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Ping Zou, Ximo Zhao, Zhonghua Meng, Aibing Li, Zhengyu Liu, Wanjie Hu, "Sample Rocks Tests and Slope Stability Analysis of a Mine Waste Dump", Advances in Civil Engineering, vol. 2018, Article ID 6835709, 17 pages, 2018. https://doi.org/10.1155/2018/6835709
Sample Rocks Tests and Slope Stability Analysis of a Mine Waste Dump
The safety and stability of waste dump are vital influencing factors to the mine sustainability and mine employees. Based on a real mine project in a certain open-pit mine waste dump in Tibet, the in situ test on waste rocks from waste dump, including measurements of density, water content, rock size, and natural repose angle, was conducted. Afterwards, these sample waste rocks, of which grain size is less than 5 cm, were selected for indoor large-scale shear test under natural and saturated conditions. By using some engineering methods, the physical and mechanical parameters of waste rocks layer were then determined accordingly. MIDAS-GTS/NX has the advantage of pre-processing modeling. FLAC3D has good computational and analytical capabilities. The process of dump accumulation is simulated numerically. According to the calculation results of FLAC3D, the distribution of stress, displacement and plastic zone in the dump is obtained. FOS (factor of safety) for each analytical step in this model was then calculated through the strength reduction method. The limit equilibrium method is used for waste dump stability analysis considering three states: only applied gravity, applied gravity and rainfall, and applied gravity and underground water. The results from this analysis show that the waste dump is stable. The potential failure modes of waste dump mainly consist of the “combined sliding mode” which has circular sliding in upper side and broken line sliding that cuts through gravel-soil layer into heavily weathered layer in the bottom. This paper documents some of the procedures and approaches utilized for waste dump life-of-mine design analysis. It provides reference for further waste dump optimization.
Mine stockpile also called mine waste dump is vital to the open-pit mine exploitation. So called by its name, mine waste dump is primarily utilized to storage the overburden and waste rock from open-pit mines . The safety and stability of waste dump refers to the mine sustainability and mine employees and should be paid enough attention. There are many factors contributing to the stability of a mine waste dump, including physical and chemical composition of the waste rock, the dumping technologies being used, engineering conditions of the landscape, hydrogeological condition, and other related parameters.
At present, a lot of research has been done on mine dump at home and abroad. It mainly includes using test and back analysis method to obtain rock mechanics parameters. The stability is determined by limit equilibrium analysis, numerical simulation analysis and simulation test. Cho and Song  studied the dumping behavior of dump slope and natural slope under dump. Linear sensors are installed at the top of the slope of the dump site to monitor the pile-up behavior of the dump site. Turer and Turer  used the two methods to determine the weight of the unit and the waste of the shear strength parameters to analyze a slope stability map. In another example, Adamczy et al.  introduce the stability of garbage sandstone open slope and choose six sections to analyze the stability of slope. Behera et al.  analyzed the stability of open-pit coal mine dump in Odisha area based on different geotechnical parameters and mineralogical composition. Verma et al.  analyzed the stability of existing dump by the analytical method. The properties of the material in the dump are measured in the laboratory, such as cohesive force, internal friction angle, permeability, volume density, particle density, particle size distribution, and field water content. In a very dry and humid environment, the stability of dump slope is simulated. Kainthola et al.  used the western coalfield limited, Nagpur, India, as an example. The shear strength reduction technique has been applied to achieve the desired factor of safety using a two-dimensional finite element code. Zhou et al.  have studied the effects, from inner drainage parameter variation of the northern slope of Haerwusu Open-pit Coal Mine, on its imbalance stress and slop stability. Cheng et al.  analyzed the slope stability of an open-pit mine under the combined effect of waste dump and blasting and hence estimated and verified the required minimum distance between mine and waste dump accordingly. Dong  invented a method in evaluating the stability of long-bench-waste-dump under heavy rainfall and applied this in Longyan Iron Mine Waste Dump in Fujian. Huang et al.  have determined the creeping-cracking failure and cracking-sliding failure for Jinduicheng Open-pit long-bench-waste-dump, analyzed the stability of this kind of waste dump under natural state, blasting vibration state, and seismic state, and finally provided some countermeasures in case of failure. The stability of rock slope is also related to different types of rock [12, 13]. The slope stability comparison between those only applied static load and those count for the extra impacts from earthquake [14–16]. The microstructure of the rock-soil in the dump has significant influence on the stability of dump [13, 17–25].
This article, based on a real mine project in a certain open-pit mine waste dump in Tibet, demonstrates the stress, displacement, and plastic zone distribution, and variation directions refer to the overall stacking process of this waste dump, as well as the possible failure mode, by conducting in situ tests and indoor rock experiments on the waste rocks from dump and establishing 3D simulation models. Thus, it provides reference for further waste dump optimization.
2. Overview of the Waste Dump
2.1. Engineering Geological Condition
The hornfels waste dump is valley type, with natural slope 10–20°, two side slopes 30–40°; the boundary altitudes in east, north, and west are 5105 m.
The exposed bedrock is beneficial to the stability of dump site. The exposure strata are mainly residual gravel, flood-accumulated rocks, strongly weathered limestone, and medium weathered limestone; the slopes are shallowly covered by Quaternary diluvial remnant gravels. The waste rocks stacked on dump are mainly slate, hornfels, limestone, granite porphyry, and Quaternary topsoil. There is no indication of negative geological development, such as landslide or collapse, or any cracking formation.
2.2. Hydrogeologic Condition
The topography of the waste dump is complex with a number of crossed valleys; its catchment area is 1.4 km2 and volume is 10800 × 104 m3. The flow of surface water varies greatly with the seasonal precipitation. During the rainy season (mainly during June till September), the seasonal flood can be easily formed in gully and hence causes negative effect on waste dump. The atmospheric precipitation can possibly form surface flows, which is finally collected in gully. The stable phreatic surface is at the depth of 0.1 to 1.0 m, mainly supplied by atmospheric precipitation and bedrock fissure leakage. The groundwater is less likely to be stored beneath these steeply slopes. The fractures developed in strong weathered limestone are filled well but with poor connections. For medium weathered limestone, the fissure is comparably developed, and mass rock body is more complete with poor permeability, which is the natural interlayer.
2.3. Overview of Design
The dumping process consists of waste rock transportation, dumping by self-discharging vehicles, and supplementing by auxiliary bulldozer. Multibench dumping starts from bottom to top and finally piles up to 5105 m with an overall heap height of 580 m and every bench height is 30 m. Set those benches be 5075 m, 5045 m, 5015 m, 4985 m, 4955 m, 4925 m, 4895 m, 4865 m, 4835 m, 4805 m, 4775 m, 4745 m, 4715 m, 4685 m, 4655 m, 4625 m, and 4595 m with the bench width of 30 m. The slope of waste dump is 1 : 1.75 and capability volume of which is 16674 × 104 m3.
3. In Situ Test on Waste Rocks
The in situ tests on waste rocks from waste dump include density, water content, rock size, and natural repose angle measurements. The rock size can greatly change the mechanical properties and stability of waste dump. Different sizes of rock were distributed to a certain height of slope automatically through sliding movement after dumps. Larger size of rocks lie regularly on the bottom while those smaller remain closer to the top; it is rare to see bigger size rock on the upper slope. At present, there are three normal methods being used to measure the rock size; they are screening, direct measurement, and photographic image analysis, which can be complement and verification of each other during experiments.
The process of in situ waste rock test can be seen in Figure 1.
4. Indoor Direct Shear Test for Waste Rocks from Dump Site
The indoor direct shear tests for waste rocks from dump site were conducted by utilizing strain-controllable direct shear device developed by Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. This device can shear the sample with grain size no more than 5 cm diameters and guarantee that the physical and mechanical parameters obtained from tests is reliable and practicable.
4.1. Test Samples
The test samples are the waste rocks with grain size less than 5 cm in diameter from pits J1, J2, N2, and N3. The detail parameters of selected samples list are shown in Tables 2 and 3. Due to the limitation of test device, additional works must be done to deal with the waste rocks with grain size equal to or even bigger than 5 cm, which exceeds the maximum allowable test sample size, normally by using the replacement method.
4.2. Test Process and Results
In the laboratory, mixing the sample rocks with different grain sizes from the same pit was firstly carried out to measure the mixture moisture content; then water accordingly until it reaches natural state, mix them fully again, and the direct shear box is treated. Another test should be conducted when those waste rocks are in saturated state by just adding adequate water into the box until those absorb sufficient water.
Applying various testing loads (Table 4) in shear tests, the parameters such as displacement-stress curves and shear stress-positive stress curve can be seen in Figure 5; other test results are shown in Table 5.
4.3. Test Data Process
Based on the field survey for waste dump and shear test results, the internal friction angle and cohesion of waste rocks in different heights along slopes can be calculated. The shear test results of fine particles ( and ) and the distribution of fine particles in different horizons have been obtained. Through the analysis of five kinds of fine particle composition, the proportion of fine particle and large particle waste rock can be calculated. The formula for calculating the cohesion and internal friction angle of waste rocks in a certain height is shown in equations (1) and (2), and the calculation results are shown in Table 6.where is the cohesion of waste rocks in crest slope (height ), is cohesion of fine waste rocks in crest slope (height ), is percentage of fine waste rocks (height ), is the internal friction angle of big rocks, which is equal to natural repose angle, and is the internal friction angle of fine waste rock (height ).
5. 3D Numerical Simulation Analysis for Hornfels Waste Dump
The computing package FLAC3D has strong computational function and simulation analysis capability, and it has been widely recognized in the world. Refer to domestic and international FLAC3D complex 3D engineering modeling method, the basic thought can be roughly summarized by using other professional 3D modeling software or finite element analysis software to build complex 3D engineering model and then load this complex model into FLAC3D for analysis and calculation. The Mohr-Coulomb strength criterion is adopted. The constraint boundary is adopted around the model, and the free boundary is used on the empty surface. This method can greatly improve the modeling efficiency, as well as save the modeling time and guarantee the authenticity and accuracy . In this article, a method combining GTS NX and FLAC3D is proved to be highly efficiency in building a 3D finite model. Firstly, GTS NX was used to complete 3D geometric modeling for Hornfels Waste Dump, tetrahedron grid model was generated, and then this model was transformed into . FLAC3D format which can be loaded into FLAC3D, using import grid function to accomplish the FLAC3D hornfels mine modeling. The final model can be seen in Figure 6; different colors represent different rock mass.
The range of the model is x = 3200 m to 5350 m (2150 m in total) in east-west direction, y = 550 m to 3900 m (3450 m in total) in north-south direction, z = 4400 m to the surface; there are totally 94063 nodes and 504907 elements being generated during modeling.
5.2. Rock Mass Mechanics Parameters
Refer to geology in Hornfels Waste Dump, there are 4 kinds of mechanics medium being considered for modeling: they are waste rocks, gravel and soil, heavily weathered rock layer, and medium weathered rock layer. After a comprehensive selection among in situ investigation, the physics and mechanics test, the rock mass quality evaluation and engineering analogy, and the rock mechanics parameters are summarized as shown in Table 7.
5.3. Analysis Process
In order to simulate the piled up process of Hornfels Waste Dump, as well as to quantify surface and waste rock deformation, totally 8 analysis steps were designed and simulated accordingly for this stacking process, which can be seen in Figure 7 and Table 8.
5.4. Analysis Results
In order to observe the internal stress, strain, displacement, and plastic zone of the model, a section view was selected right in the middle of the stacking models (Figure 8).
5.4.1. Stress Distribution
Figure 9 shows maximum and minimum principal stresses of stacking step 8. The key stress in the accumulation process increases with the depth. In the area near the slope, the principal stress contour of the slope is approximately parallel to the surface of the slope. The results also show that principle stress has been lightly disturbed by stacking process within the area near the contact surface of waste rock and gravel soil. There is no stress concentration during the overall stacking process; stresses are mainly compressive stress while only a small area of tensile stress is produced inside the waste rock layer (Table 9).
Figure 9 shows the top view and section view of displacement of step 8. The maximum displacements of all 8 steps can be seen in Figure 10 and Table 10. The key displacement changes in the accumulation process can be summarized as the displacement contours distributed in the waste rock layer. The maximum displacement points are also distributed in this region. Displacement gets bigger with increasing stacking height, and the displacement in Z direction always bigger than those in other directions; the maximum displacement points of stacking step 2 to step 7 appear in bench 4865 m or inside the waste rock layer; the maximum displacement point of step 8 locates inside the waste rock layer near 4965 m; there are two to three big displacement areas formed since step 7 and gradually gathered, having put some negative impacts to the stability of waste dump.
5.4.3. Safety Factor and Shear Strain Increment Analysis
The strength subtraction calculation function of FLAC3D was utilized in determining safety factors of waste dump slopes. The safety factors and shear strain distribution and changes in step 8 can be seen in Figure 11, and the safety factors of all 8 steps are shown in Figure 12. The key displacement changes during stacking process can be summarized as safety factor gets smaller with increasing stacking height (from 1.73 of step 2 to 1.63 of step 8); however, the decrease rate tends to be smaller (from 1.2% of step 3 to 0.6% of step 8). Luckily, these factors are all greater than the allowable safety factors (according to Chinese specification for design of nonferrous metal mining dump GB 50421–2007). The safety factor is more appropriate to be 1.15 to 1.30 when sliding mode of waste dump slope is circular sliding, plane sliding, or broken line sliding, depending on the safety regulations . The overall waste dump is stable.
The potential failure modes of waste dump mainly consist of the “combined sliding mode” that has circular sliding in upper side and broken line sliding which cuts through gravel-soil layer into heavily weathered layer in the bottom. The simulated sliding curve in each step cut through different waste dump benches in different elevations, which can be seen in Table 11.
5.4.4. Plastic Zone Analysis
Figure 13 shows the distribution of plastic zone in step 8. The tensile strength and shear-plastic zones, formed during stacking process in each step, are mainly distributed inside waste rock layer, gravel-soil layer, and heavily weathered layer near slope toe.
6. Limit Equilibrium Analysis of Hornfels Waste Dump
6.1. Analysis Method
The utilized limit equilibrium methods for Hornfels Waste Dump are the Bishop method that satisfies overall moment equilibrium about the center of the circular trial surface  and the Morgen-Prince method that satisfies overall moment equilibrium about arbitrary shape surface .
6.2. The Classification of Sliding Mode and Sliding Surface of Waste Dump
Landslide failure modes of dump are mainly divided into three types: internal landslide of dump, landslide along the interface between waste rock pile and foundation, and landslide along the weak layer of foundation of dump. The possible sliding modes in Hornfels Waste Dump are as follows:(i)Sliding occurs inside waste rock layer. The waste rocks are mainly consisting of slate, hornfels, skarn, marble (or limestone), granite porphyry, and small amounts of Quaternary soil. The dumping, using self-discharging vehicles to transport waste rocks and supplementing by auxiliary bulldozer is less likely to form weak intercalations. The internal medium inside waste rock layer is relatively homogeneous (its mechanical property is dominated by friction). Therefore, the potential sliding mode of waste rock layer inside waste dump is circular sliding or other smooth-surface sliding.(ii)Sliding along contact surface between waste rock layer and foundation layer. When the friction strength between waste rock layer and foundation is less than the shear strength of waste rock layer inside dumping site, sliding can be easily formed along this contact surface where the dipping angle of foundation is comparably big or the strength of contact is weak. The foundation of Hornfels Waste Dump and South Pit Waste Dump are mainly covered by Quaternary gravel and soil, of which strength is weak. The waste dump has a long and steep slope, and waste rock layer is thick. The potential failure mode of waste dump, regarding 3D simulation, is “combined sliding mode” which has circular sliding in upper side and broken line sliding that cuts through gravel-soil layer into heavily weathered layer in the bottom. Therefore, the waste dump is associated with a sliding threat along contact between waste dump layer and foundation.(iii)Sliding occurs in some weak intercalated part inside foundation layer. If there are some relatively weak formations or weak intercalated inside foundation, due to their weak strength or low bearing capacity, it is easy to form foundation subsidence during the dumping process or in rainfall circumstance or impacted by the other factors. The subsidence ranges and scales vary in different parts of waste dump and some parts of foundation arise by inner stress, which can subsequently cause landslides along these weak formations or intercalated inside foundation. The waste rocks in waste dump mainly consists of slate, hornfels, skarn, marble (or limestone), and granite porphyry. No unstable geological formations exist in waste dump. Therefore, it seems impossible to slide in this particular mode.
To sum up, this limit equilibrium analysis shows that the main sliding modes of waste dump are sliding occuring inside waste rock layer and sliding along contact surface between waste rock layer and foundation layer.
6.3. Assumed or Utilized Parameters
(1)Parameters of rock mass
The parameters of rock mass can be seen in Table 7.(2)The impact of earthquakes and water
The seismic peak acceleration is 0.15 g, and seismic intensity of the mine is classified into level VII. The phreatic surface, in the raining reason, is about 50 meters above the surface of foundation.(3)Allowable safety factors under different conditions
(I) Consider gravity, the allowable safety factor (K) = 1.25
(II) Consider combined impacts of gravity and seismic, the allowable safety factor (K) = 1.05
(III) Consider combined impacts of gravity and steady seepage of groundwater, the allowable safety factor (K) = 1.05
6.4. Analysis Results
The calculation results of the safety factors of typical sections are shown in Table 12. The comparison of safety factors of each year under different conditions is shown in Figure 14. From Table 12 and Figure 14, the following can be summarized: (1)Considering all three conditions each year, the slope is stable only except when it reached the year 2018 that the slope may suffer from foundation contact sliding under condition III.(2)Under condition I and II, slope safety factor is predicted to increase gradually from 2018 to 2036 and since then remain unchanged.(3)The impact of seismic, on circular sliding, can reduce safety factor by 20.10% to 20.47%, while on contact-between-foundation-and-waste-rock-layer sliding can reduce FOS by 18.54 to 19.85%. This impact can decrease overall waste dump stability by 20.32% to 20.49%.(4)The impact of steady groundwater seepage, on circular sliding, can reduce safety factor by 10.38% to 25.22%, while on contact-between-foundation-and-waste-rock-layer sliding can reduce FOS by 14.03 to 22.98%. This impact can decrease overall waste dump stability by 20.32% to 20.49%.
(1)The mechanics parameters, initially obtained from tests on the sample waste rocks of which grain size is less than 5 cm, were then derived utilizing empirical formula. This parameter determining process is ideal and could be better improved or optimized by using large-scale direct shear test devices in the future.(2)The waste rock from waste dump is noncontinuum; however, the mechanical analysis software, FLAC3D, is only capable to analyse continuum mechanics. The analysis process considered all waste rocks as a whole, that neglected the size effects and interaction forces between each rock. Better approach is required for break through of the analytical method.(3)The equivalent static load method was utilized for limit equilibrium analysis considering earthquake effect; but in a real mine, earthquake damage is associated with dynamic process and it needs further analysis.
In this paper, the tests were conducted for the waste dump in an open-pit mine in Tibet, including in situ survey and laboratory large-scale direct shear test, as well as some 3D numerical simulations for the overall stacking process. The following conclusions are made:(1)Through in situ survey, the bulk density, water content, grain size, and composition of waste rock, natural repose angle and other related parameters were obtained. The sample waste rocks in natural and saturated state were selected for large-scale direct shear test, and shear strength of fine rocks was calculated consequently. The physical and mechanical parameters of waste rocks layer were then determined accordingly.(2)There is no stress concentration during the overall stacking process; stresses are mainly compressive stress while only a small area of tensile stress is produced inside the waste rock layer. Stress is well distributed. The displacement isoline populated inside the waste rock layers, while the maximum displacement point locates in this area as well. Displacement gets bigger with increasing stacking height, and the displacement in Z direction is always bigger than these in other directions. The tensile strength and shear-plastic zones, formed during stacking process in each step, are mainly distributed inside waste rock layer, gravel-soil layer, and heavily weathered layer near slope toe.(3)The limit equilibrium method is used for waste dump stability analysis considering three states: only applied gravity, applied gravity and rainfall, and applied gravity and underground water. The results from this analysis show that the waste dump is stable.(4)The potential failure modes of waste dump mainly consist of the “combined sliding mode” that has circular sliding in upper side and broken line sliding which cuts through gravel-soil layer into heavily weathered layer in the bottom. The potential failures are mainly distributed in benches near the slope toe. Due to the thick layer of waste rock layer at the bottom of dumping site, according to the failure mode and the plastic zone analysis results, it is recommended to completely remove the gravels, soils, and overburden before dumping waste rocks.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors are grateful for the financial support from the Changsha Institute of Mining Research Co., Ltd. and Tibet Huatai Mining Development Co., Ltd. They also thank the funding support from Congo international R&D center for mineral resources development of copper and cobalt (No. 2018WK2052).
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