DUMP SLOPE RATING FOR INDIAN COAL MINING

Transcription

1 DUMP SLOPE RATING FOR INDIAN COAL MINING Rahul Sharma 1), Rajesh Rai 2) and B. K. Shrivastva 2) 1 Bharat Cocking Company Limited, Dhanbad, Jharkhand, India rahul.min06@itbhu.ac.in, ) Department of Mining Engineering, Indian Institute of Technology, (Banaras Hindu University), Varanasi , UP, India, phone.: , rajeshrai.min@itbhu.ac.in; bkshrivastva.min@itbhu.ac.in Abstract: In the present study, an attempt is made to establish a dump slope rating scheme for coal mine dump slope. The factor of safety of dump slopes has been calculated by numerical modelling method. Dump height, slope angle, numbers of benches, hydrological condition, cohesion and friction angle of dump material are used for this study. Numerical modelling has been done for various parameters of Dump slope. Principal component analysis is performed to know the weight of an individual parameter. Dump stability classes are mainly used to recommend the level of the effort for investigation, design and construction of dump slopes. Keywords:Mining, Dump slope stability, numerical modelling, Stability Classification 1 INTRODUCTION The waste generation from opencast mines has exaggerated with increasing trend in production. The amount of waste materials is increasing at an alarming rate and can pose many problems. The management of waste overburden is of prime importance for the smooth production of mineral/ ore. The storage of waste material should be done in a proper way, keeping in view the safety of man and machines. The issue relating to the stability of overburden dumps is essential for the safe working in and around mining area. Dumping of waste without any previous analysis has caused the waste dump failures [1-3]. A classification system can be used to assess the condition of slope stability. It has been applied successfully for in tunnelling and underground mining. There are several classification systems have been developed for slopes [4-6]. However, there is no rating system has been designed for dump slope stability. In this study, an effort is taken to establish a classification system for dump slope. The parameters which have been considered for the classification are dump height, overall slope angle, cohesion, angle of internal friction, foundation type, slope of foundation, degree of confinement (if any), method of construction, piezometric and climatic condition and seismicity. Numerical modelling is used as tools to establish the classification system. Various field data have been simulated by using numerical modelling. Graphs are plotted between the factor of safety and different parameters. Based on rating system dump slopes are divided into various dump stability classes. Dump stability classes are mainly used to recommend the level of effort for investigation, design and construction of dump slopes. The level of monitoring, a requirement of support, optimise the height and dump slope can be easily done based on classification. 12

2 2 SLOPE CLASSIFICATION SYSTEMS In classification system, empirical relation between rock mass properties and the behaviour of the rock mass is assessed to a particular engineering application. It has been applied successfully for in tunnelling and underground mining. Rock mass classification systems developed originally for underground excavations have been modified for slopes [4,7]. Slope Mass Rating (SMR) was developed based on field data to rating adjustment for the discontinuity orientation parameter in the RMR system. Rock slope stability is governed by the behaviour of the discontinuities. The adjustment factors for discontinuity orientation and adding a new adjustment factor for the method of excavation is used to modification the RMR [4]. This approach is suitable for preliminary assessment of slope stability in rock, including very soft or heavily jointed rock masses. The Slope Mass Rating (SMR) is obtained from RMR by subtracting a factorial adjustment factor depending on the joint-slope relationship and adding a factor depending on the method of excavation. The adjustment factors depend on the parallelism between joints and slope face strike and joint dip angle in the planar mode of failure. It also depends on the relationship between the slope face and the joint dip types of the method of excavation [6-8]. Chinese slope mass rating system (CSMR system) was developed by Romana and Zuyu. They introduce two coefficients ξ and λ and modifies slope mass rating (SMR). The ξ represents the slope height factor and λ represents the discontinuity factor. These factors are included in the system because there are several failures occurred, but SMR indicates these are stable slopes. However, this is an accepted system of classification in Chinese condition only and needs a number of corrections and modifications before using at any other place [9-10]. Rock slope rating (RSR) system has been developed for using in the evaluation of rock slope stability under a variety of geological conditions and engineering requirements. RSR system evaluates the probability of failures for plane, wedge sliding, toppling and circular failures. Probability of each mode of failure is determined individually. The main categories for input parameters are geologic features, Safety requirements, Groundwater conditions, Slope geometry, Joint characteristics and Geomechanics parameters [5]. Another rock mass classification system which is based on combining the Analytic Hierarchy Process (AHP) and the Fuzzy Delphi method (FDM) can be appropriate for rock slope stability assessment was proposed. It treats rock classification as a group decision problem and applies the fuzzy logic theory on the criterion of weighting calculations. The proposed procedure was applied to determine the rating of rock slope with the hierarchy and weighting factors that are modified for rock slopes. The Linear Discriminant Analysis (LDA) model was used to classify the stable and unstable slope. The discriminant functions which can determine the failure probability of rock slopes were carried out by the LDA procedure. The results have been compared with unstable slope hazards occurring actually, and then the relation and difference between them were discussed. Finally, the results were summarized to derive a slope rock mass classification system with the failure of probability [11-12]. The Slope Stability Rating (SSR) system considered five additional parameters whose relative effects on the stability of fractured rock slopes were precisely examined based on data retrieved from different rock slope site. The proposed system considered five additional parameters whose relative effects on the stability of fractured rock slopes were precisely 13

3 examined based on data retrieved from different rock slope sites. An overall rating for the rock mass is obtained from the summation of the individual ratings of each parameter [13]. The rating values (either positive or negative) are considered regarding their influence on the stability of jointed rock slopes. Based on the parameter ratings determined based on sensitivity analysis performed by the mentioned method, a new rock mass classification system called Slope Stability Rating (SSR) is proposed. There are some limitations in above mentioned classification systems so that it could not be used properly for the classification of dump slopes in Indian conditions. Therefore, in the present paper, a new classification system is proposed for dump stability. 3 METHODS OF ANALYSIS In the present analysis, finite element methods and principle component analysis have been used for dump modelling and classification of dump stability [14-15]. Finite element method can be used to solve the equations of equilibrium, the strain compatibility equations, and the constitutive equation for material for prescribed boundary conditions. Both the stress and the displacements can be calculated using numerical modeling techniques. The factor of safety of a slope is the ratio of actual soil shear strength to the minimum shear strength required to prevent failure, or the factor by which soil shear strength must be reduced to bring a slope to the verge of failure [16-17]. In the SSR finite element, technique elasto-plastic strength is assumed for slope materials. The material shear strengths are progressively reduced until the collapse occurs [16]. The principal component can be defined as a linear combination of optimally-weighted observed variables. Principal component analysis (PCA) is a mathematical procedure that uses an orthogonal transformation to convert a set of observations of possibly correlated variables into a set of values of uncorrelated variables called principal components. It is mostly used as a tool in exploratory data analysis and for making predictive models. The results of a PCA are usually discussed in terms of component scores (the transformed variable values corresponding to a particular case in the data) and loadings (the weight by which each standardised original variable should be multiplied to get the component score). Principal component analysis is a variable reduction procedure. It is useful when a number of variables (possibly a large number of variables) are present and believe that there is some redundancy in those variables. General form of the formula to compute scores on the first component extracted (created) in a principal component analysis is given below: C1 = b 11(X1) + b12(x 2) +... b1p (Xp) (1) Where, C1 = the subject s score on principal component 1 (the first component extracted) b1p = the regression coefficient (or weight) for observed variable p, as used in creating principal component 1 Xp = the subject s score on observed variable p. In the dump slope rating, the principal component analysis is used to calculate the importance of parameters in the calculation of factor of safety of coal mine dump slope. The data collected from the analysis of various dump slope model is used as input. PCA tool provides the variable importance spreadsheet, and we can review the modelling power of the variables in histogram format. 14

4 4 DUMP SLOPE CLASSIFICATION Dump slopes are different from the normal rock slopes, but none of the classification systems considered the parameters, which should be considered separately for the dump slope. Therefore, the present classification system has been design to deal the dump slopes only. Selection of dump slope parameter is the most important step in the establishment of the rating system. Initially, the parameters which affect the stability of slope are considered. The factor of safety is calculated by varying one parameter while other parameters kept constant. If significant changes are observed, then parameter is selected otherwise it is neglected. The determination of these parameters in labs or field is easy as well as they are more influencing parameters for dump stability. The weight of each parameter is calculated by using data gathered from the numerical simulation and principal component analysis tool. STATISTICA [17-18] is used for principal component analysis. Total estimated weight is estimated to the parameter is allocated by using the variation in factor of safety with the change in the parameter. Allocated weight of the parameter is further used to develop rating system and to establish a classification system for dump slope. In the present study, the dump slope parameters and properties have been varied as given in Table 1. Tab. 1 Parameters used in analysis for dump slope and its variation Model Parameters of Dump slope stability Range No. 1 Overall height of dump 40 to Overall Slope angle 17 0 to Number of benches 2 to 10 4 Cohesion of dump material 10 to 70 kpa 5 Friction angle of dump material 15 0 to Ground water conditions 0 to 50 % 4.1 Weight estimation and allocation of parameters Evaluation of weight of these parameters in the calculation of factor of safety is a paramount step. Principal component analysis is used to calculate the importance of parameters in the computation of factor of safety of coal mine dump slope. It is providing result concerning variable importance. It measures how well the principal components represent a variable. PCA Tools provide the variable importance spreadsheet, and we can review the modelling power of the variables in the histogram format. Height, slope angle, number of benches, water condition, cohesion and angle of friction are input as continuous variable and factor of safety as the dependent variable. The input spreadsheet is used as training data. Results have come in the form of the variable importance spreadsheet, and variable importance histogram (Table 2). 15

5 Allocation of weight to the selected parameter is one of the most important steps in the rating of dump slope. Estimated weight and variation in factor of safety with the change in value of individual parameters are mainly used for it. All selected parameters are analyzed separately and based on change in factor of safety with change in that particular parameter weights are allocated. Table 2 shows the power of variables in the rating of dump slope overall slope angle gets the highest power, and the number of benches gets the least. All the parameters, which are going to affect the stability of dump slope or factor of safety of dump slope, are now arranged according to their importance. It is difficult to use the power of variable so normalization of these powers. The normalized values are used as weight for the further study. Tab. 2 Variable importance and corresponding estimated weight Variable Variable importance (Rating of dump slope) Variable number Power Importance Estimated weight Overall slope angle Overall height Water condition Angle of friction Cohesion Number of Benches Overall height of the dump The overall height of dump plays a very important role in the failure of mine waste dump slope. Numerical method is used as a tool to calculate the factor of safety of dump slope with varying overall height. Various models have been prepared and solved. In each model, only one parameter is changing while the other parameters have been kept constant. The parameters (bench height and overall slope angle) have been varied, and overall height is ranged from 40 meters to 200 m and factor of safety is calculated in each case. The factor of safety of the waste dump decreases with increased height of the dump. Figure 1 shows the relation between factor of safety of dump slope with varying overall dump height at different overall slope angle (number of benches = 02). Similarly, the number of benches has been varied from 2 to 6 and factor of safety is determined. The figure 1 is mainly divided into five regions, before 40 m, m, m, m and after 160 m. The highest rating, which has been allocated for overall height is 20. The highest rating is given to highest stable region and the least rating to the region in which graph is almost flat. Allocated ratings are shown in Table 3. 16

6 FACTOR OF SAFETY 2,5 2,4 2,3 2,2 2,1 2 1,9 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1,1 1 0,9 0, OVERALL DUMP HEIGHT(In meter) 17⁰ 19⁰ 21⁰ 23⁰ 25⁰ 27⁰ 29⁰ 31⁰ 33⁰ 35⁰ 37⁰ 39⁰ Fig. 1 Plot for Factor of safety of dump slope with varying overall dump height at different overall slope angle (Number of benches = 2) 4.3 Overall slope angle Overall slope angle plays a vital role in the stability of dump slope. Overall height, bench height, cohesion, angle of friction, Foundation type and slope and water condition are kept constant and only overall slope angle is varied 17⁰ to 39⁰ and factor of safety is calculated for each case. Figure 2 shows the factor of safety of dump slope with varying overall slope angle at different overall dump height (Number of benches=2). Similarly, the number of benches has been varied from and factor of safety is determined. The variation is significant, and measurement of overall slope angle of dump is not very tough task, so overall slope angle is taken as a parameter for our study. Factor of safety of dump slope is continuously decreased with an increase in overall slope angle. Initially, slope of the curve is slightly steeper, and later it becomes relatively flatter. Here, the graph can be easily divided into five regions (figure 2). Slope angle region at which highest stability can be achieved is allocated with highest rating of 30. After rating is decreased continuously up to 0 for the relatively flatter region. Allocated ratings are shown in Table 3. 17

7 FACTOR OF SAFETY 2,6 2,5 2,4 2,3 2,2 2,1 2 1,9 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1,1 1 0,9 0, OVERALL SLOPE ANGLE(In degree) Fig. 2 Plot shows Factor of safety of dump slope with varying overall slope angle at different fixed overall dump height (Number of benches=2) 4.4 Number of benches Individual bench height plays a significant role in stability of dump slope. The overall height kept constant and bench height is varied keeping other parameters keeping constant and factor of safety is calculated. Figure 3 shows the effect of the number of benches for the same overall height of the dump slope. Here overall height of the slope is kept constant at 120 meter and numbers of benches vary from 2 to 8. It is evident from the figure 3 with an increase in the number of benches factor of safety of dump slope also increases. Initially when the number of benches is increased from 2 to 4 then factor of safety also increases but after that no any considerable change is seen. The maximum rating of 10 is given when factor of safety becomes almost constant (number of benches<3) and rating of 5 is awarded when the number of benches is 2 or 3. 18

8 1,21 1,2 1,19 1,18 Factor of safety 1,17 1,16 1,15 1,14 1,13 1,12 1,11 100/31⁰ 120/29⁰ 120/31⁰ 1, Number of Benches Fig. 3 Plot shows Factor of safety of dump slope with varying Number of Benches at different fixed overall dump height 4.5 Hydrological condition Water is almost always present within rock and near the Earth's surface. In waste dump slope if the material is dry as non-saturated mass, an increase in load compresses the air in the pore spaces bringing grains or rock fragments closer together, which increases its shear strength. However, when a rock mass is saturated, an increase in external pressure leads to an increase in the pore pressure, as water is relatively incompressible. This increase in pore pressure has a buoying effect which may be enough to support the weight of the overlying rock mass, thereby reducing friction and the shear strength leading to failure of the slope. Water infiltrates into dump and may develop a temporary water table in the dump. Water infiltration rate and hydraulic conductivity of dumping material and local ground water table are primary reasons behind development of the water table inside the dump. Developed water table is measured with the help of piezometer. The ground water level is varied, and factor of safety is calculated in each case. Figure 4 shows the effect of the water table on dump slope. It shows as ground water table increases the factor of safety reduces. Water accumulation or excessive pore water pressure is one of the reasons behind the failure of dump slope. Factor of safety of dump slope is continuously decreased with increasing height of ground water table (It is mainly developed due to water infiltration from the rain). The whole graph is divided into four regions, and rating is assigned (Table 3). Maximum allocated rating is 15 and the rating is given to the condition when no any ground water table is developed. 19

9 FACTOR OF SAFETY 2,1 2 1,9 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1,1 1 0,9 0,8 0,7 0,6 0, PERCENTAGE HEIGHT OF GROUND WATER TABLE(w.r.t. total dump height) CASE 1 CASE 2 CASE 3 CASE 4 CASE 5 CASE 6 CASE 7 Fig. 4 Plot shows Factor of safety of dump slope with varying height of ground water table 4.6 Geotechnical properties of dump The important geotechnical properties affecting the dump slope stability are shear strength of dump, particle size distribution, density, and angle of repose. Materials that are coarse or have a rough texture having greater opposing frictional forces, or shear strength, and resist more to movement. Shear strength depends on many factors such as the type of material, the rate of loading, the degree of compaction and the moisture content. It is the most important engineering properties of the soil to assess the stability of structures. It gives the cohesion and friction angle for the prediction of stability of the slope. Figure 5 represents that factor of safety is continuously increased with an increase in cohesion of dump material. Different lines show the change in factor of safety at different fixed value of angle of friction. Figure 6 shows factor of safety of models significantly increases with an increase in internal angle of friction of dump material. 20

10 FACTOR OF SAFETY 2,9 2,8 2,7 2,6 2,5 2,4 2,3 2,2 2,1 2 1,9 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1, COHESION OF DUMP MATERIAL(In Kpa) 20⁰ 21⁰ 22⁰ 23⁰ 24⁰ 25⁰ 26⁰ 27⁰ 28⁰ 29⁰ 30⁰ 31⁰ Fig. 5 Plot shows Factor of safety of dump slope with varying cohesion of dump material FACTOR OF SAFETY 3 2,9 2,8 2,7 2,6 2,5 2,4 2,3 2,2 2,1 2 1,9 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1,1 1 0,9 0, ANGLE OF FRICTION( In Degree) 10 KPa 20 KPa 30 KPa 40 Kpa 50 Kpa 60 KPa 70 Kpa Fig. 6 Plot shows Factor of safety of dump slope with varying internal angle of friction of dump material 21

11 5 PROPOSED DSR (DUMP SLOPE RATING) SYSTEM FOR COAL MINE Based on the parameter ratings described in the previous chapter, a new coal mine dump slope-rating scheme called DSR (Dump slope rating) system has been proposed. The final DSR value of a given rock mass is obtained after summation of the rating values of all the parameters, as illustrated in Table 3. The stability of dumps slope are divided into stability classification on the basis of factor of safety and DSR value. Table 4 shows which type of failure hazard is expected in which dump stability class. It also explains the level of investigation required by particular class and the design and construction of the dump slope. Tab. 3: Allocated weight for various parameters Overall dump height Dump height (m) Up to Above 160 Rating Overall slope angle Slope angle ( o ) < < Rating Number of Benches No of Benches 2-4 >4 Ratings 5 10 Ground water table (w.r.to total dump height) Condition of ground water No groundwater table Up to 15% height 15 to 30 % height 30 to 40% height Above 40 % height Rating Geotechnical properties of dump Cohesion (kpa) < >70 Rating Friction angle ( 0 ) < >32 Rating

12 Dump stability class A B C D Tab. 4 Dump stability classes and recommendations Failure hazard Negligible (long term stable) Low (Stable) Moderate (Short term stable) High (Unstable) Recommended level of effort for investigation, design and construction Minimum lab testing is required Minimal restriction on construction Visual monitoring is sufficient Thorough site investigation Limited lab index testing Basic stability analysis is required Limited restrictions on construction Routine visual and instrumental monitoring Detailed site investigation Undisturbed samples may be required Detailed lab testing, including shear strength and durability test Detailed stability analysis is required including parametric study Moderate restriction on construction Detailed instrumental monitoring is required Detailed and phased site investigation Undisturbed sampling probably required Detailed stability analysis is required, including parametric studies and full evaluation of alternatives Severe restrictions on construction Detailed and continuous instrument monitoring is required Range of dump rating > <40 6 CASE STUDY The lignite mine A is located at Rajasthan. The Coal block is bounded by latitude to and longitude to The mine is designed with the overall slope angle is 23 ⁰ ; the overall height of the dump is 60 m with 15 m of each bench height. The area is in the desert so any ground water table is not generated in the external dump. Cohesion of the dump material is 15 kpa and angle of internal friction is 30⁰. Calculation of the DSR value for this case is shown in table 6. The DSR value is 71 for this case, and it suggests that failure hazard in this case is low and only primary stability analysis is required time-to-time and only routine visual and instrumental monitoring is required. The factor of safety of external dump slope is 1.68 by numerical modelling which indicates that the external dump is stable for long term. 23

13 The other Mine B is located near the Singrauli area. The area lies geographically between latitudes of and longitudes The overburden rocks in this area are mostly medium to coarse-grained sandstone, carbonaceous shale and shaly sandstone. The mine is in the production stage. The external dump, the overall slope angle is 33.5 ⁰ ; the overall height of the dump is 140 m with 35 m of each bench height. In summer ground water table is not generated in the external dump. Cohesion of the dump material is 70 kpa and angle of internal friction is 39.5⁰. Calculation of the DSR value for this case is shown in table 7. For this case with DSR value=62.25, table: 7 suggests that failure hazard in this case is low and only basic stability analysis is required time to time. Overall height and overall slope angle are very high in this case but along with that material property is also very good so DSR value lies in low failure hazard zone. In this case, due to high DSR value, only few restrictions are on construction and only routine visual and instrumental monitoring is required. The factor of safety of external dump slope is It indicates that the external dump is stable for long term. Tab 5 Dump slope rating (DSR) calculation for lignite mines (Mine A) Parameter Value Rating Overall height 60 meter 15 Overall slope angle Number of benches 4 5 Ground condition water Cohesion of dump material Internal angle of friction No ground water table kpa Total Rating(DSR) 71 24

14 Tab.6 Dump slope rating (DSR) calculation for open cast coal mine (Mine B) Parameter Value Rating Overall height 140 meter 5 Overall slope angle Number of benches 4 5 Ground water No ground water table 15 condition Cohesion of dump 70 kpa 10 material Internal angle of friction Total Rating(DSR) 60 7 CONCLUSIONS The following conclusions have been drawn from the study: 1. A dump slope rating (DSR) system is proposed for assessment of the conditions dump slope. This system incorporates the six parameters, namely, the overall dump height, overall slope angle of the dump slope, number of benches, hydrological condition, cohesion of dump material and internal angle of friction of dump material. 2. The Dump slope rating (DSR) value is obtained by summation of the individual rating of each parameter, whose relative weight is calibrated based on a parametric study of each parameter and principal component analysis. 3. Dump slopes are classified into 4 stability classes A with negligible failure hazard, B with low failure hazard, C with moderate failure hazard and D with high failure hazard. 4. The stability of two different dump slope have been determined by dump slope rating and finite element method. The results from both cases were found satisfactorily. 25

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