Center for Excellence in Mining Innovation

Greater Sudbury, Canada

Center for Excellence in Mining Innovation

Greater Sudbury, Canada

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Cai M.,Laurentian University | Kaiser P.K.,Center for Excellence in Mining Innovation
Rock Mechanics and Rock Engineering | Year: 2014

It is widely accepted that the in-situ strength of massive rocks is approximately 0.4 ± 0.1 UCS, where UCS is the uniaxial compressive strength obtained from unconfined tests using diamond drilling core samples with a diameter around 50 mm. In addition, it has been suggested that the in-situ rock spalling strength, i.e., the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory tests or field microseismic monitoring. These findings were supported by back-analysis of case histories where failure had been carefully documented, using either Kirsch's solution (with approximated circular tunnel geometry and hence σ max = 3σ 1 -σ 3) or simplified numerical stress modeling (with a smooth tunnel wall boundary) to approximate the maximum tangential stress σ max at the excavation boundary. The ratio of σ max /UCS is related to the observed depth of failure and failure initiation occurs when σ max is roughly equal to 0.4 ± 0.1 UCS. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary, when interpreting the in-situ rock strength. It is demonstrated that the "actual" in-situ spalling strength of massive rocks is not equal to 0.4 ± 0.1 UCS, but can be as high as 0.8 ± 0.05 UCS when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic "as-built" excavation boundary condition is honored, the "actual" in-situ rock strength, given by 0.8 UCS, can be applied to simulate progressive brittle rock failure process satisfactorily. The interpreted, reduced in-situ rock strength of 0.4 ± 0.1 UCS without considering geometry irregularity is therefore only an "apparent" rock strength. © 2013 Springer-Verlag Wien.


Yong S.,ETH Zurich | Kaiser P.K.,Center for Excellence in Mining Innovation | Loew S.,ETH Zurich
International Journal of Rock Mechanics and Mining Sciences | Year: 2010

The Opalinus Clay is currently under investigation as a potential host rock for deep geological disposal of nuclear waste at the Mont Terri Rock Laboratory in Switzerland. Bedding in the Opalinus Clay at Mont Terri is ubiquitous and highly persistent leading to mechanical transverse isotropy. Adding to the complexity at the Rock Laboratory is the frequent occurrence of small-scale tectonic shears.This paper explores the influences of millimetre-thick tectonic shears and bedding on the development of induced fractures mapped in the EZ-B field experiment at the research facility. A series of numerical analyses were carried out by increasing the geological complexity of the host rock and comparing the redistributed stress field with geological maps of the induced fractures. The analyses show that if tectonic shears are not kinematically constrained, mobilisation of the shears can play a key role in the development of the induced fracture network and therefore, be a primary factor in the development of the excavation damaged zone. This illustrates that under certain conditions rock mass heterogeneity (in this case, resulting from the tectonic shears) may dominate over rock matrix anisotropy (in this case, resulting from bedding) and must be considered when predicting the induced fracture network of the excavation damaged zone. © 2010 Elsevier Ltd.


Amann F.,ETH Zurich | Undul O.,Istanbul University | Kaiser P.K.,Center for Excellence in Mining Innovation
Rock Mechanics and Rock Engineering | Year: 2014

Brittle fracture processes were hypothesized by several researches to cause a damage zone around an underground excavation in sulfate-rich clay rock when the stress exceeds the crack initiation threshold, and may promote swelling by crystal growth in newly formed fractures. In this study, laboratory experiments such as unconfined and confined compression tests with acoustic emission monitoring, and microstructural and mineralogical analyses are used to explain brittle fracture processes in sulfate-rich clay rock from the Gipskeuper formation in Switzerland. This rock type typically shows a heterogeneous rock fabric consisting of distinct clayey layers and stiff heterogeneities such as anhydrite layers, veins or nodules. The study showed that at low deviatoric stress, the failure behavior is dominated by the strength of the clayey matrix where microcracks are initiated. With increasing deviatoric stress or strain, growing microcracks eventually are arrested at anhydrite veins, and cracks develop either aligned with the interface between clayey layers and anhydrite veins, or penetrate anhydrite veins. These cracks often link micro-fractured regions in the specimen. This study also suggest that fracture localization in sulfate-rich clay rocks, which typically show a heterogeneous rock fabric, does not take place in the pre-peak range and renders unstable crack propagation less likely. Sulfate-rich clay rocks typically contain anhydrite veins at various scales. At the scale of a tunnel, anhydrite layers or veins may arrest growing fractures and prevent the disintegration of the rock mass. The rock mass may be damaged when the threshold stress for microcrack initiation is exceeded, thus promoting swelling by crystal growth in extension fractures, but the self-supporting capacity of the rock mass may be maintained rendering the possibility for rapidly propagating instability less likely. © 2013 Springer-Verlag Wien.


Yong S.,ETH Zurich | Kaiser P.K.,Center for Excellence in Mining Innovation | Loew S.,ETH Zurich
International Journal of Rock Mechanics and Mining Sciences | Year: 2013

In this study, the rock mass response ahead of an advancing test tunnel in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland) was investigated. Characterisation of the excavation-induced damage zone at Mont Terri is a challenging task due to the anisotropic and heterogeneous nature of the shale: pronounced bedding leads to intact rock anisotropy and prevalent small-scale tectonic shears lead to rock mass heterogeneity. Rock mass damage ahead of an experimental tunnel or niche was characterised through single-hole seismic wave velocity logging, borehole digital optical televiewer imaging, and geological drillcore mapping. Three-dimensional elastic stress analyses were completed and showed that rock mass degradation can be correlated to changes in the maximum to minimum principal stress ratio (i.e., spalling limit). Numerical results showed that close to the niche boundary, unloading lowers stress ratios, which correspond with decreasing seismic wave amplitudes and velocities; thus, indicating that strength degradation resulted from increasing crack-induced damage. Considerations of tectonic shears and distance from a previously stressed volume of rock were necessary in understanding both the damage state and extent ahead of the face. By integrating field and numerical data, the investigation showed that geological structures (i.e., bedding and bedding-parallel tectonic shears) were most influential near the entrance but played a lesser role as the niche deepened. Additionally, a portion of the niche is located in the perturbed zone of the intersecting Gallery04. © 2013 Elsevier Ltd.


Kaiser P.K.,Center for Excellence in Mining Innovation | Kim B.-H.,Laurentian University | Kim B.-H.,Itasca Consulting Group Inc.
Rock Mechanics and Rock Engineering | Year: 2014

As technologies for deep underground development such as tunneling underneath mountains or mass mining at great depths (>1,000 m) are implemented, more difficult ground conditions in highly stressed environments are encountered. Moreover, the anticipated stress level at these depths easily exceeds the loading capacity of laboratory testing, so it is difficult to properly characterize what the rock behavior would be under high confinement stress conditions. If rock is expected to fail in a brittle manner, behavior changes associated with the relatively low tensile strength, such as transition from splitting to the shear failure, have to be considered and reflected in the adopted failure criteria. Rock failure in tension takes place at low confinement around excavations due to tensile or extensional failure in heterogeneous rocks. The prospect of tensile-dominant brittle failure diminishes as the confinement increases away from the excavation boundary. Therefore, it must be expected that the transition in the failure mechanism, from tensile to shear, occurs as the confinement level increases and conditions for extensional failure are prevented or strongly diminished. However, conventional failure criteria implicitly consider only the shear failure mechanism (i.e., failure envelopes touching Mohr stress circles), and thus, do not explicitly capture the transition of failure modes from tensile to shear associated with confinement change. This paper examines the methodologies for intact rock strength determination as the basic input data for engineering design of deep excavations. It is demonstrated that published laboratory test data can be reinterpreted and better characterized using an s-shaped failure criterion highlighting the transition of failure modes in brittle failing rock. As a consequence of the bi-modal nature of the failure envelope, intact rock strength data are often misinterpreted. If the intact rock strength is estimated by standard procedures from unconfined compression tests (UCS) alone, the confined strength may be underestimated by as much as 50 % (on average). If triaxial data with a limited confinement range (e.g., σ3 ≪ 0.5 UCS due to cell pressure limitations) are used, the confined strength may be overestimated. Therefore, the application of standard data fitting procedures, without consideration of confinement-dependent failure mechanisms, may lead to erroneous intact rock strength parameters when applied to brittle rocks, and consequently, by extrapolation, to correspondingly erroneous rock mass strength parameters. It follows that the strength characteristics of massive rock differ significantly in the direct vicinity of excavation from that which is remote with higher confinement. Therefore, it is recommended to adopt a differentiated approach to obtain intact rock strength parameters for engineering problems at lower confinement (near excavation; e.g., excavation stability assessment or support design), and at elevated confinement (typically, when the confinement exceeds about 10 % of the UCS) as might be encountered in wide pillar cores. © 2014, Springer-Verlag Wien.


Amann F.,ETH Zurich | Kaiser P.,Center for Excellence in Mining Innovation | Button E.A.,Geoconsult Pvt. Ltd. India
Rock Mechanics and Rock Engineering | Year: 2012

The brittle failure behavior of an over-consolidated clay shale (Opalinus Clay) in undrained rapid triaxial compression was studied. The confining stress levels were chosen to simulate the range of confining stresses relevant for underground excavations at the Mont Terri Underground Research Laboratory, and to investigate the transition from axial splitting failure to macroscopic shear failure. Micro-crack initiation was observed throughout the confining stress range utilized in this study at a differential stress of 2.1 MPa on average, which indicates that friction was not mobilized at this stage of brittle failure. The rupture stress was dependent on confinement indicating friction mobilization during the brittle failure process. With increasing confinement net volumetric strain decreased suggesting that dilation was suppressed, which is possibly related to a change in the failure mode. At confining stress levels ≥0.5 MPa specimen rupture was associated with axial splitting. With increasing confinement, transition to a macroscopic shearing mode was observed. Multi-stage triaxial tests consistently showed lower strengths than single-stage tests, demonstrating cumulative damage in the specimens. Both the Mohr-Coulomb and Hoek-Brown failure criteria could not satisfactorily fit the data over the entire confining stress range. A bi-linear or S-shaped failure criterion was found to satisfactorily fit the test data over the entire confinement range studied. © 2011 Springer-Verlag.


Amann F.,ETH Zurich | Kaiser P.K.,Center for Excellence in Mining Innovation | Steiner W.,BS AG
Rock Mechanics in Civil and Environmental Engineering - Proceedings of the European Rock Mechanics Symposium, EUROCK 2010 | Year: 2010

Understanding the potential influence of brittle failure on swelling processes is motivated by the remarkable short and long-term heaving problems associated with tunnels constructed in anhydrite rich rock types (Gipskeuper) in Switzerland and elsewhere. Brittle failure behavior is hypothesized to be a key factor to explain the remarkable extension of water conductive zones around the excavations. In order to verify this hypothesis and to demonstrate the potential effect of brittle ground behavior on water pathway development and thus swelling, a numerical study based on ground data from several projects in Switzerland was initiated. The results obtained in the study demonstrate that brittle, extensional fractures beneath the invert of tunnels provide a remarkable and in many ways critical trigger of the swelling potential of anhydrite clay rocks. © 2010 Taylor & Francis Group.


Kaiser P.K.,Center for Excellence in Mining Innovation
Rock Mechanics in Civil and Environmental Engineering - Proceedings of the European Rock Mechanics Symposium, EUROCK 2010 | Year: 2010

This lecture focuses on brittle rock and rock mass failure modes to illustrate how highly stressed rock behaviour near deep excavations differs from rock far from an excavation, i.e., it fails predominantly by spalling rather than by shear.As a consequence, the often adopted shear failure models do not represent spallingtype failure processes and thus are often inadequate for the design of tunnels and their support systems. It is also discussed how swelling rock problems in tunnelling might be related to brittle failure processes and how a change to more appropriate modelling approaches might assist in understanding and controlling swelling. Recent developments and implications of practical importance are highlighted, particularly with respect to the selection of strength parameters for support design of underground excavations in highly stressed rock. Several case examples are used to highlight how a better understanding of the spalling behaviour may help to prevent costly delays and rehabilitation work. © 2010 Taylor & Francis Group.


Cai M.,Laurentian University | Zhao X.,Beijing Research Institute of Uranium Geology | Kaiser P.K.,Center for Excellence in Mining Innovation
Yanshilixue Yu Gongcheng Xuebao/Chinese Journal of Rock Mechanics and Engineering | Year: 2014

It is widely accepted that the field or in-situ strength of massive rocks is approximately (0.4±0.1) σc, where σc is the uniaxial compressive strength obtained from unconfined laboratory tests. In addition, it has been suggested that the in-situ rock spalling strength, i.e. the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory test or field microseismic monitoring. These findings were based on either Kirsch's solution or simplified numerical stress modeling(with smooth tunnel wall boundary) to approximate the maximum tangential stress σmax at the excavation boundary. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary. It is demonstrated that the "actual" in-situ spalling strength of massive rocks is not equal to (0.4±0.1) σc, but can be as high as (0.8±0.05) σc when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic "as-built" excavation boundary condition is honored, the "actual" in-situ rock mass strength, given by 0.8 σc, can be applied to simulate progressive brittle rock failure process satisfactorily. We conclude that the interpreted, reduced in-situ rock mass strength of (0.4±0.1) σc without considering geometry irregularity is therefore only an "apparent" rock mass strength.


Patent
Center For Excellence In Mining Innovation | Date: 2013-09-27

Disclosed is a drill and blast method for advancing the tunnel face in a mine, which makes use of a mobile canopy. The mobile canopy having vertical supports connected to a frame that supports a shield. The mobile canopy allows for face production activities and ground support activities to occur simultaneously or near simultaneously. This allows for more rapid advancement of the tunnel face compared to traditional batch drill and blast techniques.

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