IDENTIFICATION OF FAILURE SURFACES IN DRAGLINE DUMPS OF OPENCAST COAL MINES

Transcription

1 IDENTIFICATION OF FAILURE SURFACES IN DRAGLINE DUMPS OF OPENCAST COAL MINES S. Sharma, S. Sengupta and I. Roy Department of Civil and Environmental Engineering, Birla Institute of Technology, Mesra, Ranchi, India ABSTRACT Overburden dumps are an integral part of open pit coal mining. With an increase in the mining activity to meet the energy demands, the volume of overburden dumps in the open pit mines is also increasing. Draglines being the most versatile excavating machines are the favourable and most used machinery for dumping of the overburden material. Inadequacy of dumping space within the pit leads to the formation of steep and high dragline dumps which then succumbs to various failure modes following certain failure paths. This paper identifies the most probable failure paths with least factor of safety, within the overburden dragline dump masses of coal mines of Northern Coalfields Limited, Singrauli, India. This paper also identifies the different failure pattern and modes within the overburden dragline dump masses along the derived failure path. Keywords: dragline dump, circular-cum-planar failure, circular failure, open pit mining. 1. INTRODUCTION According to Kennedy B. A. (1990), dumping work is an important component of open pit mining [1]. According to Zhang D et al. (2014), under overburden - stripped and waste rocks or soils are transported to dumping areas to expose and extract coal seams [2]. Coal mines in India have high stripping ratio. According to Sanyal B (2006); the coal mines of Coal India Limited (CIL) removed about 500 million cubic metres (Mcum) of overburden (OB) to produce 260 mt of coal in at an average stripping ratio of 1.92 cu m of OB against per tonne of coal production [3]. With the continuous increase in energy requirements of the country, the demand for more and more coal is also increasing. In order to satisfy this demand of coal for energy requirements, mine operators are further going deeper into the mines for extraction of more coal. This in turn is generating and will further continue to generate huge amount of overburden waste material. According to Mathur A. D. and Ramaswamy S. (2013), draglines are the best suited equipment for handling such huge amount of overburden material generatedbecause of the high production capacity and lower cost per cum moved [4]. Draglines are versatile excavating machines deployed to remove overburden in large opencast mines. In India, dragline mining is done to excavate overburden material in strips of 25m to 70m width depending upon the boom length i.e. reach of the machine. Limitation of dumping area within the pit leads to the formation of steep and high dragline dumps. Other than height and slope of internal dragline dump material, coal rib, hydrostatic pressure and geometry of the dump also plays important role in the stability of dumps. According to Singh A. K. (2013), any compromise with these parameters results in dump slides, which had also been fatal in the past [5]. This paper identifies failure surfaces in the dragline dumps and discusses the various probable failure modes on dragline dump slopes in opencast coal mines of Northern Coalfields Limited, India that were studied throughout a CIL R and D project: development of guidelines for dragline dump profile under varying geoengineering conditions (Roy I, 2013) [6]. 1.1 Formation of dragline dump Draglines are more versatile and work independently. They can swing and no dump trucks are required. According to Kumar R. B. (2013), these machines are used to dig overburden below its level and immediately above bottom most coal seam and to sidecast the material in the form of heaps in the de-coaled area, thereby exposing the underlying coal for extraction (Figure-1) [7]. Figure-1. According to Kumar R. B. (2013), dragline in operation: formation of dragline dumps [7]. 2. STUDY AREA (Table-1) The areas studied during the investigation were; 43

2 S. No. Table-1. Study areas. Name of the mines/opencast project 1 Jayant (Figure-3) 2 Dudhichua (Figure-2) 3 Khadia 4 Bina 5 Nigahi 6 Amlohri Company Northern Coalfields Limited 3. NOTATIONS The notations of each geo-engineering parameter are presented below (Figure-5); H = Overall height of dump with respect to horizontal plane passing through dumptoe (m). L = Overall slope angle with respect to horizontal plane passing through dump toe ( 0 ). C 2 = Cohesion of dump material (kn/m 2 ). Φ 2 = Angle of internal friction of dump material ( 0 ). C 3 = Cohesion of interface material (kn/m 2 ). Φ 3 = Angle of internal friction of interface material [A layer of crushed coal, crushed rock mixed with water lies at the mine floor - this layer is called interface material ( 0 ). γ 2 = Bulk unit weight of dump material (kn/m 3 ). I = Mine floor inclination ( 0 ). D = Height of water table behind one dragline cut or 60m behind toe of the dump (m). C 4 = Cohesive resistance between coal rib and its floor (kn). Φ 4 = Frictional resistance between coal rib and its floor (kn). Ag = Ground acceleration generated in dump mass in case of earthquake (m/sec 2 ). Figure-2. Dragline dumps of Dudhichua Opencast Project. 4. GEO-ENGINEERING PARAMETERS (Tables-2, 3) The geo-engineering parameters of dumps and interface material comprise of the following; a) Geotechnical parameters. b) Hydrogeological parameters. c) Geo-mining parameters. Figure-3. Dragline dumps of Jayant Opencast Project. The geo-technical parameters like cohesion, angle of internal friction and bulk unit weight were determined from large box shear test apparatus (40x40 cm 2 ). The hydrogeological parameter has also been computed from the hydrogeological data like length of external oozing on the surface of the dump, co-efficient of permeability and also cut-width. The geo-mining parameters like mine floor inclination, general rock strata inclination, details of heavy earth moving machinery, bench configuration and existing slope geometry has been assessed from the different project specific sites. 44

3 S. No. Name of the mine Table-2. Geo-engineering parameters. H (m) L ( 0 ) I ( 0 ) C2 (kn/m 2 ) Φ2 ( 0 ) C3 (kn/m 2 ) 1 Jayant Dudhichua Khadia Bina Nigahi Amlohri Continued to Table-3 S. No. Name of the mine Table-3. Geo-engineering parameters. Φ3 ( 0 ) C4 (kn) Φ4 (kn) γ2 (kn/m 3 ) D (m) Ag (m/sec 2 ) 1 Jayant Dudhichua Khadia Bina Nigahi Amlohri RESULTS -FAILURE SURFACES (Table-4) Depending on the competency and strength characteristics of both dump and interface as well as inclination of the mine floor, most probable failure surfaces with least factor of safety were identified and failure modes (as shown in Figure 6(a)-6(l) (Table-4) were deciphered by means of self-developed computer program. The failure surfaces in the overburden dragline dumps of coal mines of Northern Coalfields Limited, followed circular and circular-cum-planar paths depending upon the mine floor inclination and competency of the interface (foundation) material. i.e. a layer of crushed coal and rock mixed with water lying at the mine floor. In case of internal dragline dumps standing over weak foundation the failure surface is through the dump material and also through the interface material (Figure-4), leading to circular-cum-planar failure instead of circular failure. On the other hand those standing over comparatively more competent foundation, the failure surface is through the dump material only (Figure-5). 45

4 Figure-4. surface through dump as well as interface material leading to circular-cum-planar failure instead of circular failure. Figure-5. surface only through the dump material leading to circular failure. 46

5 Table-4. Co-ordinates of failure surfaces and failure modes. S. No. Name of the mine Co-ordinates of failure circle mode Jayant (Figure 6A, 6B) Dudhichua (Figure 6C, 6D) Khadia (Figure 6E, 6F) Bina (Figure 6G, 6H) Nigahi (Figure 6I, 6J) Amlohri (Figure 6K, 6L) X Y XX 1-10 X Y X Y XX 1-40 X Y X Y XX 1-15 X Y X Y XX 1-10 X Y X Y XX 1-15 X Y X Y XX 1-15 X Y

6 Figure-6(a). Jayant circular-cum-planar-failure: surface through dump material and also through interface material. Figure-6(b). Jayant circular failure: failure surface only through the dump material. 48

7 Figure-6(c). Dudhichua circular-cum-planar failure: surface through dump material and also through interface material. Figure-6(d). Dudhichua circular failure: surface only through the dump material. 49

8 Figure-6(e). Khadia circular-cum-planar failure: surface through dump material and also through interface material. Figure-6(f). Khadia circular failure: surface only through the dump material. 50

9 Figure-6(g). Bina circular-cum-planar failure: surface through dump material and also through interface material. Figure-6(h). Bina circular failure: surface only through the dump material. 51

10 Figure-6(i). Nigahi circular-cum-planar failure: surface through dump material and also through interface material. Figure-6(j). Nigahi circular failure: surface only through the dump material. Figure-6(k). Amlohri circular-cum-planar failure: surface through dump material and also through interface material. 52

11 Figure-6(l). Amlohri circular failure: surface only through the dump material. 6. CONCLUSIONS The result of investigation is presented in Figure 6A-6L and Table 4 to show the paths and co-ordinates of the most probable failure surfaces in the dragline dumps of the investigated opencast coal mines. It also discourses the failure modes occurring in the dragline dump slopes of the mines. Kumar R. B Dragline mining: prospects and challenges. National Seminar on Dragline Mining: Prospects and Challenges, Singrauli. pp REFERENCES Kennedy B.A Surface Mining. 2 nd Edition, Society for Mining, Metallurgy, and Exploration, Indonesia. p Zhang D, Inoue N, Sasaoka T, Shimada H, Hamanaka and Matsui K Study on formation mechanism of dumping piles on dumping area stability. Open Journal of Geology, March 2014; ( Sanyal B Coal to stay key source for nation s energy needs. Business Line. Mathur A. D. and Ramaswamy S Experience of dragline mining in NCL. National Seminar on Dragline Mining: Prospects and Challenges, Singrauli. pp Singh A. K Dragline mining in India. National Seminar on Dragline Mining: Prospects and Challenges, Singrauli. pp Roy I Development of guidelines for safe dragline dump profiles under varying geo-engineering conditions in opencast coal Mines of Coal India, January. 53