In next two years, the current waste dump of Narynsukhait coal mine is predicted insufficient to accommodate the overburden as limited of the waste dump capacity. Thus, redesigning waste dump is paramount to increase capacity of the dump in future. This paper describes current condition of waste dump of Narynsukhait coal mine and then discusses the optimization of waste dump geometry by analyzing the effect of different waste dump’s bench configuration on slope performance. Optimization of the geometry is carried out by investigating and comparing the performance of geometrical combinations of bench height, bench angles and number of safety berm by means of numerical modeling. The model shows that increasing height of bench is able to induce shear stress in the bench and may initiate bench instable. However, the shear stress can be limited by having safety berm and/or reducing bench angle to satisfy the stability criteria.
Mining gives highest contribution on Mongolian economic growth; it accounts for 20% of Mongolian GDP [
Narynsukhait, a coal mine which located in the Omnogovi Province, is one of coal companies which have biggest fossil coal reserves in Mongolia with 380 Mt coal deposits that have great progress in regards to coal production. Narynsukhait coal mine has produced 8 Mt of coal on 2015. It is almost three times of their production on 2013, and it has increased over four times when the company just begun their production on 2008. The coal of Narynsukhait coal mine production is expected to more increase to fulfill target of Mongolian coal production.
The problem on Narynsukhait coal mine arises in regards to coal production when they face a fact that volume of their waste dump is predicted not enough to accommodate the overburden in two years. The volume of overburden is projected to be over 35 million cubic meters based on current design of the waste dump. If the volume of overburden waste material continues to increase, it may lead to dump slope vulnerability. It has been reported that over capacity or weight of deposited waste may cause slope loss and slope deformation [
Considering to aforementioned problem, Narynsukhait coal mine plans to redesign the waste dump to increase the capacity. There are several factors that together contribute to waste dump slope geometry. The most important is bench height and overall slope angle or slope angles of individual bench. This paper presents options to improve the volume of waste dump of Narynsukhait coal mine by improving the bench height. In general, the shape of a waste dump slope is governed largely by the ground conditions under the slope and the shear strength of the mine wastes [
The causes of slope failure are categorized as potential causes such as vulnerable geological properties, rainfall and ground cutting [
The Narynsukhait coal mine is situated in the complexly folded, faulted and in places, intensely metamorphosed stratigraphic sequences that host deposits of strategic minerals, oil and gas, and coal. The mining property sits north of an extensive thrust fault system that roughly parallels the Mongolian border [
that the length of the mine is around 1 km from east to south. The western part of the mining area is hilly, whereas the northern part is flat. As shown in
The study is situated in MAK field of Narynsukhait coal deposit. This deposit has 35 to 40 m thickness of coal seam with overburden that consists of shale stone, siltstone, sandstone and conglomerate as shown by stratigraphic column of Narynsukahit area in
Particle size distribution can be important in understanding physical properties of a material. It is proven that particle size distribution affects the strength and load-bearing properties of rocks and soils. Moreover, particle size distribution information can be of value in providing initial rough estimates of permeability of a rock mass since particle size influence how fast or slow water moves through it.
Particle size distribution is dependent on the method by which it is determined. The different techniques or even different equipment using the same basic technique are likely to produce different size measurements from the same sample [
Considering to particle size usually span several orders of magnitude, a logarithmic scale of particle size and statistical descriptors such as mean and percentile is preferred to descript the distribution. The result of sieving in a logarithmic scale is given in
The migration of rain water may affect pore-water pressure distribution within the slope. It also may reduce the frictional strength of the dump slopes [
permeability has been adopted in order to get information of saturated permeability of the waste rock.
Hasegawa in situ permeability test kit consists of hole cover plate, float plate, scale of 60 cm and 100 cm, fixed pin and scale guide [
After the preparation has completed that is the float has touched base of hole, amount of water was injected into the hole by siphon pump in a steady stream so that the water pressure break wall of the hole. The water injection was stopped when the water on the level of 100 mm (h1) from the hole bottom as shown in
Based on the recorded data, the permeability for the time of h3 and h4 were calculated by adopting Equation (1) as follows:
Permeabilityon i stage ( mm / hr ) = ( h i − h i − 1 ) ( t i − t i − 1 ) min × 60 (1)
where in case of this study t i − t i − 1 is 20 minutes.
Calculation result of permeability on the waste dump is given in
Moisture content is one of a major factor in the stability of slope in many unconsolidated rock such as waste dump’s material. It is moisture content which changes the rocks from liquid state to plastic state and solid states. Its value controls the shear strength and compressibility of rocks. The increasing moisture content in a rock mass may produce swelling, increasing pore pressure and a decrease in shear strength. Among basic geotechnical parameters, shear strength is of great practical importance. A notable reduction of slope stability is the usual result when the shear strength decreases. As a result, a landslide is possible to occur. Therefore, a series of test has been conducted to confirm the moisture content. Considering to the surface moisture changes with the depth, the moisture content is investigated not only at surface but also below the surface. The samples were taken from the surface up to 600 mm in depth. Twelve samples were taken from 4 locations of sections 1 and 2; 2 sampling holes for each section.
In this study, the moisture content measurement was performed by oven drying method. In this method, the sample specimen was dried by oven under temperature 60˚C to 80˚C till mass of the sample becomes constant. After drying
Material | Permeability (mm/hr) |
---|---|
Section 1: dominated by large grained (more than 0.69 mm) | 180 |
150 | |
Section 2: dominated by fine grained (less than 0.69 mm) | 12 |
30 |
process has completed, the weight of dried sample together with container and lid was measured and compared with the weight of wet sample with container and lid. Calculation of moisture content is given as follows:
W = W w W s = W 2 − W 3 W 3 − W 1 × 100 (2)
where W1 is mass of container with lid, W2 is mass of container with lid and wet soil, and W3 is mass of container with lid and dry soil. The calculation result of moisture content along the depth is given in
However, regardless to the different characteristic of moisture content in sections 1 and 2, the figure suggests that the moisture content of the research area is very low. It is possible to occur owing to low rainfall intensity. According to rainfall intensity data that recorded in this area, the rainfall intensity is 0 - 3 mm in winter and 30 - 60 mm in summer. This intensity can be categorized very low rainfall intensity. Considering to the very low moisture content of waste dump’s material, it can be assumed that the moisture content gives low influence on frictional strength of the dump slopes. In other word, the influence of moisture content to slope stability can be neglected.
Geotechnical parameters, which are considered in this investigation to determine
optimum combination between height and slope of waste dump like cohesion, angle of internal friction and bulk density after compaction are main factors for stability calculation of a waste dump. The properties of the waste rock dump from a series of laboratory tests are summarized in
When the waste rock dump’s properties is compared with the overburden’s physical properties that given in
Waste dump is possible to failure within the overburden material only or it also possible involves failure of dump foundation. In general, the most general factors affecting stability of any slope are: 1) slope’s geometry; 2) material properties; and 3) forces acting on the slope. Finite element methods and limiting equilibrium are the most common method used for slope stability analysis. Both methods are applicable to be used to analyze homogenous and inhomogeneous slopes. However, in a particular case, finite element methods are better than that of limiting equilibrium method in regards to provide more appropriate analysis. The limiting equilibrium methods often face computational difficulties in locating the critical slip surface, and moreover numerical inconsistencies may occur in this case.
Density, γ | 2 g/cm3 |
---|---|
Young’s modulus, E | 20 MPa |
Poisson’s ratio, v | 0.4 |
Tensile strength, Ts | 0.05 MPa |
Friction angle, φ | 40.6 degree |
Cohesion, c | 0.0105 MPa |
Sample | Code | Density (g/cm3) | UCS (MPa) | Young’s modulus (GPa) | Tension strength (MPa) | Poission’s ratio |
---|---|---|---|---|---|---|
Sandstone | 1 | 2.48 | 75.92 | 7.2 | 3.74 | 0.32 |
Conglomerate | 2 | 2.63 | 65.01 | 6.5 | 6.1 | 0.35 |
Siltstone | 6 | 2.51 | 70.17 | 4.5 | 3.26 | 0.3 |
Sandstone | K-1 | 2.61 | 85.94 | 7.18 | 7.02 | 0.36 |
Conglomerate | K-2 | 2.65 | 75.07 | 7.78 | 4.93 | 0.42 |
Sandstone | K-3 | 2.62 | 85.92 | 5.98 | 5.87 | 0.35 |
Owing to these inherent limitations of limit equilibrium methods, the finite element method has been increasingly used in slope stability analysis [
The analysis was begun by developing a model. The model was created based on geotechnical parameters (
According to the results of the simulation when the bench height is increased to 40 m and 60 m from 20 m high, the increasing of height is found initiate significant tensile zones at toe area of the bench due to increasing the self-load of the waste dump. Moreover, it is seen that the shear stress along the slope is also increased. The stress distribution forms like a circular sliding plane shape as shown in
In regards to slope stability, the simulation result shows that the SRF reduces with increasing the bench height from 1.31 for bench height 20 m to 0.86 and 0.85 for bench height 40 and 60 m, respectively. It means that a circular sliding plane is highly possible to occur on bench in height 40 and 60 m since the bench strength cannot support the gravity loading. Based on these result, it was considered that maximum high of the bench may not exceed 20 m.
In this study, in order to reduce stress on the bench, berm technique is adopted when the height is more than 20 m high to satisfy the stability criteria. For a
bench which has total height more than 20 m, a 10 m wide safety berm is introduced for every 20 m high single bench. The selection of the width of safety berm is, however, most often a function of the equipment size.
According to the simulation result, it has been found that the probability of a large-scale slope failure is high as increasing the bench height. The shape potential sliding surface was defined as the circular type based on the simulation analysis. The location of shear stress below the crest of waste dump was considered as the starting point of sliding surface. In this study, having safety berm is intended to reduce gravity loading, and reduce the shear stress accordingly, by limiting the single bench height that is not more than 20 m. The consequence of having a safety berm is a decreasing of overall slope angle of the bench.
A ten meter width of safety berm has been chosen not only to fit the bench angle but also designed to be capable to catching the volume of failed material from the bench faces above thus able to reduce the number of rocks that can continue to further fall onto the bench below. Other consideration for safety berms geometry is equipment size.
The simulation result of slope stability of a 40 and 60 m high of bench with a 10 m wide of safety berm is given in
Regardless to the benches that are in a critical failure point and/or have not satisfied yet the stability criteria, the shear stress is reduced by having safety berm on the bench. The SRF is increased from 0.86 to 1.02 and 0.85 to 0.97 for bench height 40 and 60 m, respectively. Different with a bench without safety berm which the high tensile stress is concentrated in the toe region and shear stress is found along the slope when the height of bench is increased from 20 m to 40 and 60 m, for the bench with having safety berm, high stress is found
concentrated only in crest region of the bench. This severe stress may be initiated by gravity loading of the waste dump. In order to reduce the stress at crest area, the bench angle is reduced.
Considering to having berm is not enough to guide the slope satisfy the stability criteria, decreasing overall slope was considered to achieve better stability of the slope. In this study, the angle of single slope was decreased 3 degrees i.e. from 36˚ to 33˚; accordingly the overall angle of slope of 40 m bench height and 60 m bench height is decreased to 29˚ and 28˚, respectively. The simulation result is described in
It is shown in
bench has already had safety berm. Meanwhile, when the bench angle is changed to 33˚ from the original angle 36˚, the shear stress zones are reduced. Based on these simulation results, it is noted that the bench configuration have a significant influence of the occurrence of shear stress.
Considering to bench design for 60 m in height with 2 safety berms constructed in it is satisfy stability criteria, the simulation is continued by increasing the height up to 80 and 100 m. Accordingly, number of safety berm increases to 3 and 4 for 80 m bench height and 100 m bench height, respectively. The simulation result is given in
increases slowly, there is an abundance of time for the stress around the dump to readjust and to interact with waste material, promoting the development of facture. Such behavior may cause overall slope failure, or may initiate failure, which may then be driven to overall slope failure.
Based on the above discussion, it is paramount to create a guideline for establishing a bench configuration that satisfies the stability criteria.
In the open pit coal mine, providing a proper waste dump is crucial to mine’s successful operation. The improper waste dump results instability issues which may affect safety and production of the mine. In order to improve the capacity of the waste dump by increasing the bench height, a study must be done in regards to analyze performances of a variety of different configurations for bench geometry such as bench height and angle to satisfy the stability criteria. The results of the simulation and analysis of the waste dump slope stability can be summarized as follows:
・ Considering to friction angle of waste dump’s material, in order to avoid bench failure, it is suggested to design the bench angle do not more than 36˚.
・ Relatively limited shear stress zones occur behind crest of the bench for a 20 m bench height.
・ Change in bench height has a major impact on stability consideration; the magnitude of shear stress increases with the bench height of the dump. According to the results of the simulation, the slope movement initiated at the crest of the waste dump and the shape of slope failure seemed to be a circular sliding plane due to gravity loading of the waste dump.
・ Having 10 m of safety berm for every 20 m in height of single bench and reducing bench angle up to 3 degrees can play important role on improving the SRF and reducing the shear stress along the slope.
・ Considering to the rock properties of the overburden, the maximum high of the bench to satisfy the stability criteria is 80 m with three 10-m-wide safety berms.
・ Besides able to reduce shear stress in the bench, a safety berms is also able to reduce the number of rocks that can continue to fall further that put safety of personal and equipment at risk.
Amarsaikhan, T., Shimada, H., Wahyudi, S., Sasaoka, T. and Hamanaka, A. (2018) Optimization of Dump Bench Configuration to Improve Waste Dump Capacity of Narynsukhait Open Pit Coal Mine. International Journal of Geosciences, 9, 379-396. https://doi.org/10.4236/ijg.2018.96024