Slope stability assessment of an open pit using lattice-spring-based synthetic rock mass (LS-SRM) modeling approach (original) (raw)
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Stability Analysis and Failure Mechanisms of Open Pit Rock Slope
Journal of the Civil Engineering Forum
Rock mass in nature tend to be unideal, for it is heterogeneous, anisotropic and has discontinuity. The discontinuity makes anisotropic strength and stress in the rock mass, and also controls the changing of the elastic properties of rock mass. This condition results to disruptions in the rock mass strength balance, and finally drives the slopes to collapse. This study aims to determine the slope failure mechanisms in the area of case study, as well as its variations based on the Rock Mass Rating (RMR), Geological Strength Index (GSI), Slope Mass Rating (SMR), kinematic analysis, numerical analysis and monitoring approach slope movement in a coal mine slope applications. The site investigations were implemented to obtain information about slope collapse. Prior to the collapse, the slope inclination was 38° with of 94 meters height, strike slope of N 245 E and direction of slope surface of 335°. After the collapse, the slope was became 25º; and after the collapse materials were clear...
Numerical Modelling and analysis of slope stability within fracture dominated rock masses
Numerical modelling of rock slopes can involve a number and variety of techniques, the selection and requirement of which depends on the factors deemed to control the potential for instability. This thesis presents a number of case studies involving slopes in fractured rock, encompassing a range of scales. The case study slopes have provided a means to question the way in which particular slope instabilities should be analysed. Currently there are few methods available for analysing the complex behaviour within slopes of fractured rock. A review of available techniques is given within this thesis, with the use of limit equilibrium, finite element and hybrid methods, to highlight their specific advantages and limitations for the chosen case study slopes. By modelling slope instability within fractured rock, the understanding of both discrete and mass behaviour increases considerably. Numerical modelling can therefore be used as a tool to help improve both the safety and efficiency of open pit mining and the management of natural rock slopes. The emphasis of the numerical modelling used in this thesis, was to assess the ability of a particular comprehensive dynamic-based code, ELFEN, for modelling fractured rock slopes. In addition, a principal objective of the research was to test the newly developed groundwater version of the code. Investigations revealed ELFEN to be effective for simulation of fracture extension due to the decreased normal stress on discontinuities, relative to pore pressure. In general, the code has the ability to simulate the full failure process in small to medium-scale slopes, providing a means to analyse rock mass and discontinuity strength, along with a representation of the failure mechanism from initiation through to deposition. At a large scale the sheer complexity of a fractured rock mass makes it impossible to model the whole slope as a representative discrete object with an embedded detailed fracture network. Subsequently an approach is presented in this thesis, whereby one can use numerical modelling to arrive at a mass strength estimate that can be used in a simpler equivalent continuum model of a large slope. Groundwater pressure was initially applied in a simple planar failure model, to provide confidence in the capability of the newly developed effective stress module within ELFEN. Following this, groundwater was implemented into two step-path failures. This highlighted the sensitivity of the specific models to the level of the phreatic surface, rock-bridge strength and discontinuity related strength. In addition, a fully drained toe-breakout failure was addressed, using various limit equilibrium and finite element methods to assess the potential strength of a rock-bridge within the toe of a 50m slope. In all numerical models it is necessary to be certain of input parameters, or to understand the implications and effects of any uncertainty. During this thesis an accumulative scheme or modelling methodology has been followed; starting with simple models so that comparisons can be made with other limit equilibrium and finite element methods, allowing calibration of the more advanced properties required within ELFEN. This calibration is made easier with the use of a staged modelling procedure. In particular it was found that, when using a dynamic-based code with fracture capabilities, an inappropriate model procedure can lead to an unrealistic simulation. In summary, particular contributions and novel aspects of the research were: i. The application of ELFEN to a variety of scale-related failures in fractured rock slopes, covering a range of failure mechanisms. In addition, direct comparisons have been made between the results of ELFEN, limit equilibrium and finite element methods, for the chosen case study slopes. This has provided an analysis and initial review of the capabilities and limitations of each of the individual approaches. ii. The newly developed groundwater version of ELFEN was tested for the first time in three of the six case study slopes, demonstrating its effectiveness in simulating mode I fracture extension and subsequent slope instability, due to a rise in the phreatic surface. iii. The development of a suitable staged approach methodology by which a fractured rock slope can be simulated efficiently and accurately, when using a dynamic fracture-based code. iv. The simulation of a large-scale case study slope using a FracMan-ELFEN approach, whereby a statistically generated fracture network is explicitly incorporated into a numerical model and mass strength is assessed on a large scale, deriving strength properties that represent an equivalent continuum of the fractured mass. Subsequently, a number of approaches were used to assess the strength of the equivalent continuum that formed a 1000m slope. This led to the comparison of the numerical and empirically derived mass strength approach for modelling of slopes.
Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering
Confidence in stability assessments of large rock slopes may be improved by greater understanding the persistence of adverse discontinuities, and the proportion and location of intact rock bridge content within the slope. This paper presents a discussion of the challenges and uncertainty in characterising discontinuity persistence and intact rock bridges, with reference to results from field investigations of open pit slopes at three mines using digital photogrammetry, ground-based LiDAR, and modified 2D window mapping methods. A conceptual numerical model is then devised, where a distinct element numerical code was applied to investigate the influence of rock bridges on brittle rock mass failure and dilation in a model large open pit slope. Distinction between co-planar or out-of-plane intact rock bridges, and larger 'rock mass bridges' between more persistent discontinuities is considered necessary and the authors suggest that a fracture network engineering approach tailored to large open pits may be helpful for their characterisation. With modified trace mapping procedures, intact rock bridges may be quantified in terms of an intensity parameter R 21 that describes the total length of inferred rock bridge traces per unit area within a mapping window. An analogous blast-induced damage intensity factor B 21 is also introduced, that describes the total length of blast-induced fracture traces per unit area in a mapping window. For numerical models, a damage intensity parameter D 21 is applied, which quantifies the intensity of fracturing that develops inside a modelled slope. Large rock slope failures rarely occur entirely along completely continuous, pre-existing basal sliding surfaces. Even if major pre-existing structures exist, deformation and failure of large slopes in hard rock is more likely to involve a combination of shearing and dilation of pre-existing discontinuities such as joints, with a degree of stress-induced brittle fracturing of intact rock (Sjöberg, 1999). The process of brittle crack initiation, propagation and coalescence is progressive (Eberhardt et al., 2004), and may be characterised by a time-dependent degradation of rock mass strength in localised zones of stress concentration, that may eventually lead to (1) the formation of a continuous sliding surface and (2) the development of kinematic freedom and finally slope failure (Stead et al., 2006). The potential complexity of slope failure mechanisms increases with the scale of the slope. In open pit mines, inter-ramp or overall slope failure surfaces may have irregular or step-path geometry, involving rupture through several structural domains with different shear strength properties and different local failure mechanisms. McMahon (1979) introduced the step-path simulation method during investigations for the Bougainville open pit mine in Papua New Guinea with the probabilistic STEPSIM code. Later, the probabilistic step-path simulation approach was further developed for a slope optimisation study at Ok Tedi mine, resulting in the STEPSIM4 code (Little et al., 1999; Baczynski, 2000). The STEPSIM code considered that a global slope failure surface could include up to five domains with different shear strength (Figure 1). https://papers.acg.uwa.edu.au/p/1308\_07\_Tuckey/ Combining field methods and numerical modelling to address challenges in characterising Z. Tuckey et al. discontinuity persistence and intact rock bridges in large open pit slopes
Arabian Journal of Geosciences
The slope stability analysis has a key role in the design, excavation, and risk prediction during a variety of rock engineering projects. The rock mass surrounding a surface excavation is inherently discontinuous, comprising rock blocks with types of fractures along their boundaries. In general, there are two ways to analyze such a medium: the discontinuum approach and the continuum approach. However, systematic investigations have rarely been carried out on the comparison between the results of these two approaches and the impact of treating a rock mass as a continuous medium. The main objective of this research is to find an answer to the question of how far the discontinuum assumption is essential for rock slope stability analysis. In this study, the topic was addressed in form of a case study. The mechanical and geometrical characteristics of rock blocks and discontinuities in the northern wall of the Golgohar iron ore mine (Iran) were considered. The Hoek-Brown equations were used to find the equivalent properties of rock masses. Then, a three-dimensional numerical model of the mine wall was made and analyzed using the both discontinuum approach and continuum approach. The safety factor for the discontinuum analysis was smaller than one (i.e., 0.84) not because of the overall instability, but due to the slide of small blocks over mine benches. The continuum modeling was not capable of simulating the local slip; hence, the safety factor in this method was calculated to be 2.31. For the first time, a new measure was proposed for the overall instability of rock structures, defined as the average displacement weighted by block volume (d *). The results demonstrated that the equivalent continuum model, in spite of having a greater safety factor, had a larger d * value compared to the discontinuum model. The results show that the equivalent continuum approach for the jointed rock masses studied here is neither capable of describing the instability of mine benches nor provides an assessment of the pit's overall behavior.
Recent advances in the characterization of complex rock slope deformation and failure using numerical techniques have demonstrated significant potential for furthering our understanding of both the mechanisms/processes involved and the associated risk. This paper illustrates how rock slope analyses may be undertaken using three levels of sophistication. Level I analyses include the conventional application of kinematic and limit equilibrium techniques with modifications to include probabilistic techniques, coupling of groundwater simulations and simplistic treatment of intact fracture and plastic yield. Such analyses are primarily suited to simple translational failures involving release on smooth basal, rear and lateral surfaces where the principle active damage mechanisms are progressive failure and/or asperity breakdown. Level II analyses involve the use of continuum and discontinuum numerical methods. In addition to simple translation, Level II techniques can be applied to complex translational rock slope deformations where step-path failure necessitates degradation and failure of intact rock bridges along basal, rear and lateral release surfaces. Active damage processes in this case comprise not only strength degradation along the release surface (e.g., asperity breakdown) but also a significant component of brittle intact rock fracture. Level III analyses involve the use of hybrid continuumdiscontinuum codes with fracture simulation capabilities. These codes are applicable to a wide spectrum of rock slope failure modes, but are particularly well suited to complex translation/rotational instabilities where failure requires internal yielding, brittle fracturing and shearing (in addition to strength degradation along release surfaces). Through a series of rock slope analyses the application of varied numerical methods are discussed. Particular emphasis is given to state-of-the-art developments and potential use of Level III hybrid techniques.
2015
Designing reliable slopes that provide safety and maximize financial return represent one of the main challenges in a mining operation. As open pits get increasingly deeper, the need to better understand the behavior of high rock slopes has become more critical. Currently, it is not clear how mining at increased depths may impact slope behavior. Similarly, the impact of in-situ stress magnitudes in slope stability is still uncertain. High horizontal stresses and increased depths can lead to unfavorable stress conditions, inducing rock mass damage and strength loss. The main goal of this research is to assess the effect of increased mining depth and in-situ stresses on slope stability. Reliable slope behavior predictions require an adequate knowledge of the local pre-mining stress setting, together with suitable numerical tools capable of capturing the brittle characteristics of rock masses. In this research, the response of the rock mass was studied using both a FEM and the Slope Mo...
Stability Analysis of a Large Fractured Rock Slope Using a DFN-Based Mass Strength Approach
The global trend towards larger open pits and block cave mining is reliant on effective design at larger scales requiring improved estimates for rock mass strength. In recent years, efforts have focussed on developing synthetic Discrete Fracture Network (DFN)-based rock mass approaches. This paper provides an example of such an approach, modelling explicitly the rock fracture network within large-scale biaxial models. A conceptual large rock slope is analysed and comparisons are made with more conventional empirical strength assessments. Finally the DFN-based mass strength was modelled within a finite element solution, providing stress paths that illustrated the onset of instability during the excavation of a large slope. This example provides an insight into some of the parameters that influence the strength for a DFN-based method. In this case, fracture network geometry, strain and confinement were particularly important. The DFN-based method also demonstrated that fracture intensity has a greater influence on mass strength in low strain environments. These factors all have relevance for the determination of mass strength by any method. The Middleton Limestone was used as the basis for a conceptual case study; in this case a reduction in intact strength was necessary to allow the development of a circular slope failure mechanism. Suggestions for further research are made which are considered important if DFN-based methods are to be used for analysing the stability of large slopes.
Characterisation of brittle rock fracture mechanisms in rock slope failures
The number of large open pits extending to increasingly greater depths requires an improved understanding of the role of brittle fracture in slope instability. It is important that the tools being used to predict slope behaviour in strong, fractured rock realistically consider the mechanics of intact rock failure. To date, stability assessments for strong fractured rock have been carried out using a variety of approaches, from simple limit equilibrium analysis to more complex numerical simulations. Where discrete structures control failure however, analyses using rock mass properties alone may be inappropriate and fractures need to be explicitly included in the models. The focus of this study is the failure mechanics of intact rock bridges that separate discrete structures within a potential failure surface. The scope is to provide the basis for effectively including the effects of intact rock bridge strength within standard slope stability analysis methods. A series of parametric studies is initially carried out using different numerical approaches to evaluate the effects of intact rock bridge geometry on slope behaviour. The second component of the study includes the construction of more realistic discrete fracture network models of rock slopes reflecting the discontinuous and heterogeneous nature of a fractured rock mass. It is hoped that the results of the current analysis will provide a significant advance in understanding slope behaviour and the validity of different analysis methods for evaluating stability. The results offer a practical approach to including the effects of intact rock bridge strength within standard slope stability analysis methods. Significant challenges presented by the explicit incorporation of rock structures in large-scale numerical models are also discussed.
Rock slope stability analysis using fracture systems
International Journal of Surface Mining, Reclamation and Environment, 2005
This paper presents a methodology whereby statistically representative fracture patterns can be used for an accurate representation of structural discontinuities in rock. This implies that field data can be used to generate characteristic fracture systems for a rock mass. It is then possible to introduce any range of slope geometry in the generated rock mass. The stability of such excavations can be evaluated using traditional limit equilibrium analysis. The advantage of this approach is that it can consider the influence of both large-scale (major) fractures whose relative location in the rock mass is well defined and minor fractures whose location and orientation is defined by probabilistic algorithms. This is demonstrated in this paper with reference to a rock slope susceptible to wedgetype failures.