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This paper focuses on ‘self-initiated’ strainbursts, and starts by providing an overview of these phenomena and their relevance to mining. This is used to set the framework for the numerical simulations presented in the second part of the paper. The numerical simulations examine and address challenges in modelling strainbursts related to the inherent difficulties in dynamic failure simulation. Existing methods either simulate dynamic failure under static or pseudo-static conditions while adopting some threshold factor (normally in terms of stress, strain, or energy) as an indicator of rockburst potential, or incorporate an assumed dynamic disturbance to cause rockburst damage. A distinct-element bonded block method is presented to simulate strainbursts. Instead of triggering failure by an assumed dynamic disturbance, the adopted method simulates the generation of a seismic event by self-initiated rock mass fracturing. This simulation of a self-initiated strainburst is interpreted using patterns of cracking, displacement and strain fields, ground velocities, and calculated kinetic and dissipated slip energies. The mechanisms of self-initiated strainbursting are successfully captured by the distinct-element bonded block method.

Numerical simulation of rockbursts remains a challenge due to the inherent difficulties in dynamic failure simulation. A novel distinct-element bonded block method is proposed and tested to simulate strainbursts. The method simulates the generation of a seismic event induced by dynamic rock cracking. The seismic event produces P- and S-waves with seismic wave properties including velocity, the ratio of Vs/Vp, and frequency that are expected for strainbursts. In this manner, the process of strainbursting by dynamic fracture processes is successfully captured. The simulations suggest that a critical stress level may exist beyond which a minor disturbance can induce strainbursts.

Although most mining companies utilise systems for slope monitoring, experience indicates that mining operations continue to be surprised by the occurrence of adverse geotechnical events. A comprehensive and robust performance monitoring system is an essential component of slope management in an open pit mining operation. The development of such a system requires considerable expertise to ensure the monitoring system is effective and reliable. Written by instrumentation experts and geotechnical practitioners, Guidelines for Slope Performance Monitoring is an initiative of the Large Open Pit (LOP) Project and the fifth book in the Guidelines for Open Pit Slope Design series. Its 10 chapters present the process of establishing and operating a slope monitoring system; the fundamentals of pit slope monitoring instrumentation and methods; monitoring system operation; data acquisition, management and analysis; and utilising and communicating monitoring results. The implications of increased automation of mining operations are also discussed, including the future requirements of performance monitoring. Guidelines for Slope Performance Monitoring summarises leading mine industry practice in monitoring system design, implementation, system management, data management and reporting, and provides guidance for engineers, geologists, technicians and others responsible for geotechnical risk management.

Experiences from deep mining have seen an increase in operational and safety challenges related to excessive rock mass bulking due to brittle fracturing and spalling. These have exposed limitations in available constitutive models used to assess post-peak deformation potential, as required for excavation support design. Despite the spalling process being driven by extensional fracturing in the low-confined rock near the excavation boundary, the yield and dilation models available in commercial numerical modelling software are dominated by shear-based models (e.g., Mohr-Coulomb, Hoek-Brown, etc.) more suitable for shear fracturing under intermediate to high confinement. In the context of plasticity theory, this corresponds to important limitations that arise from the assumptions inherent in the development of dilation models (also referred to as flow rules). Flow rules determine the relative incremental plastic straining of the yielding rock under a specific stress state and at a specific plastic straining history. The commonly-used flow rules in rock engineering have been shaped by experiences in soil mechanics and weak rock, and consequently, ignore the fact that stress-induced brittle fracturing has a strong directional component and is sensitive to 3-D confinement. In particular, the influence of the intermediate principal stress, σ 2 , on the directionality and magnitude of dilation is not accounted for. Another limitation is that many advanced dilation models require numerous empirical parameters that do not have clear physical meaning. This conflicts with the of practitioners being able to intuitively understand the influence and sensitivity of important controlling parameters on modelled deformation response. In the Part 1 companion paper to this, we presented the theoretical framework for deriving a non-potential flow rule using the concept of Cartesian-based Plastic Strain Increments Ratios (PSIRs). We demonstrated the advantages of using PSIRs for understanding the 3-D dilation/contraction behaviour of rock in relation to the underlying 3-D confinement-dependent fracturing processes. In this Part 2 paper, we first conduct a thorough review of existing dilation models and their formulation to investigate mathematical models that can be used to effectively represent both pre- and post-peak dilative behaviour in brittle rocks. We then further investigate the true dilative characteristics of brittle rock using triaxial compression test data for Lac du Bonnet Pink Granite. This data and analysis are further used to validate and parameterize the model developed. Next, we derive a σ 2 -independent dilation model based on the PSIR concept and use this to then develop a 3-D σ 2 -dependent dilation model, both of which are first-of-their-kind. The derivations also outline how the PSIR approach allows for optimization of the number of input parameters required for the dilation model, each with physical meaning. Finally, the advantages over existing dilation models used in practice is demonstrated and discussed with respect to the importance of σ 2 - and strain history-dependency in relation to 3-D brittle fracturing and bulking processes.

The high consequences of recent large open pit slope failures have increased industry and regulatory interest in establishing exclusion zones beneath an impending pit wall failure. This paper proposes a methodology to delineate an exclusion zone for an impending pit slope failure. The focus of this paper is a framework for the management of life loss risk to the most exposed individual. The probability of the landslide occurring and the probability of the resulting landslide being very or extremely rapid are described in this paper. Set in a probabilistic framework , empirical runout analysis tools are useful for estimating the spatial probability of impact throughout the open pit and establishing exclusion zones. Runout exceedance probability charts calibrated to a large dataset of pit slope failures are provided. The temporal probability that the most exposed individual is present and cannot be evacuated is also described. The resulting risk is mapped across the pit floor by dividing it into square grid cells and calculating the probability of death to the most exposed individual for each cell. The cells can be colour coded to indicate specific risk exposure ranges relative to the exclusion zone set. A conceptual case study is used to illustrate the proposed methods.

The viability and success of a deep mining or tunnelling project is dependent on a safe, reliable support design. In highly-stressed brittle rock, this design must properly account for the excessive deformations and bulking that can occur due to stress-induced extensional fracturing (referred to as spalling or slabbing) near the excavation boundary. Important is the recognition that stress-induced brittle fracturing is dominated by an extensional fracture mode with a directional dilation component that is highly sensitive to 3-D confinement. This behaviour differs from that represented by popular elastoplastic yield and dilation models used in commercial numerical modelling software (e.g., Mohr-Coulomb) that have been developed based on experiences involving ductile behaviour and shear in soils and weak rocks. Recent experiences suggest that these are inappropriate for robust design in brittle rock and can be dangerously misleading. The importance of the dilation model and its implementation through a flow rule – which determines the relative incremental plastic straining under a specific stress state and at a specific plastic straining history – has particularly been overlooked. Numerical analyses in most cases adopt the oversimplifying assumption of a uniform dilation model (that is independent of confinement and plastic strain history), using as input a constant dilation angle ψ . This imposes significant limitations in modelling the extent of the yielding zone and the resulting displacements. More advanced dilation models exist. However, they suffer from two key deficiencies: i) most do not account for the influence of the intermediate principal stress , σ 2 , on the directionality and magnitude of dilation; and ii) many require numerous empirical parameters or variables that do not have physical meaning (neither phenomenologically, nor micro-mechanically), making it difficult for practitioners to intuitively understand the influence and sensitivity of each parameter on the modelled response. This paper, presented in two parts, reviews the mathematical expression of the flow rule and its physical meaning in relation to brittle fracturing, and addresses the deficiencies in current dilation models by developing a new non-potential flow rule that accounts for the directionality and 3-D confinement-dependency of dilation (i.e., both σ 3 - and σ 2 -dependency), and uses parameters/variables that have physical meaning. In Part 1, we present a new framework for understanding and modelling the 3-D directionality of plastic deformations in Cartesian coordinates using what we call Plastic Strain Increments Ratios (PSIRs). The 3-D PSIRs are phenomenologically meaningful as they describe relative plastic straining. More importantly, we show that they are micro-mechanically meaningful as they associate the 3-D directionality of dilation with the different 3-D fracturing modes observed in intact brittle rock under various polyaxial stress states. In the Part 2 companion paper, we use these PSIRs to develop and derive the formulation of a new dilation model involving a non-potential 3-D confinement-dependent flow rule.

Objectively forecasting the runout of a potential open pit slope failure, in addition to identifying the failure itself, is a critical component of a mine’s risk management plan. Recent losses arising from large open pit slope failures demonstrate shortcomings in current practice. A dataset of 105 pit slope failures was compiled to compare open pit runout trends against established empirical runout relationships for natural landslides. Fahrböschung angle versus volume and Fahrböschung angle versus slope angle relationships provide reasonable runout estimates. Open pit slopes have the advantage of removing the influence of morphological features, vegetation, and liquefiable substrates while controlling the travel path angle and roughness. In such a controlled environment, landslide mobility has a strong sensitivity to slope angle, material properties, and fall height, and is only modestly sensitive to volume. A grouping of highly mobile open pit slope cases involving weathered, saturated, collapsible rock mass materials exceed expected runout distances when compared with established runout trends. This suggests mobility for these weaker rock masses is controlled by pore pressures mediating basal friction. The result is that two different runout exceedance trends are observed based on whether the unstable rock mass involves fresh, strong rocks or weathered weak rocks.

The evolutionary failure processes leading to large-scale mass movements in massive crystalline rock slopes are the subjects of a multidisciplinary research project in the Swiss Alps. Focus is directed towards detecting and analysing rockslide processes that involve the progressive development of a failure surface as opposed to sliding along a pre-existing one. In order to monitor the underlying mechanisms of progressive failure, several new and conventional instrumentation systems were combined with an existing in situ monitoring program at an active rockslide site in the Valais (Switzerland). Design of the instrumentation network is based on site investigations and preliminary geomechanical models of the acting rockslide processes with respect to the rate of displacements, position and orientation of geological features that delineate the unstable rockmass. The network set-up considers additional findings from borehole logging and testing. Parameters that will be measured include microseismicity, fracture patterns and the temporal and spatial evolution of 3-D displacement fields and fluid pressures.

A framework for landslide risk management for landslides and slopes is presented. A review of criteria for acceptable individual and societal risk is presented, and suggestions made for acceptable risk criteria for landsliding. Examples are given of the framework for landslide risk management for natural and man-made slopes and for landslides affecting dam reservoirs. Matters requiring discussion and further research are described.

The episodic movement of deep seated rockslides is often associated with seasonal variations in precipitation and groundwater recharge. Acceleration and deceleration phases are often observed however it is not always clear why certain acceleration phases reach alarming levels without a clear trigger (i.e. in the absence of an exceptional precipitation event). This introduces significant uncertainty in the interpretation of displacement monitoring data for early warning which in turn is further hampered by the complexity of the underlying rock slope failure processes involved. Numerous examples exist where the interpretation of early warning data without a clear understanding of the underlying mechanisms has led to variable and generally unreliable forecasts. This paper reviews recent research investigating the influence of non persistent discontinuities intact rock bridges brittle fracturing and internal shearing on the progressive failure of deep seated rockslides. These mechanisms are then linked to the concept of rock slope fatigue using coupled hydro mechanical distinct element modeling to explain the intermittent behavior observed in deep seated rockslides. Consideration is also given to the influence of seismic fatigue over longer time intervals. Seismic fatigue represents a low repeat frequency but large amplitude damage event whereas hydromechanical fatigue occurs at a higher repeat frequency but represents a smaller amplitude damage event. When calibrated against slope monitoring data the consideration of fatigue and progressive failure in numerical analyses offer a means to better support early warning forecasting and improve rockslide hazard management.

Hydraulic fracturing operations to enable production from unconventional oil and gas reservoirs have been subject to public, industry, and regulator concerns regarding induced seismicity. The injection of fluids into deep formations to generate hydraulic fractures serves to create localized increases in pore pressures and reductions in the effective normal stresses acting on critically stressed faults, resulting in fault slip and induced seismicity. Amongst the different factors influencing induced seismicity, operational factors such as injection volume and rate are potentially important, and can be controlled (in contrast to geological factors, which cannot). In this paper, an empirical study is presented examining correlations between injection rate and volume and induced seismicity events and magnitudes for data compiled for the Montney play in northeastern British Columbia. The results of the empirical analysis show that injection rate has a slightly higher correlation to induced seismicity than injection volume, and that larger events (>M3) correlate with higher injection rates (>6-8 m3/min). Three-dimensional numerical modelling was also performed to further investigate the magnitude distribution of induced seismic events as a function of different injection rates. For the modelled geological scenario, the results indicate that lower injection rates resulted in a more distributed pore pressure perturbation interacting with an adjacent critically stressed fault, resulting in multiple slip areas producing several small magnitude events. In contrast, higher injection rates resulted in a more concentrated pore pressure perturbation interacting with the fault causing a larger area to slip, producing a singular large magnitude event.

The importance of earthquakes in triggering catastrophic failure of deep-seated landslides has long been recognized and is well documented in the literature. However, seismic waves do not only act as a trigger mechanism. They also contribute to the progressive failure of large rock slopes as a fatigue process that is highly efficient in deforming and damaging rock slopes. Given the typically long recurrence time and unpredictability of earthquakes, field-based investigations of co-seismic rock slope deformations are difficult. We present here a conceptual numerical study that demonstrates how repeated earthquake activity over time can destabilize a relatively strong rock slope by creating and propagating new fractures until the rock mass is sufficiently weakened to initiate catastrophic failure. Our results further show that the damage and displacement induced by a certain earthquake strongly depends on pre-existing damage. In fact, the damage history of the slope influences the earthquake-induced displacement as much as earthquake ground motion characteristics such as the peak ground acceleration. Because seismically induced fatigue is: (1) characterized by low repeat frequency, (2) represents a large amplitude damage event, and (3) weakens the entire rock mass, it differs from other fatigue processes. Hydro-mechanical cycles, for instance, occur at higher repeat frequencies (i.e., annual cycles), lower amplitude, and only affect limited parts of the rock mass. Thus, we also compare seismically induced fatigue to seasonal hydro-mechanical fatigue. While earthquakes can progressively weaken even a strong, competent rock mass, hydro-mechanical fatigue requires a higher degree of pre-existing damage to be effective. We conclude that displacement rates induced by hydro-mechanical cycling are indicative of the degree of pre-existing damage in the rock mass. Another indicator of pre-existing damage is the seismic amplification pattern of a slope; frequency-dependent amplification factors are highly sensitive to changes in the fracture network within the slope. Our study demonstrates the importance of including fatigue-related damage history—in particular, seismically induced fatigue—into landslide stability and hazard assessments.

Earthquake-induced landslides constitute a critical component of seismic hazard in mountainous regions. While many seismic slope stability analysis methods exist with varying degrees of complexity, details of interactions between seismic waves and incipient landslides are not well understood and rarely incorporated, in particular for deep-seated slope instabilities. We present a series of 2D distinct-element numerical models aimed at clarifying interactions between earthquakes and large rock slope instabilities. The study has two main goals: 1) to explore the role of amplification in enhancing co-seismic slope deformation — a relationship widely discussed in literature but rarely tested quantitatively; and 2) to compare our numerical results with the well-established Newmark-method, which is commonly used in seismic slope stability analysis. We focus on three amplification phenomena: 1) geometric (topographic) amplification, 2) amplification related to material contrasts, and 3) amplification related to compliant fractures. Slope height, topography, seismic velocity contrasts, and internal strength and damage history were varied systematically in a series of models with a relatively simple, scalable geometry. For each model, we compute the spatial amplification patterns and displacement induced by real earthquake ground motions. We find that material contrasts and internal fracturing create both the largest amplification factors and induced displacements, while the effect of geometry is comparably small. Newmark-type sliding block methods underestimate displacements by not accounting for material contrasts and internal fracturing within the landslide body — both common phenomena in deep-seated slope instabilities. Although larger amplification factors tend to be associated with greater displacements, we did not identify a clear link between ground motion frequency content, spectral amplification, and induced displacement. Nevertheless, observation of amplification patterns can play an important role in seismic slope stability analyses, as: 1) strong amplification (related to material contrasts or compliant fractures) is an indicator of potentially large co-seismic displacements; and 2) amplification patterns can be used to constrain geological and numerical models used for seismic stability analysis. The complexity of wave–slope interactions, as well as the potential to severely underestimate hazard using Newmark-type methods, motivates use of rigorous numerical modeling for quantitative seismic hazard and risk assessment of large landslides.

Hydraulic fracturing is the primary means for enhancing rock mass permeability and improving well productivity in tight reservoir rocks. Significant advances have been made in hydraulic fracturing theory and the development of design simulators; however, these generally rely on continuum treatments of the rock mass. In situ, the geological conditions are much more complex, complicated by the presence of natural fractures and planes of weakness such as bedding planes, joints, and faults. Further complexity arises from the influence of the in situ stress field, which has its own heterogeneity. Together, these factors may either enhance or diminish the effectiveness of the hydraulic fracturing treatment and subsequent hydrocarbon production. Results are presented here from a series of two-dimensional (2-D) numerical experiments investigating the influence of natural fractures on the modeling of hydraulic fracture propagation. Distinct-element techniques applying a transient, coupled hydromechanical solution are evaluated with respect to their ability to account for both tensile rupture of intact rock in response to fluid injection and shear and dilation along existing joints. A Voronoi tessellation scheme is used to add the necessary degrees of freedom to model the propagation path of a hydraulically driven fracture. The analysis is carried out for several geometrical variants related to hypothetical geological scenarios simulating a naturally fractured shale gas reservoir. The results show that key interactions develop with the natural fractures that influence the size, orientation, and path of the hydraulic fracture as well as the stimulated volume. These interactions may also decrease the size and effectiveness of the stimulation by diverting the injected fluid and proppant and by limiting the extent of the hydraulic fracture.

The recent introduction of ground-based slope stability radar in open-pit mines to complement conventional geodetic monitoring programs provides near real-time deformation measurements over a broad coverage area; this allows geotechnical engineers to observe the spatial distribution of pit wall movements and their progression over time. This paper presents a newly proposed early warning time-of-failure (TOF) analysis procedure for use in real-time with ground-based radar measurements designed to be integrated in an open-pit mine’s trigger action response plan (TARP). The inverse-velocity and slope gradient (SLO) TOF analysis methods are applied to radar displacement measurements using a new systematic multi-pixel selection technique termed the “percent deformation method.” The utilization of the percent deformation method in the proposed real-time TOF analysis methodology gives more-reliable results than current practice by providing recommendations for pixel selections, data filtering, where and how to undertake TOF analyses, and presenting TOF results in real time. The addition of a more rigorous, methodical treatment of radar monitoring data when faced with critical slope instability will reduce uncertainty and increase confidence in any trigger action response decisions, helping to ensure a safer work environment.

Production efficiency from low permeability shale gas reservoirs requires techniques to optimize hydraulic fracture (HF) completions. This may be complicated by the presence of high horizontal in-situ stresses that result in horizontal HF, for example in parts of the Western Canadian Sedimentary Basin in northeastern British Columbia. One strategy involves the simultaneous or near simultaneous hydraulic fracturing of adjacent lateral wells to maximize the fracture network area and stimulated reservoir volume. However, changes to the in-situ stress field caused by an earlier HF on subsequent HF are not accounted for in traditional hydraulic fracturing design calculations. Presented here are the results from a set of transient, coupled hydro-mechanical simulations of a naturally fractured rock mass containing two wellbores using the discontinuum-based distinct-element method. The results demonstrate the influence of stress shadows generated by a HF on the development of subsequent HF from an adjacent well. It is shown here that these interactions have the potential to change the size and effectiveness of the HF stimulation by changing the extent of the induced fracture around the secondary well. Also, the influences of in-situ stress and operational factors on the stress shadow effect are investigated and their effects on different operational techniques are studied.

Fault slip and associated seismicity present a significant risk to the safety of underground mines. This paper investigates whether hydraulic stimulation, defined as the injection of pressurised fluid into a rock mass in advance of mining, has the potential to mitigate such risks. Fluid injected in the proximity of a criticallystressed pre-existing fault plane decreases the effective normal stress acting on the fault, initiating slip. Based on this concept, hydraulic stimulation could be used to trigger the release of built-up strain energy in advance of mining. Numerical investigations are carried out in a two-dimensional distinct element code to model the effect of injection on the slip of a pre-existing fault located adjacent to a conceptualised open stope mining operation. Injection is accompanied by slip and a significant decrease in shear stress acting on the fault. When no injection is performed, excavation triggers a series of slip events in response to the changing abutment stresses, generating a maximum Mw 1.3 event. Conversely, when hydraulic injection is performed prior to the mine excavation sequencing, the total number of slip events is reduced, and a lower maximum magnitude of Mw 1.1 is observed. Longer injection durations are observed to trigger larger areas of slip and greater total shear displacements. However, a consequence of injection activities is that stress becomes concentrated outside of the fluid propagation front, potentially triggering seismicity in areas where it was not originally anticipated. Keywords: fault slip, induced seismicity, hydraulic injection, numerical modelling, rockbursts, shearing

Slope monitoring plays an important role in the risk management of large open pit slopes. Historically, displacement data derived from measuring geodetic prisms have been relied upon to delineate the boundaries of potential slope hazards; however that data can be limited by its point-measurement nature. Localized displacements at each prism may be misinterpreted when extended to the behavior of the entire slope, and important displacements between prisms may be overlooked. New technologies like ground-based radar can provide high resolution, full area coverage of a slope in combination with near real-time acquisition and millimeter precision. As a line-of-sight instrument, these tools provide data on displacement magnitudes and rates, but not true direction hence limiting their use in gaining understanding of the kinematics and behavior of the moving slope. This paper describes a novel experiment in which two ground-based synthetic aperture radar systems were simultaneously deployed to record continuous, line-of-sight displacements of an open pit slope in “stereo”. The displacement vectors were combined to create a pseudo 3-D displacement map for the slope, which was subsequently used to interpret the influence of a major fault and the rock mass fabric in promoting different kinematic responses. The data collected demonstrates that an improved understanding of the 3-D kinematics of a large rock slope can be achieved using advanced state-of-the-art monitoring techniques to aid mine design.

The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 10 � 17 –10 � 15 m 2 at targeted depths for enhanced geothermal systems (EGS) applications, that is, >3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability. In the mining sector, fluid injection methods are being investigated to increase rock fragmentation and mitigate high-stress hazards due to operations moving to unprecedented depths. Here as well, detailed understanding of permeability and its enhancement is required. This paper reports findings from a series of hydromechanically coupled distinct-element models developed in support of a hydraulic fracture experiment testing hypotheses related to enhanced permeability, increased fragmentation, and modified stress fields. Two principal injection designs are tested as follows: injection of a high flow rate through a narrow-packed interval and injection of a low flow rate across a wider packed interval. Results show that the development of connected permeability is almost exclusively orthogonal to the minimum principal stress, leading to strongly anisotropic flow. This is because of the stress transfer associated with opening of tensile fractures, which increases the confining stress acting across neighboring natural fractures. This limits the hydraulic response of fractures and the capacity to create symmetric isotropic permeability relative to the injection wellbore. These findings suggest that the development of permeability at depth can be improved by targeting a set of fluid injections through smaller packed intervals instead of a single longer injection in open boreholes.

Natural fragmentation is a function of the fracture length and connectivity of naturally occurring rock discontinuities. This study reviews the use of a Discrete Fracture Network (DFN) method as an effective tool to assist with fragmentation assessment, primarily by providing a better description of the natural fragmentation distribution. This approach has at its core the development of a full-scale DFN model description of fracture orientation, size and intensity built up from all available geotechnical data. The model fully accounts for a spatially variable description of the fracture intensity distribution. The results suggest that DFN models could effectively be used to define equivalent rock mass parameters to improve the predictability achieved by current geomechanical simulations and empirical rock mass classification schemes. As shown in this study, a mine-scale DFN model could be converted to equivalent directional rock mass properties using a rapid analytical approach, allowing the creat...

Footwall slopes refer to unbenched rock slopes in which the slope face is parallel to a set of persistent discontinuities (e.g. bedding planes, foliation, faults). These are commonly encountered in weak, thinly bedded, orthogonally jointed, sedimentary rock sequences. Common failure mechanisms include bi-planar failures where shallow dipping crosscutting structures daylight near the slope toe, enabling sliding to occur along steep dipping bedding planes. In the absence of crosscutting structures, failure occurs through deformation and rock mass yielding involving the formation of inter block shear and toe breakout surfaces. Because of the complexity of the toe breakout mechanism, evaluation methods are not well understood. An improved understanding of the failure mechanism, the role of adverse discontinuities, and characterization of the discontinuity, intact rock and rock mass strength properties are key for a successful footwall stability analysis. This paper investigates the development of the inter block shear and toe breakout surfaces with three approaches: i) continuum-based frictional plasticity theory; ii) discontinuum-based distinct-element modelling with Voronoi tessellation using the commercial software UDEC; and iii) hybrid continuum/discontinuum finite-/discrete-element brittle fracture modelling using the commercial software ELFEN. Numerical simulations using ELFEN and UDEC demonstrated a good agreement with frictional plasticity theory. Ploughing failure of footwall slopes is also evaluated, specifically the influence of cross-cutting discontinuity dip angle relative to the slope face. The effects of different geometrical parameters (e.g., slope angle and depth/height ratio) on bi-planar and ploughing failure are assessed using a sensitivity analysis approach. A “Damage Intensity” parameter is introduced and used to quantify damage in the numerical simulations using ELFEN.

This paper reports the findings from a benchmark study testing several numerical methods, with a focus on the influence of undercut depth on block caving-induced surface deformation. A comparison is drawn between continuum v. discontinuum treatments of the modelled geology. Results were evaluated with respect to different simulated levels of ground disturbance, from complete collapse to small-strain subsidence. The results show that for a given extraction volume, the extent of ground collapse at surface decreases as undercut depth increases. The presence of sub-vertical faults was seen to limit the extent of the modelled caving zones. In contrast, the extent of small-strain surface subsidence was seen to increase with increasing undercut depth. The faults in this case did not have the same limiting effect. Overall, the findings emphasise the importance of balancing model simplification against the need to incorporate more complex and computationally demanding representations of the rock mass struc...

Several operations are considering the transition from surface mining to underground block caving to access deeper resources. Depending on the geometry of the orebody, the undercut may be positioned beneath the foot of a large open pit slope, or behind its crest. The latter scenario also arises where a natural rock slope is present. Results are reported here from a numerical modelling study investigating the mechanics of deep-seated slope displacements in response to caving. Different failure models are investigated as a function of the orientation of the jointing pattern relative to the location and progressive development of the block cave. A 2-D discontinuum modelling approach is utilised based on the distinct-element method. The results show that the cave location and the resultant strain field, plays a significant role in the rock mass interactions that develop and the subsequent kinematic response of the slope with respect to translational, rotational and toppling behaviour.

Hydraulic fracturing is today the primary means of initiating, increasing, and maintaining well productivity in unconventional shale gas reservoirs. However, hydraulic fracturing is attracting public concern over its rapid growth in use and environmental footprint, including potential hazards such as induced seismicity. To mitigate these concerns, there is a need to better understand the level of hazard present with respect to the magnitudes of the events possible, recognizing that these are not equal across different shale gas plays due to regional differences in the geological conditions or even within the same shale gas play due to local differences. An empirical study is presented here along with a series of numerical simulations to investigate the influence of the tectonic stress regime on the magnitude and magnitude distribution of induced seismicity events related to hydraulic fracturing practices. A database of determined earthquake focal mechanisms was first used to determine the tectonic stress regime for different North American shale gas basins. Next, induced events associated with hydraulic fracturing operations were identified to determine the magnitude distributions and b-values for these different shale gas basins. To support these empirical analyses, 3-D numerical modelling was performed to further investigate the mechanistic responses and event magnitudes under different simulated stress regimes. The empirical analysis results show that thrust faulting stress regimes have lower b-values than strike-slip stress regimes and therefore are more susceptible to larger induced seismicity events. The numerical simulations show that this is related to the stresses acting on a thrust fault (relative to the fault dip angle) being higher and more concentrated across a larger slip area. Slip in this case, and the stored strain energy being released, was observed to occur as a single large magnitude event. In contrast, numerical simulations for the other faulting types showed the stresses to be more distributed across the fault plane. This results in multiple slip events involving smaller slip areas and therefore with smaller event magnitudes, assuming all other factors are kept the same.

In this paper, the authors review recent applications of an integrated numerical modeling approach based on the analysis of the mechanical behavior of discrete systems. The numerical analysis includes both a more realistic representation of fracture networks and the simulation of rock mass behavior as a combination of failure through intact rock and displacement/rotation along predefined discontinuities. Selected examples are presented with respect to a variety of engineering problems, including shear testing, failure of hard-rock pillars, slope stability, and block/panel cave mining. The results clearly illustrate the importance of including natural jointing to better capture rock mass behavior in response to loading and unloading. Particular emphasis is given to modeling cave development and surface subsidence, and the proposed numerical method is shown to capture fully the complex rock mass response to caving associated with multi lift extraction. Whereas the use of relatively complex numerical...

For guidance on relationships between caving depth and surface subsidence, a comprehensive database was developed after an exhaustive search of published data from cave mining operations from around the world. The distribution of data was found to largely focus on caving angles and macro deformations; very little empirical data exists on the extent and magnitudes of smaller surface displacements. The data clearly show that caving-induced surface deformations tend to be discontinuous and asymmetric due to large movements around the cave controlled by geologic structures, rock mass heterogeneity and topographic effects. The data also show that as undercut depth increases for a given extraction volume, the magnitude and extent of the caved zone on surface decreases. However, numerical modeling indicates that this is only the case for macro deformations and the extent of smaller displacements actually increases as a function of undercut depth. The results presented caution against relying on existing empirical design charts for estimates of caving-induced subsidence where small strain subsidence is of concern, as the data being relied upon does not properly extrapolate beyond the macro deformations (i.e., caving angles) that make up the majority of the observations. The findings also suggest that the extent and magnitudes of subsidence may be underestimated if the analysis adopted neglects the influence of geological structures and assumes symmetrical surface displacements above the undercut.

List of Symbols r1 Major principal stress r3 Minor principal stress Co Uniaxial compressive strength mi Hoek–Brown material constant (intact rock) mb Hoek–Brown material constant (rock mass) s Hoek–Brown material constant a Hoek–Brown material constant GSI Geological Strength Index D Disturbance factor To Uniaxial tensile strength r3max Upper limit of confining stress r Coefficient of determination

Block caving often leads to significant ground deformations that if not properly assessed and accounted for may threaten the integrity and safety of overlying mine infrastructure. To mitigate this risk, sophisticated 3-D numerical modelling has been turned to as a means of predicting the extent and magnitudes of caving-induced surface subsidence. However, the complexity of the rock mass interactions involved, coupled with the uncertainty associated with geological heterogeneity, rock mass properties and in-situ stresses results in the need for models to be constrained and calibrated. Results are first presented here from a detailed 3-D back analysis of a caving-induced open pit mine slope failure, used to constrain the rock mass properties and far-field in-situ stresses derived from field characterisation data, as well as to bring understanding to the problem with respect to the cave-pit interactions. The “best fit” set of input properties obtained was then used for forward modelling of caving-induced subsidence for the period 2009–2010. Further calibration of this model was performed using high-resolution InSAR monitoring data. The close fit achieved between the predictive 3-D numerical model and InSAR data demonstrates the promise of InSAR as a means to calibrate sophisticated numerical models, and thereby contribute to managing block caving associated subsidence hazards.

Uncertainty is inherent in geotechnical design. In regard to estimating the stability state of dip slopes, most of the uncertainty lies in the geologic model assumed and the geotechnical parameters used in the evaluation. Biplanar (or active-passive) sliding in dip slopes occurs along a slope-parallel sliding surface with toe breakout occurring at the base of the failure. Internal shearing is required to facilitate kinematic release. All three of these release surfaces work together for the slope to fail, but with different degrees of importance depending on the dip-slope inclination. Increased efficiency and value with respect to the site investigation resources can be gained by working toward minimizing the uncertainty of those parameters that have the greatest bearing on the outcome of the slope stability analysis. This can be done quickly and inexpensively by performing scoping calculations facilitated by the use of Spearman rank correlation coefficients. This paper demonstrates that for shallow-dipping dip slopes, stability is primarily dictated by the shear strength of the slope-parallel sliding surface, and therefore, efforts should be focused on constraining the shear strength of this surface. For steep dip slopes, the shear strength related to the toe breakout and internal shear release surfaces becomes dominant, and therefore, the rock-mass shear strength and that for any adversely dipping persistent discontinuities should be the focus of the geotechnical investigation. DOI: 10.1061/(ASCE)GT.1943-5606.0000515. © 2012 American Society of Civil Engineers. CE Database subject headings: Slope stability; Uncertainty principles; Rocks; Rock masses; Site investigation. Author keywords: Dip slope; Uncertainty; Rock engineering; Reliability; Rock-mass rating; Geological strength index.

This paper reports a detailed rock slope hazard investigation of the Portillo Rock Avalanche site located in the rugged mountains of the Andean Cordillera of central Chile. The site is important as it lies along the International Santiago–Mendoza Highway Corridor connecting Chile and Argentina, and a major ski resort is located on its deposits. A number of large lobate-shaped diamicton deposits were mapped and dated by cosmogenic nuclides ( 36 Cl), with the results showing that they correspond to two significant prehistoric rockslide events. An integrated field mapping and numerical modelling investigation was subsequently carried out to assess the threat posed to the area by further rocksliding activity. Distinct-element modelling was used to back analyze the failure mechanism and identify the geological model that best reproduced the Portillo Rock Avalanche failure surface. Results show that a stress-controlled failure at the toe of the slope followed by sliding along volcaniclastic bedding was the likely failure mechanism. A 3-D dynamic runout analysis was carried out to back analyze which combinations of rheologies, material properties and rockslide sequencing were best able to reproduce the current distribution of rockslide deposits. Results indicate that two separate sliding events originating from different sources had occurred, with each involving different combinations of frictional and Voellmy rheologies depending on the level of entrainment that occurred along each travel path. Insights gained from the back analysis were then used to carry out a forward analysis to assess the potential for a recurring major rockslide under several different triggering scenarios. Results suggest that a low probability M 7.8 earthquake would be required to trigger another rockslide from the original source area. The rock slope was otherwise found to be stable, even following high precipitation events. Nevertheless, runout simulations for the estimated large-magnitude earthquake-triggered rockslide volume showed that for both a highly frictional and non-saturated path (i.e. dry season) and a snow covered path (winter), the leading edge of the flow would override part of the International Santiago–Mendoza Corridor with the debris coming to rest in a flat-lying area in the upper part of the valley. Overall, the results from this integrated hazard assessment suggest that the hazard level is low.

This paper presents the geometry, kinematics and temporal deformation characteristics of a deep-seated rockslide system, the “Hochmais–Atemkopf”, situated above the Gepatsch dam reservoir in Northern Tyrol, Austria. Results from surface and subsurface geological investigations and deformation monitoring indicate that the Hochmais–Atemkopf rockslide system involves several sliding masses, one on top of the other, characterized by different velocity characteristics with displacements being greater for the shallower slide bodies. During the initial impounding phases of the Gepatsch reservoir, uplift forces beneath the foot of the slope led to the activation of one of these shallower slide bodies, moving it more than 10 m downslope in 2 years. After continuous deceleration of the sliding mass, the deformation rates reduced to about 2 to 4 cm per year. These were found to show seasonal fluctuations that correlated with reservoir levels and drawdown conditions, with induced slope accelerations peaking when reservoir levels were at their lowest. This suggests, in part, a controlling mechanism based on seepage forces where reservoir drawdown drives the episodic rockslide deformation behaviour. Together, the data and analyses presented demonstrate the importance of integrating detailed geology and monitoring data to derive a basic understanding of the kinematics and controlling mechanisms of a deep-seated rockslide system in advance of undertaking comprehensive numerical modelling.

Hazard assessments involving large rock slopes are often problematic, given the influence of geology on failure kinematics and the subsequent influence of the failure kinematics on the rockslide runout. The 2003 Afternoon Creek rockslide in northwest Washington is one such example, where 750 000 m3 of rock slid from a steep ridge harmlessly into Afternoon Creek. However, topographic and structural controls at the source area unexpectedly redirected a small volume (

This paper presents a detailed field investigation and hazard assessment for an unstable rock slope above an industrial site in the Riviera valley in the Canton of Ticino in southern Switzerland. An integrated framework was used to counter issues of geological complexity and uncertainty, linking geological mapping and numerical modeling to develop an understanding of the acting instability mechanism to 2-D and 3-D dynamic runout simulations to predict the travel path and reach in the event of a large volume rockslide. The results from the numerical stability analysis provided a means to constrain failure volume estimates, whereas a series of calibration simulations were used to constrain the input parameters required by the rheological model used for the runout analysis. Results from this assessment suggest that current protection measures in place may not be sufficient, helping local authorities to define hazard zones and aid further development plans for the region.

Fractures and the networks they form have a major impact on the mechanical, thermal and hydraulic behaviour of a rock mass. For this reason, practically all geotechnical engineering projects conducted in fractured rock require quantitative descriptions of the properties of the fractures. A case in point is the project that motivated this study: the simulation of the surface subsidence resulting from the drawdown in pore-pressure about a deep tunnel in crystalline rock due to drainage into the tunnel [1,2]. Hydro-mechanically coupled discontinuum models of the medium required properties to be assigned, amongst which was the stress-dependent normal stiffness characteristics. Since there were no estimates of such for the fractures within the rock mass in question, a survey of published studies of normal stiffness was undertaken to guide the model parameterisation. Most studies measured fracture closure as a function of a change in applied effective normal stress. In laboratory experiments, the change was usually cyclical. A key problem in rendering such diverse data to be collectively interpretable is posed by the fact that the normal stiffness of fractures, kn, is strongly stress dependent, particularly at low effective normal stress levels (Fig. 1). This means that the closure-normal stress curves must be approximated by some function that fits the curves, preferably with as few free parameters as possible.

Surface subsidence associated with tunnelling in fractured crystalline rock masses is rarely considered to be large enough to be a cause for concern. Recent high precision levelling measurements along the Gotthard Pass road in Central Switzerland, however, have revealed up to 12 cm of subsidence along sections that pass several hundred metres above the Gotthard highway tunnel. Large-scale consolidation associated with pore-pressure reduction in the rock mass arising from tunnel drainage is believed to be the contributing mechanism. Although these settlements may appear to be small compared to those associated with groundwater or oil and gas withdrawal from more compliant porous media, they are large enough to adversely affect the structural integrity of sensitive concrete structures on the surface (e.g. thin-arch concrete dams). This is a concern for the 57 km long Gotthard Base Tunnel, which is currently under construction, as it will pass through similar geological conditions as the Gotthard highway tunnel and underneath several important dams. The prediction of the expected settlements requires a more complete understanding of the processes underlying the settlements than exist at present. This paper (Part 1) and a companion paper (Part 2) are a contribution towards this end. In Part 1 we examine the question of consolidation in fractured crystalline rock, introduce the geodetic data and the geological setting of the subsidence associated with the Gotthard highway tunnel. Possible mechanisms for the consolidation are described and estimates obtained for the key hydro-mechanical parameters that govern their action, such as the normal stiffness of fault zones. These data are used to condition analytical scoping calculations of expected subsidence based upon poro-elastic theory, and to provide input for a series of numerical models used to obtain a more detailed understanding of the data, which are presented in Part 2. r 2008 Elsevier Ltd. All rights reserved.

A recent high precision levelling survey several hundred metres above the Gotthard highway tunnel in central Switzerland has revealed up to 12 cm of subsidence. Subsidence of this magnitude in relation to a deep tunnel excavated in fractured crystalline rock is unexpected and appears to be related to large-scale consolidation resulting from groundwater drainage and pore-pressure changes around the tunnel. This is a concern for the 57 km long Gotthard Base Tunnel currently under construction, as its alignment will pass through similar rock mass conditions and under several important concrete dams. Thus, the assessment and prediction of potential surface displacements are of paramount importance. This paper, the second of two parts, presents results from an extensive and thorough numerical modelling study focussing on the hydro-mechanical processes responsible for the measured subsidence above the Gotthard highway tunnel. Results derived from 2-D continuum and discontinuum numerical models (i.e. finite- and distinct-element, respectively) show that discrete fracture deformation and poroelastic consolidation of the intact rock matrix both contribute to the observed subsidence. Moreover, the explicit inclusion of geological structures in the distinct-element models enabled a better fit of the width and shape (asymmetry, small-scale inflections, etc.) of the measured subsidence profile to be achieved. Continuum models, although able to reproduce the maximum settlement when constrained by field observations, could not reproduce the asymmetric shape of the subsidence profile leading to under prediction of vertical displacements away from the centre of the subsidence trough.

6m 3 of material, occurred in fractured gneissic rock whose principal foliation, faults and fracture zones dip into the slope. Above the scarp of the second rockslide, the mountain continues to move at rates of up to 2 cm/year. In an attempt to understand failure mechanisms in fractured crystalline rock masses, we have carried out a comprehensive multidisciplinary study that included detailed geological mapping, borehole logging and imaging, and surface and borehole geophysical surveys. An integrated interpretation of the resultant data has revealed the presence of three prominent fault- and fracture zone sets that define the internal structure of the rock mass. Some faults and fracture zones could be traced up to 85 m into the subsurface using borehole radar reflection methods. The dips of the faults and fracture zones estimated from surface mapping matched those from radar reflection and borehole televiewer surveys. No evidence was found for significant faults or persistent weakness planes that paralleled the 1991 rockslide failure surfaces. Instead, the new observations were consistent with dominant failure mechanisms that involve the progressive development of major shear and brittle fracturezones and the eventual destruction of intact rock bridges immediately prior to catastrophic collapse. This integrated study provides the first reliable model of the 3-D fault structure associated with and contributing to a large rock slope instability in a brittle crystalline rock mass.