Dissertation
In order to receive a Master's degree
Rock Mechanics
Abstract
In open pit mining where many steps are created, it is necessary to conduct stability studies of the wall slope. By increasing the slope angle of the mine, the ratio of tailings is reduced, which will save the overall costs of the mine, but on the other hand, it will reduce the safety, in case of wall destruction, hundreds of thousands of tons of soil and peat will flow; Therefore, an angle should be determined for the mine that considers the above two issues simultaneously. In this project, by using limit equilibrium methods and two-dimensional numerical methods, the maximum slope of the walls of Tagui 2 bauxite mine of Jajarm was obtained for the northern, eastern and western walls of 55 degrees, and due to the slope of about 45 degrees of the bauxite layer, the slope of the southern wall was considered equal to the slope of this layer. Then, the final limit of the mine was modeled in 3DEC software for stability analysis, and the stability of the walls was checked under horizontal acceleration of 0.35 gravity acceleration in different directions. The results of this software show drops of several blocks in each case; But in the rest of the blocks, a maximum displacement of 20 mm was obtained, which finally can be said that this mine is stable with an angle of 45 degrees for the south wall and an angle of 55 degrees for the other walls.
Key words: stability analysis, Tagoi 2 bauxite mine, 3D numerical analysis, numerical modeling, 3DEC.
Introduction
Today the issue of slope stability is one of the main and determining parameters in the economics and safety of open pit mines. It is not correct to assign a slope for the entire mine walls in most mines because the mine walls are usually made of different materials and with different structural conditions, and therefore, the design of the slope should be determined after determining the geotechnical parameters, different lithologies and defining the geotechnical limits. Among these cases, we can mention the collapse of the Chuquicamata mine wall [1] in Chile. The height of this wall at the time of its collapse was 284 meters and its angle was 43 degrees. The main cause of this fall is the tremors caused by the earthquake. [1]
On the other hand, obtaining the maximum possible profit from the extraction of minerals under safe conditions has been one of the main goals of mining throughout history. Although safety and profit appear to be two goals in opposite directions (which means that when one increases, the other decreases), but experience has shown that increasing safety to an acceptable level in mines has prevented the occurrence of accidents, which in turn indirectly increases the profit that can be collected for the miner; Therefore, one of the manifestations of the mutual effect of safety and profit is the issue of slope stability in open pit mines. Increasing the overall slope of open pit mines, on the one hand, reduces the ratio of tailings removal and, as a result, increases mine earnings, and on the other hand, increasing the slope increases the possibility of instability in the slope. Therefore, in the first stage of mine design, complete geotechnical, structural geology and hydrogeological studies of the mine should be carried out, so that based on these studies, as well as sufficient knowledge of the possible fallout in different parts of the mine, the maximum safe slope angle for the mine can be obtained in the second stage. Of course, in these stages, the effect of different stabilization methods such as flushing on the safe slope angle and also the costs they impose on the miner should be carefully examined.
The open pit mining method is one of the mining methods with a relatively low mining cost, in which the mechanization capability and the amount of production can be very high. Therefore, it is possible to extract minerals with very low grade, which are uneconomical to extract by underground methods. In recent decades, the depth of open pit mines has increased and depths of more than 500 meters are no longer uncommon. Since the underground mining method is still more expensive than the open-pit mining method, it is unlikely to extract such deposits by the underground method and ignore the open-pit mining in the future.Therefore, it is expected that the depth of open pit mines will increase in the future, provided that the cost of production decreases and the price of metal remains stable. An important complication that arises with increasing depth is the risk of large-scale instability. Large-scale failure potentially occurs in the entire height of the summit and in the final limit of the mine. Therefore, keeping the mine wall as steep as possible is very vital in reducing the tailings ratio, which in turn has a close relationship with the cost of mining. So, in this case, the design of the final range does not depend only on the grade distribution and production cost, but also on the overall strength of the rock mass and stability. In each mine, the potential for collapse should be evaluated and coordinated with the final pit design.
There are several slope angles for an open pit mine. The slope angle of the stair wall, the angle of the wall between the ramps and the angle of the end wall should be determined based on the stability assessment in each unit separately. In general, the instabilities created in open pit mines are relatively common and do not have much effect on the pit design. Individual steps and walls between ramps in an open pit mine can be stable at the same time, while the overall wall may not be stable [2].
Proposed methods for checking the stability of roofs are divided into two groups:
A- Methods based on limit equilibrium.
B- Methods based on on numerical analysis.
One ??of the most widely used methods for analyzing the stability of roofs is the limit equilibrium method. The reason for the wide application of this method is the ease of assumptions and history of its creation. This method was first used by Coulomb in the late 18th century in soil mechanics problems. In this method, by assuming an optional sliding surface, the rigidity of the ruptured part and using the Mohr-Coulomb rupture criterion, the reliability factor is defined as a comparison of the available shear strength with the required shear strength for equilibrium. In this method, if the reliability coefficient is greater than 1, the rock mass is stable, and if it is less than 1, it is unstable. If the confidence factor is equal to 1, it will be a critical or limit state (beginning of rupture). Finally, the surface that has the lowest stability coefficient will be considered as the most likely sliding surface.
It should be noted that although limit equilibrium methods are widely used to analyze slope stability problems, there are several weaknesses in these methods, the most important of which is not considering the stress-strain relationship of materials in the analysis. Also, due to non-observance of the laws of mechanics of continuous environments, the location of the answers to the problems in relation to the real answer is not clear in this method [3]. Another disadvantage of these methods is that these methods do not consider whether the slope is an embankment or a natural slope or an excavated slope, and they ignore the effects of structural development, initial stress, stress-strain behavior, etc., and probably slope stability is cautiously predicted in these methods [4].
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Abstract
Stability analysis of the mine slope, in particular jointed rock slopes are among the most important issues in designing surface mines. Slopes in open pit mines must be considered as geotechnical structures. Therefore their design and implementation must be conducted with all considerations including technical, economic, environmental and safety issues. In this project, maximum overall slopes of Taguee 2 Mine of Jajarm Bauxite Deposit by using analytical and 2D numerical method for the Northern, the Eastern and Western walls 55 degrees was obtained and due to dip of the bauxite layer, for the Southern wall 45 degrees was chosen. For stability analysis, by using geotechnical testing results, the geometric and mechanical models of mine were made in the 3DEC software. Finally, the model has been analyzed under horizontal acceleration of 0.35 in different directions. The results of this software is the slide as much as a few blocks in each case.