Blast Design for Controlled Augmentation of Muck Pile Throw and Drop

Transcription

1 터널과지하공간, 한국암반공학회지제 20 권제 5 호, 2010 년 10 월, pp. 360~368 TUNNEL & UNDERGROUND SPACE, Vol. 20, No. 5, 2010, pp Journal of Korean Society for Rock Mechanics Blast Design for Controlled Augmentation of Muck Pile Throw and Drop Piyush Rai 1), Hyung-Sik Yang 2) * 발파석의비산과낙하를조절하기위한발파설계 피유시라이, 양형식 Abstract The paper presents a case study from a surface mine where the controlled augmentation of throw and drop of the blasted muck piles was warranted to spread the muck piles on the lower berm of the bench. While the augmentation of throw increased the lateral spread and the looseness of the broken muck, the augmentation of drop significantly lowered the muck pile height for easy excavation by the excavators. In this light, the present paper highlights and discusses some pertinent changes in the blast design parameters for such specialized application of cast blasting in a surface mine, where a sandstone bench, with average height of m was to be made amenable for excavation by 10 m 3 rope shovels, which possessed maximum digging capability of up to 14 m. The results of tailoring the blast design parameters for augmentation of throw and drop are compared with the baseline blasts which were earlier practiced on the same bench by dividing the full height of the bench in 2-slices; upper slice (10-14 m high) and lower slice (12-15 m high). Results of fragment size, its distribution and total cycle time of excavator (shovel) are presented, and discussed. Key words Throw, Drop, Muck pile 초록이논문은발파석의비산과낙하를조절하여파쇄석이사면의낮은쪽소단에분산될수있도록한노천발파에대한사례연구이다. 비산이횡적인분산과파쇄석더미의이완을초래하는동안낙하는굴착기에의한굴착이용이하도록파쇄석더미의높이를낮춰주는역할을한다. 이런면에서이논문에서는몇몇발파설계변수들을조정하는데주안점을두었다. 대상사면은사암벤치로서벤치의평균높이는 22~24 m 이다. 이사면은굴착심도가 14 m 인 10 m 3 용량의로우프쇼벨작업이가능하도록조성되었다. 비산과낙하를조절한새로운발파설계결과를이현장에서적용되고있던상단 (10~14 m) 과하단 (12~15 m) 의이단식발파결과와비교하였다. 파쇄석의입도와그분포및쇼벨의굴착싸이클시간들을비교하였다. 핵심어비산, 낙하, 발파석 1. Introduction Blast casting is an established technique to move a substantial percentage (40-80%) of the overburden rocks to the disposal site inside the pit using the explosives. 1) 전남대학교방문교수, 인도바나라스힌두대학교광산공학과부교수 2) 전남대학교에너지자원공학과교수 * 교신저자 : hsyang@jnu.ac.kr 접수일 : 2010년 9월 29일심사완료일 : 2010년 10월 19일게재확정일 : 2010년 10월 20일 The technique aims at controlled amplification of throw distances by using the explosive energy. On the other hand, in normal rock blasting operations the broken rock should be thrown only far enough to allow room for expansion of the broken rock. Still, there are certain specialized situations which stipulate the throw distances which are intermediate to the throw in blast casting and normal blasting operations. These specialized situations, for instance, refer to considerable muck pile loosening by increasing the throw in order to facilitate their loading by the front-end-loaders or similar machines 360

2 터널과지하공간 361 which have low diggability (Rai & Imperial, 2005). Furthermore, there are instances where the height of benches overshoots the maximum digging height of the designated excavators. Controlled augmentation of muck pile throw by blasting, as discussed in this paper, refers to case in which an increased throw of the muck pile was imperative to spread the muck pile on the lower berm of the bench. This, on one hand, increased the looseness of the muck pile and also lowered its height significantly for easy excavation by the excavators. Additionally, in contrast to cast blasting, as the entire blasted muck was excavated by the rope shovels, post-blast fragment size also needed to be controlled for efficient loading by the excavators. In this light, the present paper highlights and discusses a few pertinent changes in the blast design parameters for such specialized application of throw and drop augmentation in an overburden bench of a surface coal mine. 2. Scope of present study Fig. 1. Desired muck pile shape The present study was conducted on a high sandstone overburden bench section of a major surface coal mine. Although the average height of the overburden bench normally ranged from m, occasionally it reached even up to 28 m. The bench was mined by dividing it in 2-slices; upper slice m high and lower slice, m high. The mine was divided into two sections, east and west. In each section there were 11 benches and only 14 nos. of 10 m 3 rope shovel, which were equally divided in the 2-sections of the mine. As such, the study section (west section) with 11 benches had only seven shovels. The paucity of shovels led to their frequent marching from one bench to the other, which, in turn, reduced their availability and the productivity. To overcome this constraint, the mine management decided to work the said overburden in a single slice, m high after combining the upper and lower slices. However, with the existing blasting practices, it was envisaged that the height of blasted muck pile would be enormous for safe excavation by the specified 10 m 3 rope shovels possessing a maximum digging height of up to 14 m only. Excavation of such high muck piles by the given shovel was considered hazardous and inefficient in view of large overhangs. Furthermore, the procurement of larger sized shovels, with greater digging reach was not an economically viable proposition. Giving due cognizance to these issues, the current research project was undertaken to with the objective of modifying the important blast design parameters for blasting the combined bench (22-24 m high) to augment the throw in a controlled manner, which in turn, ought to laterally spread the muck pile and drop it down for safe and convenient excavation of the muck pile by the specified rope shovel. The desired shape of the muck pile after controlled augmentation of throw and drop is illustrated in Fig. 1. Additionally, it was also important to keep the fragmentation within acceptable limits for excavation by bucket of 10 m 3 rope shovels. 3. Brief description of the mine The mine, where the studies were conducted, is one of the largest and well-mechanized surface coal mines, in India, having large capacity HEMMs including 4 draglines. A major fault divided the mine into 2-working sections; east and west section. The stripping ratio was about 1:3 and annual coal production was almost 10 Mte. It consisted of eleven overburden benches and 3 coal seams. The mine belongs to the Singrauli coa3- coal seams separated by substantial overburden partings. lfields of Coal India Limited, which consisted of The overburden rocks were of Gondwana formation. There were 11-overburden benches in each section of the mine. The overburden bench, on which the blasting studies were conducted, was m thick (occasionally 28 m at certain locations). It consisted of medium to coarse-grained sandstone with uniaxial compressive strength of MPa, tensile strength of 1-2 MPa and shear strength of 1-5 MPa. The bench was fairly

3 362 Blast Design for Controlled Augmentation of Muck Pile Throw and Drop uniform and did not possess any significant geological anomaly. The overburden, after blasting, was removed by 10 m 3 rope shovels in conjunction with 85 &120 ton-rear dump trucks. The coal seam underlying this overburden bench comprised E-F grade coal with average heat value of 4818 Kcal. The average ash, moisture and fixed carbon percentages in the coal were 26.1, 8 and 30.8% respectively. 4. Research methodology To commence with, the baseline blast data and results were recorded by conducting two blasts on the said bench while it was worked in two separate slices (as described in section 2). The important baseline parameters and data are tabulated in Table 1. Thereafter, a series of blasts were conducted on the combined bench, m high to meet the slated objectives. The salient blast parameters and data of these blasts are summarized in Table 2. Keeping the prime objectives of controlled augmentation of throw and drop to lower down the blasted muck height, while maintaining the fragment size within acceptable limits for handling by the designated 10 m 3 rope shovels, as aforementioned, the modification attempted in the blast design and their justifications are explained as follows: Row-to-row firing pattern was discussed and implemented. In the row-to-row type firing pattern, all the blast holes in a row were fired simultaneously and the consecutive rows were fired at higher delay interval. This type of firing pattern was designed in view of its known superiority of providing adequate relief to the burden rock. Provision of adequate relief was considered to be very instrumental in casting the mucks. The row-to-row firing pattern as implemented for the blasts is illustrated in Fig. 2. Burden to spacing ratio was kept almost as 1 in line with the recommendations cited by Chironis (1981); Giltner & Worsey (1986) and Dupree (1987), who attached importance to the fact that radial fracturing extends out and links adjacent holes before reaching the free face, resulting in more efficient use of explosive energy in form of propulsion. Also, with this ratio, there is less chance of over-confinement and stemming Table 1. Blast design parameters and data for baseline blasts Parameters B1 B2 Compressive strength of rock (MPa) Hole diameter (mm) Mesh area, S x B (m) 9x8 9x8 Avg. bench height (m) Avg. sub-grade drilling (m) 1 1 Avg. stemming length (m) Avg. deck length (m), 1 1 Total no. of holes No. of rows Total explosive quantity (Kg) Avg. column charge length (m) Initiator D-cord D-cord Drilling pattern Rectangular Rectangular Firing pattern Diagonal Diagonal Inter-row delay timing(ms)* 0/100/125/150 0/100/125/150 Mean fragment size, K50 (m) P.F. (kg/m 3 ) Avg. total cycle time (s) 48 46

4 터널과지하공간 363 Table 2. Blast design parameters and results for the modified blasts Parameters B3 B4 B5 B6 Compressive strength of rock (MPa) Hole diameter (mm) Mesh area, S x B (m) 9x9 9x8.5 9x8.5 9x9 Avg. bench height. (m) Avg. sub-gradedrilling (m) Avg. stemming length (m) Total no. of holes Total explosive quantity (Kg) Initiator D-cord D-cord D-cord D-cord Drilling pattern rectangular rectangular rectangular rectangular Firing pattern Row-to-row Row-to-row Row-to-row Row-to-row Deck Vol. of broken muck (m 3 ) No. of rows fired Inter row delays 0/150/175 0/150/175 0/150/175 0/150/175 Column charge length (m) P.F. (kg/m 3 ) M.F.S. (m) Avg. total cycle time (s) Fig. 2. Row-to-row type firing pattern ejections. The fixing up of burden value was done by engineering calculation as per Konya s formula for estimation of burden (Konya, 1995) after taking into account the related rock parameters, geological properties and explosive factors. Furthermore, the past experiences of burden fixation for normal production blast (base line blasts) on the given bench also provided useful insights. The inter-row delay interval was enhanced to provide sufficient time for proper burden movement such that burdens get fully detached and adequately displaced before firing of the next row commenced. The inter-row delay was kept at ms/m of firing burden. This delay timing is in line with that prescribed by Tracy (1985), Rollins & Givens (1987), Konya (1995), Jimeno et al., (1995). The primary thrust was given on burden movement with ample burden velocity. In case of tight inter row delays, the throw and subsequently the drop results could be poor because successive rows of holes, fired with shorter delays have a tendency to retard the desired displacement of the blasted muck pile. The blast round were slightly overcharged to ensure that sufficient energy was available from the explosive for adequate throw on one hand and acceptable fragmentation on other hand for given 10 m 3 rope shovels. Singh et al. (1996) suggested charge factor of kg/m 3 in coal mine overburden rocks for moderate casting. Although this change resulted in increased explosive consumption, it was justifiable from viewpoints of considerable savings in the shovel operating times and costs on such high benches. The explosive

5 364 Blast Design for Controlled Augmentation of Muck Pile Throw and Drop selected for such applications, must be of requisite properties such as high VOD, adequate RBS, high gas energy etc. in order to provide ample burden velocity to the rock mass. Although ANFO would have been the best choice, site- mixed emulsion explosive was used in the blasts due to non-availability of ANFO. In order to marginalize the premature venting of explosive energy, 3 to 4 gunny bags (225 mm dia. and 0.8 m long) filled with drill cuttings were inserted within the stemming column to provide better confinement. The stemming column was well-compacted with wooden rod. This practice helped in achieving good fragmentation and controlling the fly rocks. The possibility of high wall damages such as back break, over break etc. were not completely ruled out as high amount of explosive was detonated in these large-scale blasts. To this end, wall-control technique of line drilling of 251/269 mm dia. drill-holes, 7-10 m deep at the boundary of the blast rounds was practiced with success. Almost straight ( ) face of the high wall was maintained, which is always desirable for good and proper throw. Such gigantic blasts are always endangered with ground vibrations, which called for a detailed study before attempting the blasts at field-scale. Accordingly, the predictor equations were established by conducting trial blasts. To limit the blast induced ground vibrations, 3-4 holes were blasted at one time in a row with 25 ms delay. The blast rounds consisted of 4-drilling rows on a rectangular pattern. Each row had holes. This drilling pattern was diagonally fired in 12 diagonal rows. The delay timing of 0/100/125/150 means that the starting from the first hole (also known as initiation point of the blast round) at 0 delay, next 4-firing rows were fired at inter-row delay timing of 100 ms. Subsequently, 3-firing rows were fired at inter-row delay timing of 125 ms and thereafter the last 5-rows were fired at inter row delay timing of 150 ms. The incremental delays with the blast progression are very useful in providing adequate relief to the subsequent rows. To measure the fragment size in post-blasted muck piles, image capturing, processing and analysis technique was deployed. The imaging technique has been in use in the field of blasting for last almost two and a half decades. Needless to state that with the use of computers, the quantification process has become largely simplified, quick and almost inexpensive. The basis of this technique is to capture scaled images of the blasted muck pile using a high resolution camera in the field, and then to digitize and measure the delineated fragments to provide a measure of the particle size distribution. In the present work, blasted muck piles were documented with scaled images captured from front of the blasted bench. It is understandable that a single image or even a couple of images are incapable of evaluating blasted muck piles. Hence, a series of images were periodically captured (every hour) to cover the entire excavation sequence of the blasted muck pile. Besides, in the event of any exceptional situation, such as, occurrence of large boulders, large amount of fines, evidences of geological features, etc., a few additional images were captured. Typically an image frame could capture broken rock fragments. Analysis of almost images for each muck pile was considered suitable for yielding a statistically representative sample for characterizing the fragmentation in one muck pile. These guidelines are per the recommendations published by various researchers (Reid, 1976; Maerz et al. 1987; Exadaktylos and Tsoutrelis, 1991; Palangio and Franklin, 1996; Scott et.al, 1996, Wang et al., 1996). A suitable scale marker (square scale 0.2 x 0.2 m, painted in red color/ a wooden ruler 0.5 m long, painted in brown color) was placed in the image frames for calibration. For the purpose of processing and analysis of captured images to characterize the fragment sizes in the blasted muck piles, commercial state-of-art image analysis software Fragalyst TM was used. The stopwatch was used to precisely record the total cycle time of rope shovels operating on the blasted muck piles. The average total cycle time was estimated from an adequately good number of observations. This data is quite consequential in order to indirectly assess the impact of implemented blast design changes on the total cycle time of the designated shovels, to highlight the excavation condition in the blasted muck piles. 5. Results and discussion A comparative perusal of blast results given in tables

6 터널과지하공간 & 2 and also the complete fragment size distribution curves for blasts 1-6 (as represented in figs. 3-8) reveal that the mean fragment sizes in the baseline blasts B1-B2 were quite large in comparison to the modified blasts B3-B6. Consequently, the total cycle time of shovels, operating on the blasts B1-B2, was also quite high. The presence of large to very large sized boulders in the crust portion of the muck piles corresponding to the collar region (Fig. 9) and the back rows of these blasts (Fig. 10) is observed clearly. However, in the mantle portion of these blasts, the fragmentation was better (Fig. 11). On the other hand, both the fragment size and the total cycle time of the shovels were lower in the blasts B3-B6, which were tailored to increase the throw and drop. The tailoring of the blast design parameters assisted in augmenting the throw, which, in turn, augmented the lateral spreading of the muck piles (Fig. 12). Con- Fig. 3. Complete fragment size distribution for blast 1 Fig. 6. Complete fragment size distribution for blast 4 Fig. 4. Complete fragment size distribution for blast 2 Fig. 7. Complete fragment size distribution for blast 5 Fig. 5. Complete fragment size distribution for blast 3 Fig. 8. Complete fragment size distribution for blast 6

7 366 Blast Design for Controlled Augmentation of Muck Pile Throw and Drop sequently, considerable drop of the muck piles can be seen (Fig. 13). The field measurements made on these blasted muck piles, immediately after blast, revealed that the blasted mucks had laterally spread to approximately m. and dropped down by 8-10 m. While the lateral spreading increased the looseness of the muck piles, the drop resulted into vertical lowering of these muck piles. Accordingly, the height of these muck piles was reduced to abide the maximum digging height of the given 10 m 3 rope shovels. Additionally, lateral spreading and the vertical lowering of the combined high bench was helpful in sharply cutting down the unproductive marching and non-availability hours of the shovels in contrast to working the same bench in 2-slices. Post blast wall-control, which was considered as very critical to the success of these blasts, appeared quite good with almost vertical high wall (Fig. 14). The straight and damage free high walls can be attributed to the line drilling of holes as explained in the previous section, which provided a weak plane for neat shearing of strata. Nevertheless, it is important to mention here that in such large-scale blasts maximum three drilling rows, each containing not more than Fig. 9. Presence of boulders in the collar region Fig. 10. Large boulders along the back rows in the crust and smaller fragments in the mantle Fig. 12. Laterally spread muck pile immediately after blast Fig. 11. Fairly good and uniform fragmentation inside the muck pile (in the mantle) Fig. 13. Vertical lowering of the muck pile by 8-10 m

8 터널과지하공간 holes, can only be fired in order to limit the possibility of back breaks and ground vibrations. Although, as seen in the Fig. 15, some large sized fragments were always present in these blasts, the mean fragment size results appear to be within acceptable limits vis-à-vis excavation by 10 m 3 rope shovel. The maximum fragment size for some blasts was quite high (about 0.75 m); still it was much lower than the baseline blasts with maximum fragment sizes (K 100) of over 0.96 and 0.84 m. The coarse fragment size (K 90) also revealed the similar trends. Similarly, the total cycle time of the shovels was recorded to be within seconds, which is commendable. The literature as well as the manufacturers suggest the total cycle time of up to 45 sec. The total cycle time of shovels operating on the baseline blasts Fig. 14. Almost straight high-wall was significantly higher as the muck piles were not loose and remained tightly close to the face. To commence with, the changes there was perceptible fear of unduly increasing the cycle time due to presence of large proportion of boulders in the blasted muck piles. The results, on the contrary, indicate that the muck handling time by the excavator did not increase significantly to affect the excavator performance. Conversely speaking, the cycle time study corroborates the earlier mentioned findings pertaining to the overall degree of fragmentation in the blasted muck pile. With further site-specific refinements in the suggested blast designs, the given 80 m wide bench was excavated successfully for 1500 m strike length in the west section of the mine. 6. Conclusions 1) Row-to-row firing with increased delay timing between the rows, increased explosive charge and its proper distribution in each hole are important design parameters for controlled augmentation of throw and drop of the blasted muck piles. 2) The fragment size analysis, as done by imaging technique, reveals good degree of fragmentation in the laterally spread and vertically lowered blasted mucks. 3) Augmentation of throw and drop results into considerable spreading and loosening of the muck piles. 4) Spreading, lowering and loosening of muck piles have been positively manifested by the performance of excavator (rope shovels) in terms of total cycle time. 5) Controlled augmentation of throw and drop of the blasted muck of the combined high bench was useful in sharply cutting down the unproductive marching and non-availability hours of the shovels, while working the same bench in 2-slices. Acknowledgement Fig. 15. Occurrence of some boulders inside the muck piles The excellent support and co-operation rendered by the management and staff of Jayant open-cast mines of Northern Coal Fields ltd., Dist. Sidhi, India, is gratefully acknowledged.

9 368 Blast Design for Controlled Augmentation of Muck Pile Throw and Drop References 1. Chironis, N.P., 1981, Dragline computer improves control, Coal Age, pp Dupree, P.D., 1987, Applied drilling and blasting techniques for blast casting at Trapper mine, potential to save on overburden removal, Mining Eng., V. 26, No. 1, pp Exadaktylos, G.E. and C.E. Tsoutrelis, 1991, Fragmentation analysis using the photographic method, Int. Journal of Sur. Min. & Reclamation, V.5, pp Giltner, S.J. and P.N. Worsey, 1986, Blast monitoring using high speed video research equipment, Proc. Am. Conf. on Expl. & Blasting Techniques, SEE, pp Jimeno, C.L., E.L. Jimeno and F.J.A. Carcedo, 1995, Drilling and Blasting of Rocks, A.A. Balkema, Rotterdam, the Netherlands. 6. Konya, C.J., 1995, Blast Design, Intercontinental Development Co., Ohio 44064, USA. 7. Maerz, H.N., J.A. Franklin, L. Rothenburg and D.L. Coursen, 1987, Measurement of rock fragmentation by digital photo analysis, 5 th Int. Cong. Int. Soc. Rock Mech., pp Palangio, T.C. and J.A. Franklin, 1996, Practical guidelines for lighting and photography, Proc. Fragblast 5, Montreal, Canada, pp Rai, P. and F.L. Imperial, 2005, Mesh area vis-à-vis blast performance in a limestone quarry a case study., Fragblast, Vol. 9, No.4, pp Reid, P.E., 1976, Fragmentation analysis and its use in blasting cost optimization programs in open pit mining, B.Sc. thesis, Queen s University, Kingston, Ontario, Canada. 11. Rollins, R.R. and R.W. Givens, 1987, Blast casting at an Eastern strip mine, AIME Trans., V.282, pp Scott, A., A. Cocker, N. Djordjevic, M. Higgins, D. La Rosa, K.S. Sarma and R. Wedmaiser, 1996, Open Pit Blast Design Analysis and Optimization, JKMRC monograph series in mining and mineral processing, (Eds. A. Scott and T.J. Napier, Munn.) 13. Singh, D.P., M.M. Singh & A. Katkar, 2003, Blast casting in surface excavations-a global scenario, Min. Tech., pp Tracy, J.M., 1985, Increased production through cast blasting in surface coal mines, J. Exp. Eng., V.3, No. 2, pp Wang, W., Bergholm, F. and Stephansson, O., 1996, Image analysis of fragment size and shape, Proc. Fragblast-5, Montreal, pp Piyush Rai 1987 인도 Jodhpur 대학광산공학과학사 1992 인도 Banaras 대학광산공학과석사 2002 인도 Banaras 대학광산공학과박사 양형식 1979 서울대학교자원공학과학사 1981 서울대학교자원공학과석사 1987 서울대학교자원공학과박사 Tel: piyushrai.bhu.@gmail.com 현재 Banaras 대학광산공학과 Associate Professor 현재전남대학교에너지자원공학과초빙교수 Tel: hsyang@jnu.ac.kr 현재전남대학교에너지자원공학과교수