I also declare that the intellectual content of this dissertation is the product of my own work, except to the extent that the assistance of others in the design and conception of the project or in its style, presentation, and linguistic expression is acknowledged. In view of the dissertation research, a two-dimensional finite element method was chosen for the analysis of the Bozshakol open-pit mine. In particular, the south wall of the pit is of most concern as, based on the data collected, this appears to be the critical zone prone to possible instabilities.
Initially, the results of mining activities include uplift from the pit bottom and subsidence of rock from the lateral walls of the pit section (eg, excavation, blasting, benching). Most importantly, higher displacements and strains are observed along the south wall of the pit than along the north wall. Further three-dimensional research and improved sensitivity analysis are recommended for advancing the project.
I would like to express my gratitude to KAZ Minerals and The Bozshakol Copper Mine for allowing me to access and use the geological and geotechnical data relating to their respective mines for the purposes of this research effort. I would like to express my deepest gratitude to Bayuprima Adiyansyah, the Geotechnical Supervisor at the mine, as well as Jose Gonzalez Borja, the Geotechnical Manager, for their support in the compilation of this data.
INTRODUCTION
- Background
- Objectives of the Thesis
- Main Objectives
- Specific Objectives
- Hypotheses
- Justification of the R&D
- Scope of Work
In this sense, the Bozshakol copper mine is a perfect place to assess the condition of the slopes using numerical analysis. In particular, safety factors and kinematic analyzes performed for individual geotechnical sectors may not represent the entire pit condition. Finite element modeling, on the other hand, is one of the most popular tools used in numerical simulations.
The secondary aim of the thesis is to reveal the deformation-prone zones and mechanisms of possible faults. Third, the change of the rock strength parameters to different extents should show the sensitivity of the model. Estimate the sensitivity of the model to different levels of changes in modulus of elasticity, cohesion and friction angle.
Determine any observable critical zones in the model while presenting the chosen two-dimensional cross-section of the well. The south wall of the pit may be more critical and prone to failure modes than the north wall due to the low strength parameters of the gabbro.
LITERATURE REVIEW
- A Brief Overview of Slope Stability
- Empirical Methods
- Limit equilibrium Methods
- Analytical Methods
- Numerical Modelling
- Classification
- Input Parameters
- Finite Element and Finite-Difference Methods
- The Scale of the Project
- Advantages of Numerical Modelling and Finite Elements
- Limitations of Numerical Modelling and Finite Elements
This is justified by the simplified assumptions used in determining the safety factor. Unlike the slice method, Bishop's simplified version does not satisfy the force balance for any of the slices. Although the safety factor may not look rigorous, it has been a stability indicator for most geotechnical problems in the industry.
The parameters included in the analysis may vary depending on the type of numerical modeling and research objectives. The reality of slope stability problems, however, as the authors suggest, does not always meet this condition. Depending on the scale of the analysis, two-dimensional or three-dimensional monitoring can be used.
The number of elements in the model is large and affects the accuracy of the simulation. Similar to the objectives of the thesis research, the evaluation of stress changes at the global level is very important. Numerical modeling enables convenient investigation of the influence of various parameters on the voltage level.
Contrary to the several advantages mentioned above, most literature overlooks the specific disadvantages of the numerical calculations.
METHODOLOGY
- Bozshakol Copper Mine
- Finite Elements
- Theory
- Stress Reduction Technique
- Geotechnical Data
- Modelling strategy
- Sensitivity Analysis
- Input parameters
Some of the papers also highlight disadvantages of the SSR, which are not necessarily only applied to numerical methods. The long-evolving Bozshakol fault (or fault zone) confines ore-bearing intrusive bodies (mainly granitoid dikes) of the deposit. The post-Ore West Mysore fault of the northeast strike bounds the Bozshakol deposit from the east.
The frames of the structural elements were subsequently tested for consistency with the images of the core. The earliest application of the finite element approach was in the study of aeronautical structures. Most of the literature on SSR assumes that slope materials have Mohr–Coulomb strength (Hammah et al, 2004; Read & Stacey, 2009; Zinkiewicz et al, 2014).
Based on the assumption that the geotechnical sectors 2 and 6 have a potentially unstable wall and steeper slope angles than any other sectors presented in the datum, a perpendicular two-dimensional slice of this region was specifically selected for the modelling. However, the most important thing is to keep the model as simple as possible due to high risks associated with improper evaluations given the complexity of the project. The mesh, with respect to the size of the model, is set uniformly by triangular elements, resulting in approximately 5000 commutation zones.
The main contribution of this project is the appropriate modeling of all production stages of the Bozshakol mine, as shown in the figure. The cave's sensitivity to changes in various parameters could reveal the weaknesses of the slopes. Accordingly, it helps to determine the tendency of the wall to collapse based on these changes in properties.
Overall wall angles are usually predetermined during preliminary geotechnical design as they play a major role in production safety. The analysis was performed on the basis of deviations in RMR of andesite (see Table 3). This is due to the fact that andesite is the host rock and covers most of the model.
RESULTS
Rock Mass Design Parameters
Numerical Simulation Results
- Pre-mining Conditions (Stage 0)
- Production Stages
The progression of the deformation scenarios between the low and high SRF values could occur considering the time dimension. Given the critical SRF of 2.15, as the results of the FE display indicate, the current production phase of the Bozshakol is sensitive to a small deformation in the southern (right) wall (see Figure 19). A wide range of movements around the bottom and sides of the model demonstrate the impact of mining and excavation.
It is worth noting that a maximum displacement of 0.035 m can indicate the sufficient safety of the south wall. The results also imply that the northern wall is relatively safer than the southern wall at the moment. However, given the maximum SRF of 2.6, the south wall can be severely deformed with displacements of up to 3.3 m.
This means that the strength of the rock can decrease over time and stresses begin to build up. The voltage levels actually start to increase near the slopes in the time dimension (see Appendix C). The production stage 3 scenario is relatively similar to that shown in stage 2 (see Figure 20).
The upper and southern part of the cave seems to be very sensitive to large deformations. Movements are directed towards the bottom of the pit, which means a high probability of a downward planar or circular collapse. FE analysis of production stages 4-5 showed larger displacements with a maximum of 0.07 m compared to the previous stage.
The bottom of the cave and the remaining zones of the southern wall show movements between 0.05 m and 0.07 m. The most worrisome scenario among all analyzed could occur in production phase 6, as shown in the figure. As can be seen, the northern (left) wall of the cave is less prone to major deformations even during the final excavation of the cave.
Sensitivity Analysis
- Change in Elasticity Modulus
- Change in Cohesion
- Change in Friction Angle
The reduction of the main deformation zones as well as the maximum displacement of 0.018 m at the critical SRF are observed in Fig. As the cohesion was reduced to 1.5 MPa from 3.3 MPa, the displacements reached a higher maximum of 0.7 m given the critical SRF of 2.21 . Another similar pattern feature is the progression of deformation on the south wall.
The stress begins to accumulate around the upper part of the wall, causing large rock movement at the SRF of 2.4. With the increase in cohesion from 3.32 MPa to 4.52 MPa, the displacements reached a larger maximum of 0.05 m when the critical SRF was 2.2. With SRF = 2.7, the stress begins to accumulate near the top of the wall, causing significant bedrock displacement above 5.4 m.
It is worth noting that a friction angle increase led to higher displacements compared to the base case results (see Fig. 27). The increase in friction angle of andesite, remarkably, also resulted in higher displacements compared to base value. 28 shows the similar sliding mechanism as illustrated in previous production stage 2 models.
First, the analysis of the base cases in the production phases clearly showed the weaknesses on the south wall. Mining activities have resulted in higher stress levels in the bottom and sides of the pit. Finally, the stress reduction method revealed possible moderate deformations on the south face, reaching 0.14 m of rock movement.
The graph also highlights the rapid growth of maximum displacements after the critical SRF in all cases. Given the displacements at the critical SRF (i.e. at the point of failure), factors of safety can now be determined that would provide the best safety at the site. Nevertheless, the deformation limits of the stages indicate the possibility of a minor rock fall, which is undoubtedly unfavorable for production (see Annexes C, F, I, L).
CONCLUSIONS AND RECOMMENDATIONS
As mentioned, three-dimensional models are capable of providing substantial information about the entire well and possible deformation zones. That would also allow a comparative analysis with the results of the 2D FE analysis to distinguish the main differences and features. Using the results of this project and other geotechnical analyzes (i.e. LEM, analytical and empirical), it is recommended to develop a convenient support design to avoid, minimize or mitigate the impact of the deformations presented in this thesis.
It is also recommended to analyze other parts of the pit that might be prone to deformation zones. More numerical calculations would help to have a broader view of the critical areas of the pit. It could be interesting to perform a sensitivity analysis by varying other properties in addition to the rock mass parameters.
APPENDICES
