The Main Pit at Koolan Island consists in general of a bermless footwall from 26 mRL to -86 mRL a... more The Main Pit at Koolan Island consists in general of a bermless footwall from 26 mRL to -86 mRL and a benched hanging wall containing the haul roads. The footwall above 26 m RL was benched at 20 m intervals and the top crest of the footwall ranges from 140 m RL to 110 m RL. The current pit is approximately 2000 m long and 500 m wide. The batter angle of the footwall varies from 45 at the western end to 60 at the eastern end. Mount Gibson Iron Limited (MGI) is currently re-opening the operation with a plan to deepen the pit to around -186 mRL. The width of the hanging wall cutback will be about 80 m and the footwall will be continued without any berms. Extensive zones of extremely weak, friable material and the tabular nature of the footwall formation are the major challenge of overall footwall stability. This paper will discuss the analysis of the footwall slope stability of the proposed pit including rehabilitation requirements to the existing footwall. The numerical modeling packa...
<p>Growth of internal cracks in compression is a primary mechanism of catas... more <p>Growth of internal cracks in compression is a primary mechanism of catastrophic rock failures.  Since the cracks are internal, only indirect methods are currently available for investigation and monitoring of failures of this kind.  In order to understand the fundamentals of 3D (internal) crack growth in compression, special physical models using transparent polymer prisms with embedded penny-shaped flaws were performed.  A biaxial stress field was applied, to simulate conditions near the face or walls of deep tunnels.  Previously, such experiments were conducted within a fully-enclosed polyaxial testing machine, preventing observation of the growing crack itself.  Recent experiments at UWA utilized a plane-strain restraint device, which develops the secondary principal stress (σ<sub>2</sub>) by limiting the lateral expansion that would otherwise result from the imposition of the primary principal stress (σ<sub>1</sub>).  This device leaves clear line-of-sight to the sample free face, allowing visual observation and the installation of other instrumentation. </p><p>As expected, the biaxial stress field resulted in growth of an extensive, nearly planar crack, parallel to the σ<sub>1</sub>-σ<sub>2</sub> plane.  Growth of this crack was observed using high-speed video.  The crack surface morphology shows similarity to many natural and excavation-induced fractures in geomaterials.  This similarity justifies the usage of the transparent materials for investigating rock failure. Furthermore, the observed morphological features can be linked to specific events in the crack growth process.</p><p>In addition, experiments incorporated acoustic emission sensors, both on the sample, and in the air near the sample.  Two categories of vibration were observed at the sample’s rear free face: a short-duration wavelet, representing the crack initiation, and a long-duration high-amplitude “ringing” waveform.  The “ringing” was also observed in air (as audible sound) and in video (as movement of the whole sample). The observed vibrations could be utilized for monitoring dangerous rock failures.</p>
<p>Rock samples tested in uniaxial compression tests are often observed to have spa... more <p>Rock samples tested in uniaxial compression tests are often observed to have spallation failure at the lateral surface of the sample caused by buckling of layers developing parallel to the direction of loading. These layers are caused by extensive crack growth. Yet, the experiments show that extensive crack growth in real 3D situations requires the presence of intermediate compressive principal stress (biaxial loading). We check the hypothesis that the intermediate principal stress is generated by the direct contact with metal loading platens, where the “frictional” boundary conditions are developed at the sample ends, creating non-uniform stress distributions near the sample ends. By the use of the finite element method (FEM), we analyse the stress distributions at the immediate lateral surface of the cylindrical sample in uniaxial compression in the polar coordinate system. It is found that near the ends the sample is actually biaxially loaded: the circumferential stress could be induced to play the role of the intermediate principal stress. The sizes of the zones and the maximum magnitude of the circumferential stress depend upon the friction coefficient and the Poisson’s ratio of the rock. The biaxial load ratio is defined by the ratio between the circumferential compressive stress and the axial stress. Comparing the biaxial load ratio determined in numerical models with the critical biaxial load ratio inducing extensive crack growth, the spallation of rock samples in uniaxial compression tests can be interpreted from a new perspective.</p>
This study presents an unprecedented campaign of measurements (magnitudes and orientations) of th... more This study presents an unprecedented campaign of measurements (magnitudes and orientations) of the in situ stresses in itabirite rocks of the Brazilian Iron Quadrangle region using the hydraulic fr...
International Journal of Rock Mechanics and Mining Sciences, 2014
The sources of nonlinear deformation in hard rock under short term uniaxial compression can be at... more The sources of nonlinear deformation in hard rock under short term uniaxial compression can be attributed to crack closure, sliding, compaction and crack generation. The common approach to finding the Young's modulus of a "hard rock" is to determine the linear part of stress-strain curve. However, it is usually a difficult task, although several methods of resolving it have been proposed in the past. We believe that in some rock types there is no linear part as such and provide evidence to support the fact that nonlinear deformation contributed by crack closure, sliding and compaction in different degrees could co-exist within the same stress range. We demonstrate that the change in tangent modulus under different stress levels is attributable to the combination of crack closure, sliding and dilatancy. The difference in tangent modulus under different stress levels could reach more than 20GPa in some rocks. We have also found that the nonlinear deformation contributed by irreversible sliding, compaction and even dilatancy increases the rock stiffness in the second loading. This phenomenon was found under loads ranging from 15% to 95% of UCS.
This study presents an unprecedented campaign of measurements (magnitudes and orientations) of th... more This study presents an unprecedented campaign of measurements (magnitudes and orientations) of the in situ stresses in itabirite rocks of the Brazilian Iron Quadrangle region using the hydraulic fracturing (HF), deformation rate analysis (DRA) and diametrical core deformation analysis (DCDA) methods. The previous studies of this rock mass consider estimated values of k, and the practice of using in situ stress tests to support geotechnical analyses of the rock mass of this region has not been adopted. The study site is located at a depth of up to 400 m around a pit. The rock mass under study shows the presence of different geological structures, which made performing the tests difcult. The DRA results show a marked difference compared to the HF and DCDA results; the horizontal stresses were greater than the vertical stresses, which is a fnding that may occur due to local geodynamic influences. The results contribute to the understanding of the strains and stresses induced by mining act...
The Main Pit at Koolan Island consists in general of a bermless footwall from 26 mRL to -86 mRL a... more The Main Pit at Koolan Island consists in general of a bermless footwall from 26 mRL to -86 mRL and a benched hanging wall containing the haul roads. The footwall above 26 m RL was benched at 20 m intervals and the top crest of the footwall ranges from 140 m RL to 110 m RL. The current pit is approximately 2000 m long and 500 m wide. The batter angle of the footwall varies from 45 at the western end to 60 at the eastern end. Mount Gibson Iron Limited (MGI) is currently re-opening the operation with a plan to deepen the pit to around -186 mRL. The width of the hanging wall cutback will be about 80 m and the footwall will be continued without any berms. Extensive zones of extremely weak, friable material and the tabular nature of the footwall formation are the major challenge of overall footwall stability. This paper will discuss the analysis of the footwall slope stability of the proposed pit including rehabilitation requirements to the existing footwall. The numerical modeling packa...
<p>Growth of internal cracks in compression is a primary mechanism of catas... more <p>Growth of internal cracks in compression is a primary mechanism of catastrophic rock failures.  Since the cracks are internal, only indirect methods are currently available for investigation and monitoring of failures of this kind.  In order to understand the fundamentals of 3D (internal) crack growth in compression, special physical models using transparent polymer prisms with embedded penny-shaped flaws were performed.  A biaxial stress field was applied, to simulate conditions near the face or walls of deep tunnels.  Previously, such experiments were conducted within a fully-enclosed polyaxial testing machine, preventing observation of the growing crack itself.  Recent experiments at UWA utilized a plane-strain restraint device, which develops the secondary principal stress (σ<sub>2</sub>) by limiting the lateral expansion that would otherwise result from the imposition of the primary principal stress (σ<sub>1</sub>).  This device leaves clear line-of-sight to the sample free face, allowing visual observation and the installation of other instrumentation. </p><p>As expected, the biaxial stress field resulted in growth of an extensive, nearly planar crack, parallel to the σ<sub>1</sub>-σ<sub>2</sub> plane.  Growth of this crack was observed using high-speed video.  The crack surface morphology shows similarity to many natural and excavation-induced fractures in geomaterials.  This similarity justifies the usage of the transparent materials for investigating rock failure. Furthermore, the observed morphological features can be linked to specific events in the crack growth process.</p><p>In addition, experiments incorporated acoustic emission sensors, both on the sample, and in the air near the sample.  Two categories of vibration were observed at the sample’s rear free face: a short-duration wavelet, representing the crack initiation, and a long-duration high-amplitude “ringing” waveform.  The “ringing” was also observed in air (as audible sound) and in video (as movement of the whole sample). The observed vibrations could be utilized for monitoring dangerous rock failures.</p>
<p>Rock samples tested in uniaxial compression tests are often observed to have spa... more <p>Rock samples tested in uniaxial compression tests are often observed to have spallation failure at the lateral surface of the sample caused by buckling of layers developing parallel to the direction of loading. These layers are caused by extensive crack growth. Yet, the experiments show that extensive crack growth in real 3D situations requires the presence of intermediate compressive principal stress (biaxial loading). We check the hypothesis that the intermediate principal stress is generated by the direct contact with metal loading platens, where the “frictional” boundary conditions are developed at the sample ends, creating non-uniform stress distributions near the sample ends. By the use of the finite element method (FEM), we analyse the stress distributions at the immediate lateral surface of the cylindrical sample in uniaxial compression in the polar coordinate system. It is found that near the ends the sample is actually biaxially loaded: the circumferential stress could be induced to play the role of the intermediate principal stress. The sizes of the zones and the maximum magnitude of the circumferential stress depend upon the friction coefficient and the Poisson’s ratio of the rock. The biaxial load ratio is defined by the ratio between the circumferential compressive stress and the axial stress. Comparing the biaxial load ratio determined in numerical models with the critical biaxial load ratio inducing extensive crack growth, the spallation of rock samples in uniaxial compression tests can be interpreted from a new perspective.</p>
This study presents an unprecedented campaign of measurements (magnitudes and orientations) of th... more This study presents an unprecedented campaign of measurements (magnitudes and orientations) of the in situ stresses in itabirite rocks of the Brazilian Iron Quadrangle region using the hydraulic fr...
International Journal of Rock Mechanics and Mining Sciences, 2014
The sources of nonlinear deformation in hard rock under short term uniaxial compression can be at... more The sources of nonlinear deformation in hard rock under short term uniaxial compression can be attributed to crack closure, sliding, compaction and crack generation. The common approach to finding the Young's modulus of a "hard rock" is to determine the linear part of stress-strain curve. However, it is usually a difficult task, although several methods of resolving it have been proposed in the past. We believe that in some rock types there is no linear part as such and provide evidence to support the fact that nonlinear deformation contributed by crack closure, sliding and compaction in different degrees could co-exist within the same stress range. We demonstrate that the change in tangent modulus under different stress levels is attributable to the combination of crack closure, sliding and dilatancy. The difference in tangent modulus under different stress levels could reach more than 20GPa in some rocks. We have also found that the nonlinear deformation contributed by irreversible sliding, compaction and even dilatancy increases the rock stiffness in the second loading. This phenomenon was found under loads ranging from 15% to 95% of UCS.
This study presents an unprecedented campaign of measurements (magnitudes and orientations) of th... more This study presents an unprecedented campaign of measurements (magnitudes and orientations) of the in situ stresses in itabirite rocks of the Brazilian Iron Quadrangle region using the hydraulic fracturing (HF), deformation rate analysis (DRA) and diametrical core deformation analysis (DCDA) methods. The previous studies of this rock mass consider estimated values of k, and the practice of using in situ stress tests to support geotechnical analyses of the rock mass of this region has not been adopted. The study site is located at a depth of up to 400 m around a pit. The rock mass under study shows the presence of different geological structures, which made performing the tests difcult. The DRA results show a marked difference compared to the HF and DCDA results; the horizontal stresses were greater than the vertical stresses, which is a fnding that may occur due to local geodynamic influences. The results contribute to the understanding of the strains and stresses induced by mining act...
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