise risk and costs.
Numerical modeling is performed using FLAC3D (Fast Lagrangian Analysis of
Continua) software [5]. The application of a suitable constitutive model is critical
when assessing high stress conditions [1, 13, 14]. Consequently, an elastoplastic strain-softening constitutive model has been developed and is applied. This model uses rock mass cohesion as a proxy for micro-seismic activity, by assuming that a rapid drop in cohesion drop after the peak strength is reached is associated with energy release and event generation (see Fig. 3). This approach has been successfully applied at Morrison Mine to assess likely levels of micro-seismic activity and risk. This has led to the development of seismic risk maps for active mining areas, which indicate expected regions of low, moderate, high, and severe risk conditions which dictates the type and extent of ground reinforcement to be installed (see Fig. 3).
Fig. 3. Generalised Image of Strain Softening Behavior [5]
2.3. Micro-Seismicity Monitoring
A micro-seismic monitoring array system was installed at Morrison Mine shortly after micro-seismic activity commenced in late 2013. The array features a combination of uniaxial accelerometers, triaxial geophones, and 15 Hz geophones installed underground, with one Strong Ground Motion (SGM sensor) located on the surface. The relatively high frequency monitoring range of the accelerometers installed at Morrison Mine limits them to accurately measuring events with magnitudes less than -0.5 Moment Magnitude (Mw), while the SGM sensor can only measure events greater than 0.5 Mw. Consequently, additional 15 Hz triaxial sensors were installed to cover the frequency ‘gap’ between the accelerometers and the SGM sensor (see Fig. 5).
2.4. Dynamic Ground Reinforcement Systems
Due to the micro-seismic activity at Morison Mine, a dynamic ground reinforcement system has been developed to withstand micro-seismic loading conditions. The principal element in this system is the Swellex rock bolt. Introduced by Atlas Copco in the 1980’s the Swellex bolt and is made from a folded thin-wall steel tube. Bushings are pressed onto both ends of the bolt, which are sealed by welding. The lower bushing has a small hole through which water is injected into the bolt at high pressure to expand the steel tube. During the expansion process, the Swellex bolt compresses the rock surrounding the hole and adapts its shape to fit the irregularities of the borehole (see Fig. 6). Two variation of the Swellex bolt are used at Morrison Mine; standard and Super Swellex. Standard Swellex rock bolts are 2.1 m long and have capacity of 16 tonnes (157 kN), while Super Swellex rock bolts are considerably longer from 2.4 m to 4.8 m with an increased capacity of 215 kN.
Fig. 4. A Typical Seismic Risk Map at Morrison Mine
Fig. 5. The Application of Different Sensor Types in Measuring Magnitude Ranges ESG Solutions, 2013
A dynamic reinforcement system was designed to withstand loads associated with a peak particle velocity (PPV) of 0.4 m/s. This PPV can be induced by 2.5 Mw and 2 Mw micro-seismic events located at a distance from an excavation boundary of 13 m and 3 m respectively. The dynamic reinforcement system is installed in areas that are assessed as being at ‘high’ and ‘sever’ levels of micro-seismic. It is also applied in areas which have a history of large micro-seismic event(s). Steel mesh straps are added to the dynamic reinforcement system in areas which are deemed prone to pillar burst. These straps are 2.1 m long and 45 cm wide, manufactured from 7.6 mm diameter wire, welded into 10 10 cm squares.
Within this dynamic reinforcement system, two different classes of dynamic support are considered. Areas where pillar burst is expected feature Super Swellex rock bolts, while those exposed to strain burst apply standard Swellex rock bolts.
Fig. 6. Swelllex Bolt and the Interaction With Rock;
(A) The Swellex Bolt is Placed in the Drill Hole; (B) The Bolt is Expanded Under High Water Pressure; (C) The Water Pressure is Released and the Surrounding Rock Contracts,
Providing the Swellex Bolt ‘Locking’ Effect [7]
Overall, the dynamic reinforcing system has performed well at Morrison Mine. In cases where it has not (including dynamic rock burst and static falls of ground), it was found that the integrity of the installed system was compromised. For example, bolts and / or steel mesh straps were not correctly installed along the overlap of standard screen sheets. This was been addressed with the application of improved operator training. In other cases, coverage of the dynamic reinforcing system has been extended further down excavation walls. Both approaches have been successful in limiting damage associated with rock burst events.
2.5. Re-Entry Protocols (Exclusion Periods and Zones)
Considerable levels of micro-seismic activity are typically associated with the release of energy from production blasting and / or re-equilibration of the local stress field to the changed excavation void configuration. In order to mitigate risk associated with re-entry after blasting, protocols which limit access to specific zones for a minimum time period are applied until the background micro-seismic activity and magnitude decay to a ‘background level’. These periods may be extended if the decay process takes longer than expected.
Fig. 7. Seismic Risk Levels, Support Classes and Re-Entry Time [9]
A rate of 5 small events (Mw<-1.0) per hour has been proposed for the threshold for establishing re-entry procedures. It is typical for seismic frequency to decay to background levels very quickly after blasting (within a few hours), however there have been a few occasions where extensive delay periods have been observed. Figure 7 summarizes the anticipated seismicity, expected damage intensity, and exclusion time for each of the categorized micro-seismic risk levels, as well as the recommended support standards. This trigger action response plan (TARP) has been developed using site-specific data and is regularly review as the character of micro-seismicity at Morrison Mine changes.
In addition to the re-entry protocols, seismic work rate and seismic strain rate [8] in conjunction with clustering parameter have recently been trialed to clear large events and blasts and reduce stand-off time. This method has been successful in more than 95% of trials and enabled Morrison Mine to reduce exclusion times significantly.
Seismic work is the cumulative summation of the square root of the seismic moment.
=
Seismic moment is a measure of event strength, and is defined as: =
where is shear modulus at source, is average displacement of source and A is the source area.
Fig. 8. An example of a seismic work response curve [8]
When seismic work is plotted versus time reference to a blast or a large event, the slope of the graph represents seismic event generation. Non-linear slope is an indication of an unstable state induced by critical phenomena such as rockburst sequences. A linear slope represents a non-critical condition and returning to background. Figure 8 is an example of a seismic work response curve.
3. Application to the Victoria Project
As outlined above, the ground conditions (including far-field stress) of the Morrison Mine and Victoria Project are comparable. Active mining at Morrison Mine occurs between approximately 870 to 1,400 m below surface. With proposed mining depths at the Victoria Project expected to occur between 1,220 m and 1,830, it is reasonable to assume that micro-seismic risk will be at least as significant as that encountered at Morrison Mine. Therefore, it will be critical to incorporate risk mitigation measures at the earliest stages of design process.
3.1. Mitigation of Micro-Seismic Risk at the Design Stage
The Victoria Project is currently at the Feasibility Level of design. Consequently, several of the mitigation methods used at Morison Mine (such as re-entry protocols and dynamic reinforcement systems) will not become relevant at the Victoria Project until mining approaches depths where micro-seismicity becomes apparent. Nonetheless, risk mitigation measures associated with mine design and layout can be applied at the initial stages of the Victoria Project design process. Significantly,
based on the hierarchy of hazard controls (Table 3), design controls are the most effective tools available to manage micro-seismic hazard in underground mines.
Table 3. Hierarchy of Hazard Control [11]
Hierarchy of Controls Description
Most Effective Elimination Remove the hazard
Substitution Reorganise the task to reduce risk
Isolation Perform the task using remote mining methods
Engineering Controls Install guards or use monitoring
Administration Develop general operating procedures
Least Effective Personal Protective Equipment Provide minimum and additional equipment
3.2. Mine Design
The first step to mitigate micro-seismic risk to the Victoria Project mine design was to select a mining method, stope geometry, and mining sequence to reduce the adverse effects of expected micro-seismic activity. Based on the assumed orientation of the far-field stress tensors, the recommended orientation for the stope extraction is a longitudinal extraction as outlined in Fig. 9. It should be noted that while this layout is not optimal to maximize the deposit extraction rate (limits the access to the resource) it was deemed necessary to effectively manage the anticipated adverse effects of mining-induced micro-seismic risk.
An optimised stope extraction sequence is also critical to successful management of micro-seismic risk. Consequently, production stope extraction has been sequenced from the center out with a breakthrough of the sill pillar between mining horizons scheduled as soon as practical.
Fig. 9. Plan View of Extraction Orientation (Longitudinal Versus Transverse)
Elastoplastic 3D numerical models will be constructed to analyse the effects of orebody extraction on nearby excavations and infrastructure, including:
The layout of permanent underground excavations Stope extraction sequencing
The influence of major geological features:
Regional-scale Creighton Fault (located close to mining at depth) Small scale structures which transect the Victoria Project resource. Numerical simulations will also be used to identify areas that are likely to be exposed to micro-seismic activity and stress-induced damage as the resource is extracted over the LOM. As outlined above, the application of an appropriate constitutive model will be critical to correctly simulate time-dependent effects, including migration of stress (and micro-seismicity) as the rock mass responds to extraction of the resource. Based on these results, permanent infrastructure will be placed outside of the zone of mining-induced high stress and micro-seismicity activity. This is particularly important for vertical development (i.e. shafts and ventilation raises).
3.3. Micro-Seismic Monitoring Array
Several surface micro-seismic sensors have been installed nearby as a part of the Sudbury Regional Seismic Network (SRSN). This system will augment a site-specific micro-seismic monitoring array which is planned to be installed before shaft sinking progresses beyond a depth where micro-seismic response to mining is observed. It is expected that the initial micro-seismic monitoring array will be focused on the sinking of the production and ventilation shafts. However, the array will be extended as the mining front moves deeper and micro-seismic activity (i.e. event frequency and magnitude) increases.
3.4. Shaft Lining Design
At the early stages of the Victoria Project, primary consideration must be given to the shaft liner design. This design must be capable of withstanding significant changes to the local stress field which is expected as the deposit is extracted over the life of mine. Therefore, reliable forecasting of local stress conditions is required at various stages of the LOM to ensure that liner design performance at future stress and micro-seismic conditions is acceptable (i.e. exceeds Factor of Safety). This is most reliably achieved through preliminary numerical analysis. For example, preliminary numerical simulations have been carried for pattern bolting and concrete liner. Early results suggest that this approach will be effective in reducing failure around the shaft by providing confinement to rock at the excavation boundary (Fig. 10). Support capacity diagrams plotted in axial force versus shear force space and compared to the capacity envelopes indicates the FoS of the liner exceeds the design factor of safety.
Fig. 10. Total displacement contours and capability chart for rock bolt pattern and concrete liner
Conclusions
The effective management and mitigation of micro-seismic hazard will be critical for the safe and successful extraction of the Victoria Project resource. Many such mitigation measures are currently applied at the Morrison Mine, which has comparable ground conditions (including far-field stress) and is mining at similar depths to those proposed for the Victoria Project. Many of these mitigation approaches can be applied during design phase of the Victoria Project.
Given the current Feasibility Level of study at the Victoria Project, engineering controls are the most effective applicable at this time, including mine design and layout. The application of an appropriate constitutive model when using 3D numerical software will be critical to correctly simulate time-dependent effects, including migration of stress (and micro-seismicity) as the rock mass responds to extraction of the resource. This, together with the initial installation of a micro-seismic array during shaft sinking, will ensure that the associated hazards are effectively managed from the commencement of the mine life. Such proactive approaches will maximise safety, while optimising the resource.
Acknowledgments
The authors would like to thank KGHM International Ltd and KGHM Polska Miedź S.A. for allowing access to geotechnical data from the Morrison Mine and Victoria Project.
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