Open Pit Mine Design Research Papers (original) (raw)

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.