Comparison of continuum-discontinuum approaches to stability evaluation of open pit slopes: introduction of new measure of instability (original) (raw)
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Block Model Approach for Analysis of Rock Bench Stability in Open Pit Mines
2010
Wedge failure analysis conventionally adopted for the design of global angle of open pit mines is not always applicable when the area consists of jointed rock benches. In this analysis, the unstable wedge is assumed intact while, in fact, the rock bench consists of an assemblage of blocks generated by the intersections of well-defined discontinuities. Block model approach adopted in Resoblok provides an alternative solution for the design and stability analysis of a rock bench. The model analyzes the stability of individual block by limit equilibrium method while the discontinuity aspect is considered statistically based on geometrical modelling of the individual fractured rock mass. This paper presents the stability analysis of five rock benches from two mining sites. The analysis shows that Resoblok model provides a good indication of the distribution, the number and the volume of unstable blocks; therefore, it is useful for the design of open pit mines. The result of this study i...
Journal of Rock Mechanics and Geotechnical Engineering, 2020
Discontinuity waviness is one of the most important properties that influence shear strength of jointed rock masses, and it should be incorporated into numerical models for slope stability assessment. However, in most existing numerical modeling tools, discontinuities are often simplified into planar surfaces. Discrete fracture network modeling tools such as MoFrac allow the simulation of non-planar discontinuities which can be incorporated into lattice-spring-based geomechanical software such as Slope Model for slope stability assessment. In this study, the slope failure of the south wall at Cadia Hill open pit mine is simulated using the lattice-spring-based synthetic rock mass (LS-SRM) modeling approach. First, the slope model is calibrated using field displacement monitoring data, and then the influence of different discontinuity configurations on the stability of the slope is investigated. The modeling results show that the slope with non-planar discontinuities is comparatively more stable than the ones with planar discontinuities. In addition, the slope becomes increasingly unstable with the increases of discontinuity intensity and size. At greater pit depth with higher in situ stress, both the slope models with planar and non-planar discontinuities experience localized failures due to very high stress concentrations, and the slope model with planar discontinuities is more deformable and less stable than that with non-planar discontinuities.
Evaluation of safety factors in discontinuous rock
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1996
Safety factors for kinematically admissible failure mechanisms in jointed rock masses have been defined with linear and nonlinear failure criteria for rock discontinuities. Data required to compute these safety factors are obtained by means of two finite element analyses of the effects of selfweight and external (structural) loading, respectively. Both types of analysis are closely linked since they share a common geometry. Joint elements are used to simulate the behaviour of rock discontinuities. If kinematically admissible mechanisms are possible under field conditions, the finite element mesh should also allow them to develop. Different aspects of the methodology have been illustrated through the safety evaluation of a 150m high arch dam and its foundation in fractured cretaceous limestone. Special attention has been paid to the modelling of a realistic geometry including three-dimensional rock blocks and discontinuities. The paper discusses the effect of initial state of stress, the evolution of safety as the external load increases and the relation between the defined safety factors. It also provides practical guidelines for conducting this type of analysis in complex situations.
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
The effect of discontinuity orientation on the stability of rock masses
The shear stresses generated in an excavated rock slope are usually very low in comparison with the shear strength of the intact rock material. However, existing discontinuities such as bedding and cleavage planes and joints, in rock masses may have sufficiently low shear strength to act as failure plane. Geometry together with the frequency of the discontinuities are then the dominant factors governing the potential mode of failure, namely: rotational, translational, or toppling mode. This paper discusses the effect of the orientation and shear strength of internal discontinuities in the sliding body on the stability of a slope rock mass. The factor of safety decreases if the inclination angle of internal discontinuities in a sliding body increases from dipping "with" to dipping "against" the slope face. Sensitivity analyses are given to explain the influence.
Probabilistic assessment of slope stability at ore mining with steep layers in deep open pits
Mining of Mineral Deposits
Purpose. A methodology development for predicting the geomechanical situation when mining an ore deposit with steep-dipping layers, taking into account the uncertainty in determining the rock properties, which is a consequence of the rock mass heterogeneity. Methods. The assessment of the open-pit wall stability is based on a combination of numerical simulation of the rock stress-strain state (SSS) and probabilistic analysis. The finite element method is used to determine the changes in the SSS that occur at various stages of mining operations due to design changes in the overall open-pit slope angle. The elastic-plastic model of the medium and the Mohr-Coulomb failure criterion are implemented in the codes of the 3D finite element analysis program RS3 (Rocscience). Stochastic simulation is used to assess random risks associated with natural object state variations. Findings. The distribution of maximum shear strains, which localizes the real or potential sliding surfaces in the ope...
International Journal of Mining Science and Technology, 2016
Rock fall related accidents continue to occur in coal mines, although artificial support mechanisms have been used extensively [1]. Roof stability is primarily determined in many underground mines by a limited number of methods that often resort to subjective criteria. It is argued in this paper that stability conditions of mine roof strata, as a key factor in coal mines, must be determined by a survey which proactively investigates fundamental aspects. Failure of rock around the opening happens as a result of both high rock stress conditions and the presence of structural discontinuities. The properties of such discontinuities affect the engineering behavior of rock masses causing wedges or blocks to fall from the roof or slide out of the walls [2]. A practical rulebased approach to assess the risk of a roof fall is proposed in the paper. The method is based on the analysis of structural data and the geometry and stability of wedges in underground coal mines. In this regard, an accident causing a huge collapse in a coal mine leading to four fatalities is illustrated by way of a case study. A comprehensive investigation of the hanging wall has been gathered through a systematic collection of evidence. The investigation results are then analyzed, and an interpretation of the evidence gathered is provided. For this purpose, horizontal and vertical profiles are prepared by geophysical methods to define the falling zone and its boundaries. The collapse is then modeled by the use of sophisticated computer programs in order to identify the causes of the accident and hence recommend corrective actions to prevent or minimize reoccurrence probability of similar accidents. The corrective measures are placed into a rule-based framework for the benefit of the mine operators.
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...