Introduction and Background Itasca Consulting Group, Inc. (Itasca) has been participating in the geomechanical design of the underground 118-Zone at the Capstone Minto Mine (Minto) in the Yukon, in northwestern Canada This presentation focuses on the large-scale 3DEC simulations conducted to assess the effects of persistent shallow-dipping joints on the post-pillars in the cut-and-fill stopes 2
Background 3DEC vs. FLAC3D Initial analyses of the 118 Zone were completed with FLAC3D After discussion with Minto staff, it was decided to build and run a 3DEC model of Zone 118 to: A. Assess the effects of the persistent shallow-dipping (30-60 ) joint set on the stability of the post-pillars B. Better capture the behaviour of the large faults, which are expected to behave more like fractures than weak zones (as they were modelled in FLAC3D) Zones are represented as tetrahedrals in 3DEC and hexahedrals in FLAC3D. This makes 3DEC better-suited to model the behaviour of discontinuities, and FLAC3D better at capturing plasticity and associated deformations within the rock mass As a result, the work described in this presentation focuses mainly on the effects of the faults and joints 3
3DEC Code Overview 3DEC simulates the response of discontinuous media that are subjected to static and/or dynamic loading The discontinuous media are modelled as assemblages of convex or concave polyhedra (tetrahedrals in this case) individual blocks may be rigid or deformable, and the discontinuities are treated as boundary conditions between these blocks. All blocks were zoned in this analysis. Motion along discontinuities are governed by linear and non-linear force-displacement relations for movements in both the normal and shear directions 4
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3DEC Model Overview Large faults included in the model 640 m (Isometric view) 6
3DEC Model Overview (Isometric view) Zone 118 as modelled in 3DEC the different colours represent the different mining steps 7
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Property Encasing Waste material Ore material Bulk modulus K (GPa) 26 26 0.025 - Shear modulus G (GPa) 16 15 0.015 - RMR 89 /GSI 74/69 76/71 - - Fill Faults and joints Peak friction angle (deg) 1 61.9 60.5 43 30 Peak cohesion (MPa) 1 3.24 2.82 0 - Peak tensile strength (kpa) 466 387 0 - Critical plastic strain interval (%) 2 0.92 0.86 - - Residual friction angle (deg) 43.3 42.4 - - Residual cohesion (kpa) 162 141 - - Residual tensile strength (kpa) 0 0-1 Established with a tangential fit at sig3max = 3.3 MPa (for a mining depth of 140 m max), and a linear failure envelope. This overestimates friction, but provides a conservative estimate of cohesion, which matters most in our case. 2 Strain-softening interval between peak and residual strength (the same for the cohesion and friction). Based on GSI. The tensile critical strain was set to 0.00001% to reflect the more brittle behaviour of tensile failure. 9
Joints and Large-Scale Faults The persistent steeply dipping joint set observed underground was randomly added in the post-pillars with the following rules (based on information gathered during an Itasca site visit): Dip: 30 to 60 * Dip Direction: 0 to 100 * Joint spacing: 1 to 10m * Joint diameter: 15m *All values were randomly assigned, based on uniform distributions Three faults also were included in the 3DEC model as planar features, based upon 3D information provided by Minto in dxf format 10
In-Situ Stress Field 11
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Mining Sequence Implementation The cut-and-fill stopes were excavated with the following approach: 1. Run the model to initial equilibrium before removing any material 2. Excavate rooms level by level starting from the bottom of Zone 118. All the rooms on a given level are excavated and then backfilled before excavating the next level above. 13
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Results Presentation The next slides show plasticity states, displacements and 1 stress magnitudes on two cross-sections The first cross-section was selected because it was showing signs of instability in the FLAC3D model due to interactions between the stopes and one of the major faults The second cross-section was selected because it goes through some of the taller pillars and is located approximately in the middle of the mine 16
Plasticity Results X-Section 1 Tensile failure in the past in all pillars and the roof of some stopes (Cross-section looking east) (Cross-section location on plan view) 17
Displacement Results X-Section 1 Little displacement around the rooms (< 1cm) (Scale in metres) (Cross-section looking east) (Cross-section location on plan view) 18
1 Results X-Section 1 (Scale in Pa, with a negative compression convention) Lower σ 1 magnitudes in the roof. This could result in structural instability. (Cross-section looking east) (Cross-section location on plan view) 19
Plasticity Results X-Section 2 The plasticity plot shows tensile failure in the past for all the pillars and the roof of some rooms (Cross-section looking east) (Cross-section location on plan view) 20
Displacement Results X-Section 2 Low displacement values around the excavations (again < 1cm) (Scale in metres) (Cross-section looking east) (Cross-section location on plan view) 21
1 Results X-Section 2 This plot shows 1 stress concentrations in the post-pillars above 15 MPa (the highest values are only around 22 MPa) (Scale in Pa, with a negative compression convention) (Cross-section looking east) (Cross-section location on plan view) 22
Conclusions of These Results The first cross-section was showing signs of instability in the FLAC3D model due to intersections with one of the major faults. It does not show the same level of instability in the 3DEC model because the fault is now represented as a planar feature instead of a weak zone with a thickness. Although some shear is predicted along the fault by the 3DEC results, the associated displacements are small. The second selected cross-section does not show signs of significant instability in the 3DEC model 23
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Plasticity Results in Post-Pillars Shear now (in purple) (Isometric view) 25
Plasticity Results in Post-Pillars The plasticity plot on the previous slide shows tension past in all the post-pillars, as well as some tension now in the upper portion of some pillars Some shear now also can be observed towards the top of some pillars, on their north side As expected, because tetrahedral zones are used in 3DEC, this model shows less plasticity than was indicated by FLAC3D 26
Cohesion Loss in Post-Pillars (Scale in Pa) Peak cohesion was set at 2.82 MPa for the ore material Red means still at peak cohesion (Isometric view) 27
Cohesion Loss in Post-Pillars The plot on the previous slide shows cohesion loss in the post-pillars as a result of yielding Cohesion remains very close to peak value (set at 2.82 MPa for the ore) in most of the pillars Many pillars very locally undergo enough failure to reduce their cohesion down to about 2 MPa this represents a loss of about 30% from peak strength in these regions 28
1 Magnitude Results in Post-Pillars (Scale in Pa, with a negative compression convention) (Isometric view) 29
1 Magnitude Results in Post-Pillars The 1 magnitude plot on the previous slide shows elevated values towards the top of the post-pillars, on their north side This is where the shear now plasticity indicators were located in the plot of Slide 25 Some pillars on the north side of the zone also show elevated stress levels through their core 30
Shear Displacement Results (On Joints) (Scale in metres) (Isometric view) 31
Shear Displacement Results (On Joints) The plot on the previous slide shows the shear displacement computed on the persistent shallow-dipping joints that intersect many post-pillars Although there is some movement, especially within the larger post-pillars towards the centre of the zone, the displacement values are low (with a maximum of only around 2 cm) 32
Shear Displacement Results (On Large Fault) (Scale in metres) Post-pillar locations (Isometric view) 33
Shear Displacement Results (On Large Fault) The plot on the previous slide shows the shear displacement on one of the large faults cutting through 118 Zone The arrows in the plot indicate the locations where the fault intersects post-pillars The white holes within the fault surface indicate excavated areas. Although some shear is computed, the amount of displacement remains small. 34
Slip and Tensile Failure Results (On Joints) Slipping now Tensile failure (Isometric view) 35
Slip and Tensile Failure Results (On Joints) The plot on the previous slide shows slip, as well as tensile failure on the persistent shallow-dipping joints that intersect some of the post-pillars All joints experience some level of slip in the past, with some joints slipping now (shown in green) and some subjected to tensile failure (shown in blue) 36
Slip and Tensile Failure Results (On Faults) Slipping now Slipping now (Isometric view) 37
Slip and Tensile Failure Results (On Faults) The plot on the previous slide shows slip and tensile failure on the two large faults in the vicinity of Zone 118 Both faults experience slip in the past in post-pillars and above the excavations Some slipping now (in green) can be seen in one post-pillar (associated with the east fault) and in the roof of the excavations (by the west fault) 38
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The 3DEC model shows that all the post-pillars experience some degree of plasticity, some of them exhibiting tension now and shear now The plasticity plots also show that most room backs and walls behave well. However, the FLAC3D model was better-suited to evaluate intact rock mass performance. The 1 plots show stress concentrations in the post-pillars in the order of 20 MPa The shear displacement and slip plots show that many of those persistent shallow-dipping joints that intersect post-pillars can be expected to sustain some damage, but the associated displacements remain small Similarly, shear displacements along the two major faults are expected to be small The 3DEC model assumes that all pillars are 5m by 5m and comprised of undisturbed rock. Poor blasting practices and/or local discrete structures could result in pillars of smaller dimensions and/or lower strength. This would affect the stability of these pillars, and, potentially, that of Zone 118. 40
Limitations of the Analyses The following important limitations apply to the 3DEC results shown in this presentation: 1. Only two homogenous and isotropic geological units were considered 2. Properties and stresses were derived from a limited set of data (no data in the case of the in-situ stress field) 3. o calibration of strength and/or stresses could be conducted 4. The persistent shallow-dipping joints were added based on the information gathered from one site visit, and values for their dip, dip direction and spacing were assigned randomly from a uniform distribution 5. Possible hydrology/hydrogeology-related effects were not considered Limitations 1, 2, 3 and 4 could have a significant effect on the results if what was included in the model is significantly different from what will be encountered in the field Limitation 5 also could significantly affect the geomechanical performance of the excavations, if prominent 41