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Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams
International Journal of Mining Science and Technology xxx (2017) xxx–xxx
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Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams Zha Zhengao a, Ma Li b,c,⇑, Li Kemin a, Ding Xiaohua a, Xiao Shuangshuang b a
School of Mines, China University of Mining and Technology, Xuzhou 221116, China School of Energy Engineering, Xi’an University of Science and Technology, Xi’an 710054, China c State Key Laboratory for Geomechanics & Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China b
a r t i c l e
i n f o
Article history: Received 2 December 2016 Received in revised form 12 February 2017 Accepted 13 April 2017 Available online xxxx Keywords: Ultra-thick coal seam End wall in surface mine Highwall Mining System Local steep slope Resource exploitation
a b s t r a c t The paper aims to identify a reasonable method for mining ultra-thick coal seams in an end-slope in surface mine. With a case study of Heidaigou surface coal mine (HSCM), the paper conducted a comparative research on three mining methods, namely Underground Mining Method (UMM), Highwall Mining System (HMS) and Local Steep Slope Mining Method (LSSMM). A model was firstly established to simulate the impact that UMM and HMS exert on monitoring points and surface deformation. The way that stripping and excavation amount varies with different slope angle, and the corresponding end slope stability were analyzed in the mode of LSSMM. Then a TOPSIS model was established by taking into account six indicators such as recovery ratio, technical complexity and adaptability, the impact on surface mining production, production safety and economic benefits. Finally, LSSMM was determined as the best mining method for mining ultra-thick coal seams in end slope in HSCM. Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Most coal seams of surface coal mines in China are thick or ultra-thick, and the occurrence depth in large-scale surface coal mines could normally reach 300–600 m. Hence, as mining depth increases, a large amount of coal resources would be buried beneath the end slope. In addition, even if this part of coal resources could be extracted, due to the temporal and spatial correlation of coal mining and dumping, they have to be extracted in a timely manner. Otherwise, with inner dump developing and advancing, they could gradually end up being stagnant coal reserves whose economic benefits and mining feasibility would be both reduced. Therefore, provided the slope stability is ensured, it is of great significance to increase resource recovery ratio by recovering the reserves beneath end slope (RBES) using reasonable mining method and technology. Since end slope area is influenced by surface coal mining and dumping engineering practice, the selection of RBES exploitation method should be based on the characteristics of end-slope and meanwhile should be in combination with open-pit mining and haulage system. Three typical methods in this regard are
⇑ Corresponding author at: School of Energy Engineering, Xi’an University of Science and Technology, Xi’an 710054, China. E-mail address: [email protected] (L. Ma).
Underground Mining Method (UMM), Highwall Mining System (HMS) and Local Steep Slope Mining Method (LSSMM), respectively. To be specific, UMM refers to the combination between underground coal mining method with surface mining, HMS refers to a special mining system applied in end-slope working face, and LSSMM means surface mining method applied in the end-slope area with the end slope angle enlarged to be steep. Meanwhile, other factors such as the geological conditions of the mines, coal seam occurrence conditions, technical and economic conditions etc. shall be taken into full consideration as well. Besides, the three RBES exploitation methods mentioned above are quite different in terms of their technical process and their impact on normal production organization. (1) From the perspective of resource recovery rate, in the mode of UMM and HMS, a large number of coal pillars have to be retained and additional equipment is also required to reach a recovery rate of 60%–70%. While in the case of LSSMM, the end slope angle could be enlarged using current technological method of surface coal mining and its enlargement range directly determines the corresponding recovery rate. Therefore, no additional mining equipment is required in comparison with the former two. Nevertheless, coal resources excavated using this method would be limited and there would be a large amount of in situ RBES left unexploited.
https://doi.org/10.1016/j.ijmst.2017.10.002 2095-2686/Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
(2) From the perspective of production mode and economic investment, both UMM and HMS require additional investment of mining equipment and personnel and the investment is relatively large. But this is not the case in the mode of LSSMM. Since it could take full advantage of existing surface coal mining equipment to realize the exploitation of RBES, its production organization and management is relatively simple. In addition, UMM method takes many specialized technical staff and HMS also takes three to four technicians to complete production. So both modes would exert certain effect on normal surface coal mine production and that how to make the coordination has to be considered. Furthermore, the layout of the first two modes needs to meet certain requirements about open pit bottom width, while in the case of LSSMM, the smaller bottom width, the more favorable for local steep mining and slope stability. Che et al., Liu et al., Xu et al., and Shang et al. conducted analytic research on end-slope coal mining process system and mining method [1–5]. They also probed into the relationship between the end slope coal extraction and the dumping and mining of open pit in Anjialing surface mine. They further proposed a mining method named ‘‘Top Coal Carving Method with Supportive Coal Wall”. According to this method, a working face is set in the end slope using underground mining process, which could greatly improve the RBES recovery rate to as high as 45% and the entire coal resources recovery rate would also be greatly enhanced. In spite of this advantage, the room and pillar mining method would greatly disturb slope rocks and bring about ground surface subsidence, slope instability and deformation during extraction process. In recent years, mine production personnel and R&D department also began to set their eyes on HMS study and its application analysis. For instance, Cai et al., and Liu et al. analyzed its characteristics and explored its application prospect in end-slope coal mining in China [6,7]. Shimada et al. analyzed its applicability in end slope of surface coal mines, and they also analyzed and calculated the feasibility of HMS system in Antaibao surface coal mine using numerical simulation [8]. On top of these, based on the fact that sectional mining and inner dumping with end wall covered practice could effectively enlarge end-slope angle, Cai et al. proposed techniques such as ‘‘Steep End Slope Mining” and ‘‘Time-sensitive Slope Design”. These attempts could enlarge end slope angle so as to increase coal resources recovery, reduce land occupation and improve economic benefits [9–11]. As a matter of fact, each mining method is characteristic of different advantages in terms of RBES exploitation and the economic benefits are remarkable especially in the case of mining thin coal seams. But when it comes to thick or ultra-thick coal seam extraction, these mining methods are yet put into application. Under this situation, this paper probed into the mining method for extracting ultra-thick coal seams in end slope. The objective of this study herein is the RBES in nearly horizontal surface coal mine with ultra-thick coal seams. The applicability of various mining methods to this end was discussed in the paper, respectively.
2. Brief introduction of end slope in HSCM HSCM came into use in 1992 and its initially designed production capacity was 12 Mt/a. After two technological modifications, the current production capacity has exceeded 35 Mt/a. The main minable coal seam in HSCM is 6# coal seam. Its average mining depth is 185 m and its average thickness is 28.8 m. HSCM is divided into three mining districts, as shown in Fig. 1a. Currently, it is in the transitional period from Mining District I to Mining District II. Since the west end slope of Mining District I is influenced by
anticlinal axis, the coal seam is steeply inclined downward. And due to lacking in effective method to timely exploit end-slope resources, the exposed face has been buried again. The end-slope length of Mining District II and Mining District III are 8200 m and 5800 m, respectively. Both of their RBES is approximately 200 m wide and the coal resources buried in end slopes on both sides are 69.43 Mt and 49.11 Mt, respectively. And their addition is approximately the total production of raw coals for four years in HSCM. Therefore, their recovery represents substantial economic and social benefits. Specifically, if RBES excavation is taken into account simultaneously in early stage of coal mining in Mining District II, it would not only improve recovery ratio, but would play a guiding role in the future RBES exploitation in Mining District III. In HSCM, semi-continuous mining system is adopted, which includes single-bucket excavator, truck, semi-fixed crushing station and belt conveyor. For the rock steps that are 40 m above the coal seam roof, casting blast and draglines stripping technique have been used. As for the stripping work of the other rock steps and topsoil, it is accomplished by a discontinuous process consisting of single-bucket excavators and trucks. The operation bench height for single bucket excavator is 15 m and the working bench of inner dump is 30 m. A transport berm is set on the end slope step at an interval of every 30 m. Trucks deliver the stripped overburdens to inner dump via transport berms. Fig. 1b illustrates the end-slope patterns at current excavating position in HSCM. 3. Mining schemes for RBES excavation 3.1. Mining scheme of UMM According to strip mining theory, the width of excavated strip should be less than a third of the buried depth, and the width of remaining strip should meet the following Eq. (1) [12–14]:
as P 6:56 103 Hh þ
2
b b 3 3:6H
ð1Þ
where as is the remaining strip width, m; H the buried depth, m; h the mining height, m; and b the excavated strip width, m. The coordination between end-slope underground mining and surface coal mining is mainly composed of underground mining time, working face moving and shifting time, new roadway excavating time and surface mining-and-dumping advancing speed, as shown in Fig. 2. In order to minimize the mutual interaction between UMM and surface mining, the following relations should be considered:
(
D P ðb þ as þ 2bh Þn nðt b þ tc þ tk Þ 6 12D a
ð2Þ
where D is the bottom width, m; tc the mining time of working face, month; n the strip amount at end-slope exposed face; a the advancing speed of surface mining, m/month; tb the movement and shifting time of working face, month; tk the new roadway excavating time, month; and bh the roadway width, m. The burial depth of 6# coal seam in HSCM is around 150 m and the RBES width is 200 m. Based on Table 1 and Eq. (2), the mining width was determined as 20 m, and the coal pillar width is 20 m, among which the roadway width was 3.5 m. As coal seams in HSCM is ultra-thick, the mining height realized using UMM is far from that of surface coal mining. With mining height assumed as 10 m, the surface deformation resulting from underground strip mining in end slope was analyzed and simulated. The vertical displacement curve at monitoring points is shown in Fig. 3. As more and more strips became excavated, the subsidence at monitoring points increased as well.
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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Fig. 1. Sketch diagram of mining districts division and end-slope patterns in HSCM.
the economic benefits for RBES excavation is expected to reach 1.56 billion yuan. 3.2. Mining scheme of HMS
Fig. 2. Diagrammatic drawing of UMM in end-slope.
Table 1 Relation between mining depth, mining width and coal pillar width (m). Mining depth H
Mining width b
Coal pillar width a
>400 200–400 100–200 70–100
0.1H 30 20–25 12–15
0.1H 30 20 8–10
HMS integrates the features and technique of surface mining with underground mining. And the retaining of unexploited coal pillars is a basic guarantee for operation safety. In order to ensure that coal pillars manage to support rock mass as well as prevent the occurrence of group collapse and failure, permanent coal pillars should be set among supporting coal pillars at certain interval [15,16]. The width of coal pillars is mainly affected by factors like thickness and density of overlying rock and soil, mining width, in-situ strength of coal rock mass, coal seam thickness, and mining height, etc. [17]. Based on the coal pillar strength formula given by MarkBieniawsk, the supporting and permanent coal pillar strength would be respectively given by equations derived below:
8 SF wp rv hc 0:64rt hc > > W wp ¼ > > 1:08rt > > > qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi > > 2 > > ð0:64rt hc SF wp rv hc Þ þ2:16rt hc SF wp rv W E > > > <þ 1:08rt > SF r h 0:64 rt hc > W ¼ bp v c > > bp > 1:08 r > t > qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi > > > > > r h SF rv hc Þ2 þ2:16rt hc SF bp rv ½NW wp þðN þ1ÞW E ð0:64 t c > bp > :þ 1:08rt ð3Þ
Fig. 3. Vertical displacement of monitoring points in different stages.
The strips before exerted quite limited the impact on vertical displacement of monitoring points at current strip, yet once mining practice began at the current strip, the corresponding vertical displacement was gradually increased and the scope of influence was around 400 m. According to the analysis results, the monitored subsidence at central strip roof caused by single layer strip mining was around 24 cm. And the maximum vertical displacement at surface monitoring point was about 20 cm. With single-layer stripping mining method, approximately 12 Mt of RBES could be exploited on one side and the resource recovery rate could reach 17.36%. Considering estimated equipment investment of 356,000 yuan and strip mining cost of 50 yuan/t,
To make sure HMS operates smoothly under the condition of surface mining and dumping, it is a necessity to guarantee that pit bottom is as wide as at least one mining strip. Meanwhile HMS’s mining period for excavating a cut width should be no longer than the advancing time of opencast coal mining and dumping.
(
tk þ t b 6 12D a D P W bp þ NW wp þ ðN þ 1ÞW E
ð4Þ
where tb denotes the movement-and-shift time of HMS, month; and tk the HMS’s mining time for a cut width, month. According to the parameters of HMS equipment, when one strip has been finished mining in end-slope, a rectangular roadway that is 3.05 m high and 3.5 m wide would come into being. With the stability coefficient of coal pillar assumed as 1.5, and according to the constraint relation of Eqs. (3) and (4), seven mining caves could be laid out
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
Fig. 4. Moving state cloud within overlying strata.
in HSCM end-slope whose bottom width was 80 m. And the width of supporting coal pillar and permanent coal pillar were determined as 5.12 m and 17.6 m, respectively. As illustrated in Fig. 4, the mining effected boundary continuously extended with increasing cave amount, yet the rock strata beyond the boundary remained unaffected; the largest vertical displacement appeared at the center of cave roof, and a vertical downward displacement zone formed between two caves; in the meantime, affected by lateral and bottom stress, vertical upward displacement took place at bottom rock strata; the retained supporting coal pillars bore the gravity shifted from upper rock strata. The center of each cave roof was taken as a monitoring point to record the excavation amount change of each mining cave. These mining caves were spread among permanent coal pillars. And the deformation at monitoring points was illustrated in Fig. 5. The analysis results show that surface deformation caused by HMS mining method was relatively small. Specifically, the vertical displacement at central cave roof among a single unit of supporting coal pillars ranged from 0.007 m to 0.01 m, and caves mining in front and at the back caused even smaller impact on the vertical displacement of current cave’s roof center. HMS application in ultra-thick coal seam excavation in HSCM allows to exploit 2.25 Mt raw coal from one side of the endslope, and the recovery ratio is merely 3.25%. Assuming that the investment for HMS equipment is 200 million yuan per set, and the mining cost is 12 yuan/t, then the economic benefits achieved on one side of end slope would be 178 million yuan. 3.3. Mining scheme of LSSMM When the mining method of inner-dumping with end-wall covered is applied in surface coal mines with nearly horizontal occur-
rence, end slopes are normally buried soon after being excavated. So they are not those permanent slopes that exist in the entire mining process. As a matter of fact, their being and disappearing are periodic. Generally speaking, rock slope stability is inclined to decrease with the passage of time. The traditional slope angle is usually designed under the assumption that the open pit slopes are permanent. Hence, if they only exist for a short time, the originally designed slope angle would seem far too conservative. As mentioned earlier, with the mining method of inner-dumping with end-wall covered, the exposure time of end slopes is shortened to a certain extent. And this feature serves as technical support for improving end slope stability and enlarging end slope angle. According to surface mining engineering features, when establishing the relationship between rock mass strength and existence time of end-slope, the only requirement is to guarantee the end slope stability within its life span [18]. LSSMM method is based on such slope timeliness. By enlarging end slope angle, more coal resources could be excavated without extra overburden stripping. The end slope angle in HSCM is 33°. With lower benches generated by casting blast, the end-slope angle could be changed by adjusting upper benches. The end-slope stability results at different slope angles were analyzed, as listed in Table 2. At present, the end slope angle in HSCM is 33° and the corresponding stability coefficient can reach 1.4, representing relatively high stability. However, the stability coefficient is inclined to decrease as slope angle goes up. For instance, when end slope angle is enlarged to 38°, the stability coefficient would be reduced to 1.21. In this case, recoverable resources on one side of the endslope is 11.6 Mt and the recovery ratio is 17.2%. Meanwhile, 19.95 106 m3 overburden rock need to be stripped. Assuming that the pure stripping cost is 16 yuan/m3, the pure coal mining cost is 13 yuan/t, and the selling price is 180 yuan/t, the directly increased economic benefits would reach almost 1.618 trillion yuan. 4. Optimum mining method of RBES 4.1. Key indicators for selecting RBES mining methods
Fig. 5. Vertical displacement curve of mining cave roof center by HMS.
(1) Resource recovery ratio Resource recovery ratio is an important reflection of sustainable resource development, and it is also a significant index to weigh up resource recovery method. The major objective of end-slope resource recovery is to improve overall coal resource recovery ratio. With differentiated technical methods, processes, equipment, etc., each mining method could achieve varying degrees of
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Table 2 Overburden amount of end-slope angle adjustment per meter. Angle (°)
Overburden (m3)
Coal extraction (t)
Stripping ratio (m3/t)
Stability coefficient
33 34 35 36 37 38
0 418.567 1054.863 1389.990 1925.766 2432.832
0 339.058 606.575 885.512 1149.383 1414.366
1.23 1.74 1.57 1.68 1.72
1.40 1.33 1.29 1.27 1.23 1.21
Table 3 Preferred evaluation indicators for end-slope mining method. Indicator
UMM
HMS
LSSMM
Resource recovery ratio Technical process complexity Technical process adaptability Impacts on surface mining production Production safety Economic benefits
X11 X12 X13 X14 X15 X16
X21 X22 X23 X24 X25 X26
X31 X32 X33 X34 X35 X36
Table 4 Scaling method of judgement matrix. Scale
Meaning
1 3
Means that the two factors are of equal importance Indicates that one factor is slightly more important than the other one Indicates that one factor is obviously more important than the other one Indicates that one factor is significantly more important than the other one Indicates that one factor is extremely more important than the other one The median value of the two adjacent judgments above
5 7 9 2,4,6,8
end slope resource recovery. And a higher recovery ratio translates into an advantage that a certain mining method possesses in this regard.
angle and rock mass strength. Specifically, HMS is only applicable to recover crop coal or end-slope coal resources with flat occurrence and a less-than-8° dip angle, but it is especially effective in thin seam extraction. When it comes to thick coal seam mining, the efficiency of UMM and HMS is restricted; yet the thinner the coal seam, the fewer resources can be excavated by LSSMM. (4) Impact on surface mining production In the aspect of working face layout, both UMM and HMS application require minimum bottom width of open pit coal mine. In addition, working face moving and shifting in end-slope should be temporally consistent with the advancing of open pit mining and dumping. In this sense, the effect that RBES excavation and haulage system exerts on regular open pit haulage system is something that cannot be neglected. As a matter of fact, the implementation of LSSMM is accompanied by the amalgamation, revocation and reconstruction of end-slope haulage berm, hence, the economic rationality of raw coal haulage system and the end slope stability would both be affected to a certain degree. (5) Production safety After the introduction of end-slope mining methods, their own production safety issue along with their impact on normal open pit mine production security would bring about higher probability of accidents. For example, mining disturbance might cause surface deformation and undermine slope stability, and end-slope resources haulage system may be coupled with that of raw coal.
(2) Technical process complexity (6) Economic benefits Different mining methods require different technologies and devices. The production organization and management patterns are quite differentiated in the following aspects such as working face layout, roadway excavation, haulage system arrangement and working face moving and shifting. There is also a remarkable disparity in technical complexity, equipment amount and personnel matching. (3) Technical process adaptability
Economic benefits represent an important indicator to determine whether end-slope resources could be mined. Specifically, UMM cost and investment are closely related to technical mining process. HMS is characteristic of large one-time investment and low production cost, while LSSMM requires no additional equipment investment. 4.2. Establishment of optimum model based on TOPSIS
Mining method and process selection for excavating resources beneath end-slopes are influenced by their occurrence characteristics such as coal seam thickness range and its change, seam dip
By calculating the proximity between evaluated objects and ideal targets, TOPSIS ranks a finite number of evaluated objects based on their pros and cons, thus obtaining the optimal mining
Table 5 Preliminary evaluation indexes for end-slope mining methods in HSCM. Indicator
UMM
HMS
LSSMM
Resource recovery ratio Technical process complexity Technical process adaptability Impacts on surface mining production Production safety Economic benefits
17.36% Medium Medium Medium Relatively poorer 1.56 trillion yuan
3.25% Medium Difficult Relatively greater Relatively higher 0.178 trillion yuan
17.2% Simple Simple Relatively smaller Safe 1.618 trillion yuan
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
Table 6 Evaluation indicators for end-slope mining schemes of HSCM. Indicator
Weight
UMM
HMS
LSSMM
Recovery ratio Technical process complexity Technical process adaptability Impact on surface mining production Production safety Economic benefits
0.248 0.124 0.057 0.181 0.210 0.181
0.1736 3 5 5 3 15.60
0.0325 3 3 3 7 1.78
0.172 5 7 7 5 16.18
Table 7 Relative proximity of index value and optimum value and the sorted result. Method
Di+
D i
Ci
Result
UMM HMS LSSMM
0.302 0.467 0.101
0.392 0.207 0.490
0.565 0.307 0.830
2 3 1
method. According to TOPSIS calculation principle, firstly, normalization treatment is carried out on original data matrix to obtain the optimal vector and the worst vector. Secondly, the distance between every single evaluated object and the optimal/worst vector is calculated, respectively. The calculation results are used to determine the proximity between them and the optimal method [19,20]. (1) Table 3 is a list of evaluation indicators mentioned above. High merit indicators are usually converted using reciprocal method, e.g. Xij0 = 1/Xij, which is assumed as a high merit indicator converted from a low merit one. And evaluation indicators list with same trending were obtained below. Qualitative indicators are quantified using the scaling method of the constructive judgment matrices in Table 4. (2) Normalization treatment was conducted on original data matrix and the corresponding matrix was established.
8 .qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 > > < aij ¼ X ij i¼1 X ij ðOriginal superior indicatorsÞ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P n > 0 0 2 > : aij ¼ X ij i¼1 ðX ij Þ ðOriginal inferior indicatorsÞ
ð5Þ
where aij denotes the value of a certain evaluated object i in the jth indicator; Xij the value of ith evaluated object in the jth indicator; and Xij0 the value of the ith evaluated object after conversion in the jth indicator. The normalized matrix is shown as follows:
2
a11
6a 6 21 A¼6 4 an1
(
a12
a1m
3
a22
a2m 7 7 7 5
an2
anm
A ¼ ai1 ; ai2 ; ; aim
ð8Þ
where Dþ i and Di denote the distance from the optimum method and the worst method to the ith evaluated object, respectively; and xj the weight of the jth indicator. (5) The proximity Ci between each evaluated object and the optimum method was given by Eq. (9):
Ci ¼
Dþi
Di þ Di
ð9Þ
(6) The evaluated objects ranked according to their respective proximity. The higher value of Ci, the better assessment results. 4.3. Optimal mining method selection for RBES To identify the optimal mining method for extracting ultrathick coal seam in HSCM, factors such as technical process adaptability, mining parameters, resource recovery ratio and economic benefits of different mining methods were analyzed, respectively. And the preliminary evaluation indicators are as shown in Table 5. In addition, those qualitative indicators were further treated according to the scaling method of constructing judgment matrix in Table 4, thus the final evaluation indicators were obtained in Table 6. All indicators but recovery ratio and economic benefits in Table 4 are qualitative. Hence, they need to be converted into quantitative ones by the scaling approach in Table 4 before getting involved in solution optimization. Based on calculation principle of TOPSIS, the results were obtained as shown in Table 7. Rank the evaluated objects based on the value of Ci. The closer to 1 the value of Ci, the closer to the optimum level the method. Of course, if the value of Ci is close to 0, the corresponding method is approaching the worst level. According to the calculation results, LSSMM was assessed as the best method, which indicates that under the current technical process conditions, LSSMM is the optimum method for mining HSCM ultra-thick coal seam along with the process of mining and stripping engineering. 5. Conclusions
ð6Þ
(3) The maximum value of each column is the optimum vector A+ and the minimum value is the worst vector A:
Aþ ¼ aþi1 ; aþi2 ; ; aþim
8 ffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pm 2 > þ < Dþi ¼ j¼1 xj ðaij aij Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi > : D ¼ Pm xj ða aij Þ2 i j¼1 ij
ð7Þ
(4) Indicator weight was determined using set-valued iteration method. And the distance between indicator value of every single evaluated object and its optimum method Dþ i , and the distance between indicator value and its worst method D i were calculated respectively, as shown in Eq. (8):
(1) Based on advancing relations of open-pit coal mining and dumping engineering, the temporal and spatial characteristics of open pit end-slope were analyzed. The stability coefficient of end-slope decreases dramatically with increasing exposed area, and a smaller exposed free face is beneficial to the improvement of slope stability coefficient. The free face area of end-slope is closely related to pit bottom width, the advancing speed of operating lines at working slope and inner dumping site. (2) This paper analyzed and summarized three mining methods, namely UMM, HMS and LSSMM. Mining parameters regarding UMM and HMS were identified and the impacts that mining activities exert on surface deformation were also
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
obtained through analysis and simulation. Specifically, in the case of UMM application, the subsidence of monitoring points at central strip roof was around 24 cm and the maximum vertical displacement of surface monitoring points was 20 cm. In comparison, the application of HMS method had a smaller influence on surface deformation. Besides, with the aid of analysis software, the critical end-slope stability angle in HSCM was identified as 38° and the recoverable resources in unilateral end-slope was 11.6 Mt. (3) On the basis of TOPSIS method, a model was established for the sake of selecting the optimal mining method for mining resources beneath end-slope. Resource recovery ratio, technical process complexity, technical process adaptability, production safety and economic benefits were taken as evaluation indicators in the model. The final results indicate that LSSMM was superior to the other two methods except that its resource recovery ratio was merely 17.2%.
Acknowledgments Financial supports for this work, provided by the National Natural Science Foundation of China (No. 90510002) and the Science and Technology Research of the Ministry of Education of China (No. 306008), are gratefully acknowledged. References [1] Che ZX, Yang H. Application of open-pit and underground mining technology for residual coal of end-slopes. Mining Sci Technol 2010;20(2):266–70. [2] Liu ZM, Wang SL, Cai QX. Application of top-coal caving method in Anjialing surface mine. J China Univ Mining Technol 2001;30(5):515–7. [3] Xu ZY, Cai QX, Shang T. Study of end-slope coal mining and its system layout. J Mining Saf Eng 2006;23(4):406–10.
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[4] Shang T, Cai QX, Che ZX. End-slope mining method and system arrangement of Anjialing surface coal mine. Opencast Mining Technol 2007;22(5):4–6. [5] Shang T, Shu JS, Cai QX, Che ZX. Space-time relationship between end-slope coal extraction and dumping and mining of open pits. J China Univ Mining Technol 2001;30(1):27–9. [6] Cai L, Sun JB, Ma ZY. Application of HMS in surface coal mine. Modern Mining 2011; 27(11): 63–64, 74. [7] Liu WG, Wang LS, Fu Q. SHM highwall mining technology and key issues of application. Coal Eng 2012;44(6):1–4. [8] Shimada H, Chen YL, Hamanaka A, Sasaoka T, Shimada H, Matsui K. Application of highwall mining system to recover residual coal in end-walls. Procedia Earth Planetary Sci 2013;6:311–8. [9] Cai QX, Zhou W, Che ZX, Liu Y, Peng HG. Research on the mining method and stripping ratio of steep end-slope mining in surface coal mines with flat coal deposit. J China Univ Mining Technol 2007;36(6):743–7. [10] Liu Y, Che ZX, Li ZQ, Cai QX. Analyses of end slope remnant coal mining and slope exposure time. J China Univ Mining Technol 2006;35(6):727–31. [11] Cai QX, Zhou W, Shu JS, Liu Y, Peng HG. Analysis and application on end-slope timeliness of internal dumping under flat dipping ore body in large surface coal mine. J China Univ Mining Technol 2008;37(6):740–4. [12] Liu KG, Wang JC, Xu JH. Study on mining system and stability of coal pillar in short wall mechanized mining. J China Univ Mining Technol 2005;34(1):24–9. [13] Du JP. Mining Science. Xuzhou: China University of Mining & Technology Press; 2009. [14] Chen YL, Cai QX, Shang T, Peng HG, Zhou W, Chen SZ. Zonal extraction technology and numerical simulation analysis in open pit coal mine. Int J Mining Sci Technol 2012;22(4):487–91. [15] Liu L. End slope mining technologies introduction. Opencast Mining Technol 2012;27(1):36–7. [16] Wang SG, Yao GL, Bao H. Application analysis on end slope shearer in open-pit mine. Opencast Mining Technol 2014;29(10):28–30. [17] Zipf JRK, Mark C. Ground control for highwall mining in the United States. Int J Mining, Reclamation Environ 2005; (19)3: 188–217. [18] Afrapoli AM, Osanloo M. Determination and stability analysis of ultimate open-pit slope under geomechanical uncertainty. Int J Mining Sci Technol 2014;24(1):105–10. [19] Zhao KN. The evaluation about validity of regional innovation system based on Topsis method. World Regional Stud 2009;18(2):71–5. [20] Chen X, Wu YH. The evaluation and comparison of corporation’s independent innovation ability in different regions based on TOPSIS method. Sci Technol Prog Policy 2009;26(15):131–4.
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams Zha Zhengao a, Ma Li b,c,⇑, Li Kemin a, Ding Xiaohua a, Xiao Shuangshuang b a
School of Mines, China University of Mining and Technology, Xuzhou 221116, China School of Energy Engineering, Xi’an University of Science and Technology, Xi’an 710054, China c State Key Laboratory for Geomechanics & Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China b
a r t i c l e
i n f o
Article history: Received 2 December 2016 Received in revised form 12 February 2017 Accepted 13 April 2017 Available online xxxx Keywords: Ultra-thick coal seam End wall in surface mine Highwall Mining System Local steep slope Resource exploitation
a b s t r a c t The paper aims to identify a reasonable method for mining ultra-thick coal seams in an end-slope in surface mine. With a case study of Heidaigou surface coal mine (HSCM), the paper conducted a comparative research on three mining methods, namely Underground Mining Method (UMM), Highwall Mining System (HMS) and Local Steep Slope Mining Method (LSSMM). A model was firstly established to simulate the impact that UMM and HMS exert on monitoring points and surface deformation. The way that stripping and excavation amount varies with different slope angle, and the corresponding end slope stability were analyzed in the mode of LSSMM. Then a TOPSIS model was established by taking into account six indicators such as recovery ratio, technical complexity and adaptability, the impact on surface mining production, production safety and economic benefits. Finally, LSSMM was determined as the best mining method for mining ultra-thick coal seams in end slope in HSCM. Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Most coal seams of surface coal mines in China are thick or ultra-thick, and the occurrence depth in large-scale surface coal mines could normally reach 300–600 m. Hence, as mining depth increases, a large amount of coal resources would be buried beneath the end slope. In addition, even if this part of coal resources could be extracted, due to the temporal and spatial correlation of coal mining and dumping, they have to be extracted in a timely manner. Otherwise, with inner dump developing and advancing, they could gradually end up being stagnant coal reserves whose economic benefits and mining feasibility would be both reduced. Therefore, provided the slope stability is ensured, it is of great significance to increase resource recovery ratio by recovering the reserves beneath end slope (RBES) using reasonable mining method and technology. Since end slope area is influenced by surface coal mining and dumping engineering practice, the selection of RBES exploitation method should be based on the characteristics of end-slope and meanwhile should be in combination with open-pit mining and haulage system. Three typical methods in this regard are
⇑ Corresponding author at: School of Energy Engineering, Xi’an University of Science and Technology, Xi’an 710054, China. E-mail address: [email protected] (L. Ma).
Underground Mining Method (UMM), Highwall Mining System (HMS) and Local Steep Slope Mining Method (LSSMM), respectively. To be specific, UMM refers to the combination between underground coal mining method with surface mining, HMS refers to a special mining system applied in end-slope working face, and LSSMM means surface mining method applied in the end-slope area with the end slope angle enlarged to be steep. Meanwhile, other factors such as the geological conditions of the mines, coal seam occurrence conditions, technical and economic conditions etc. shall be taken into full consideration as well. Besides, the three RBES exploitation methods mentioned above are quite different in terms of their technical process and their impact on normal production organization. (1) From the perspective of resource recovery rate, in the mode of UMM and HMS, a large number of coal pillars have to be retained and additional equipment is also required to reach a recovery rate of 60%–70%. While in the case of LSSMM, the end slope angle could be enlarged using current technological method of surface coal mining and its enlargement range directly determines the corresponding recovery rate. Therefore, no additional mining equipment is required in comparison with the former two. Nevertheless, coal resources excavated using this method would be limited and there would be a large amount of in situ RBES left unexploited.
https://doi.org/10.1016/j.ijmst.2017.10.002 2095-2686/Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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(2) From the perspective of production mode and economic investment, both UMM and HMS require additional investment of mining equipment and personnel and the investment is relatively large. But this is not the case in the mode of LSSMM. Since it could take full advantage of existing surface coal mining equipment to realize the exploitation of RBES, its production organization and management is relatively simple. In addition, UMM method takes many specialized technical staff and HMS also takes three to four technicians to complete production. So both modes would exert certain effect on normal surface coal mine production and that how to make the coordination has to be considered. Furthermore, the layout of the first two modes needs to meet certain requirements about open pit bottom width, while in the case of LSSMM, the smaller bottom width, the more favorable for local steep mining and slope stability. Che et al., Liu et al., Xu et al., and Shang et al. conducted analytic research on end-slope coal mining process system and mining method [1–5]. They also probed into the relationship between the end slope coal extraction and the dumping and mining of open pit in Anjialing surface mine. They further proposed a mining method named ‘‘Top Coal Carving Method with Supportive Coal Wall”. According to this method, a working face is set in the end slope using underground mining process, which could greatly improve the RBES recovery rate to as high as 45% and the entire coal resources recovery rate would also be greatly enhanced. In spite of this advantage, the room and pillar mining method would greatly disturb slope rocks and bring about ground surface subsidence, slope instability and deformation during extraction process. In recent years, mine production personnel and R&D department also began to set their eyes on HMS study and its application analysis. For instance, Cai et al., and Liu et al. analyzed its characteristics and explored its application prospect in end-slope coal mining in China [6,7]. Shimada et al. analyzed its applicability in end slope of surface coal mines, and they also analyzed and calculated the feasibility of HMS system in Antaibao surface coal mine using numerical simulation [8]. On top of these, based on the fact that sectional mining and inner dumping with end wall covered practice could effectively enlarge end-slope angle, Cai et al. proposed techniques such as ‘‘Steep End Slope Mining” and ‘‘Time-sensitive Slope Design”. These attempts could enlarge end slope angle so as to increase coal resources recovery, reduce land occupation and improve economic benefits [9–11]. As a matter of fact, each mining method is characteristic of different advantages in terms of RBES exploitation and the economic benefits are remarkable especially in the case of mining thin coal seams. But when it comes to thick or ultra-thick coal seam extraction, these mining methods are yet put into application. Under this situation, this paper probed into the mining method for extracting ultra-thick coal seams in end slope. The objective of this study herein is the RBES in nearly horizontal surface coal mine with ultra-thick coal seams. The applicability of various mining methods to this end was discussed in the paper, respectively.
2. Brief introduction of end slope in HSCM HSCM came into use in 1992 and its initially designed production capacity was 12 Mt/a. After two technological modifications, the current production capacity has exceeded 35 Mt/a. The main minable coal seam in HSCM is 6# coal seam. Its average mining depth is 185 m and its average thickness is 28.8 m. HSCM is divided into three mining districts, as shown in Fig. 1a. Currently, it is in the transitional period from Mining District I to Mining District II. Since the west end slope of Mining District I is influenced by
anticlinal axis, the coal seam is steeply inclined downward. And due to lacking in effective method to timely exploit end-slope resources, the exposed face has been buried again. The end-slope length of Mining District II and Mining District III are 8200 m and 5800 m, respectively. Both of their RBES is approximately 200 m wide and the coal resources buried in end slopes on both sides are 69.43 Mt and 49.11 Mt, respectively. And their addition is approximately the total production of raw coals for four years in HSCM. Therefore, their recovery represents substantial economic and social benefits. Specifically, if RBES excavation is taken into account simultaneously in early stage of coal mining in Mining District II, it would not only improve recovery ratio, but would play a guiding role in the future RBES exploitation in Mining District III. In HSCM, semi-continuous mining system is adopted, which includes single-bucket excavator, truck, semi-fixed crushing station and belt conveyor. For the rock steps that are 40 m above the coal seam roof, casting blast and draglines stripping technique have been used. As for the stripping work of the other rock steps and topsoil, it is accomplished by a discontinuous process consisting of single-bucket excavators and trucks. The operation bench height for single bucket excavator is 15 m and the working bench of inner dump is 30 m. A transport berm is set on the end slope step at an interval of every 30 m. Trucks deliver the stripped overburdens to inner dump via transport berms. Fig. 1b illustrates the end-slope patterns at current excavating position in HSCM. 3. Mining schemes for RBES excavation 3.1. Mining scheme of UMM According to strip mining theory, the width of excavated strip should be less than a third of the buried depth, and the width of remaining strip should meet the following Eq. (1) [12–14]:
as P 6:56 103 Hh þ
2
b b 3 3:6H
ð1Þ
where as is the remaining strip width, m; H the buried depth, m; h the mining height, m; and b the excavated strip width, m. The coordination between end-slope underground mining and surface coal mining is mainly composed of underground mining time, working face moving and shifting time, new roadway excavating time and surface mining-and-dumping advancing speed, as shown in Fig. 2. In order to minimize the mutual interaction between UMM and surface mining, the following relations should be considered:
(
D P ðb þ as þ 2bh Þn nðt b þ tc þ tk Þ 6 12D a
ð2Þ
where D is the bottom width, m; tc the mining time of working face, month; n the strip amount at end-slope exposed face; a the advancing speed of surface mining, m/month; tb the movement and shifting time of working face, month; tk the new roadway excavating time, month; and bh the roadway width, m. The burial depth of 6# coal seam in HSCM is around 150 m and the RBES width is 200 m. Based on Table 1 and Eq. (2), the mining width was determined as 20 m, and the coal pillar width is 20 m, among which the roadway width was 3.5 m. As coal seams in HSCM is ultra-thick, the mining height realized using UMM is far from that of surface coal mining. With mining height assumed as 10 m, the surface deformation resulting from underground strip mining in end slope was analyzed and simulated. The vertical displacement curve at monitoring points is shown in Fig. 3. As more and more strips became excavated, the subsidence at monitoring points increased as well.
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
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Fig. 1. Sketch diagram of mining districts division and end-slope patterns in HSCM.
the economic benefits for RBES excavation is expected to reach 1.56 billion yuan. 3.2. Mining scheme of HMS
Fig. 2. Diagrammatic drawing of UMM in end-slope.
Table 1 Relation between mining depth, mining width and coal pillar width (m). Mining depth H
Mining width b
Coal pillar width a
>400 200–400 100–200 70–100
0.1H 30 20–25 12–15
0.1H 30 20 8–10
HMS integrates the features and technique of surface mining with underground mining. And the retaining of unexploited coal pillars is a basic guarantee for operation safety. In order to ensure that coal pillars manage to support rock mass as well as prevent the occurrence of group collapse and failure, permanent coal pillars should be set among supporting coal pillars at certain interval [15,16]. The width of coal pillars is mainly affected by factors like thickness and density of overlying rock and soil, mining width, in-situ strength of coal rock mass, coal seam thickness, and mining height, etc. [17]. Based on the coal pillar strength formula given by MarkBieniawsk, the supporting and permanent coal pillar strength would be respectively given by equations derived below:
8 SF wp rv hc 0:64rt hc > > W wp ¼ > > 1:08rt > > > qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi > > 2 > > ð0:64rt hc SF wp rv hc Þ þ2:16rt hc SF wp rv W E > > > <þ 1:08rt > SF r h 0:64 rt hc > W ¼ bp v c > > bp > 1:08 r > t > qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi > > > > > r h SF rv hc Þ2 þ2:16rt hc SF bp rv ½NW wp þðN þ1ÞW E ð0:64 t c > bp > :þ 1:08rt ð3Þ
Fig. 3. Vertical displacement of monitoring points in different stages.
The strips before exerted quite limited the impact on vertical displacement of monitoring points at current strip, yet once mining practice began at the current strip, the corresponding vertical displacement was gradually increased and the scope of influence was around 400 m. According to the analysis results, the monitored subsidence at central strip roof caused by single layer strip mining was around 24 cm. And the maximum vertical displacement at surface monitoring point was about 20 cm. With single-layer stripping mining method, approximately 12 Mt of RBES could be exploited on one side and the resource recovery rate could reach 17.36%. Considering estimated equipment investment of 356,000 yuan and strip mining cost of 50 yuan/t,
To make sure HMS operates smoothly under the condition of surface mining and dumping, it is a necessity to guarantee that pit bottom is as wide as at least one mining strip. Meanwhile HMS’s mining period for excavating a cut width should be no longer than the advancing time of opencast coal mining and dumping.
(
tk þ t b 6 12D a D P W bp þ NW wp þ ðN þ 1ÞW E
ð4Þ
where tb denotes the movement-and-shift time of HMS, month; and tk the HMS’s mining time for a cut width, month. According to the parameters of HMS equipment, when one strip has been finished mining in end-slope, a rectangular roadway that is 3.05 m high and 3.5 m wide would come into being. With the stability coefficient of coal pillar assumed as 1.5, and according to the constraint relation of Eqs. (3) and (4), seven mining caves could be laid out
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
Fig. 4. Moving state cloud within overlying strata.
in HSCM end-slope whose bottom width was 80 m. And the width of supporting coal pillar and permanent coal pillar were determined as 5.12 m and 17.6 m, respectively. As illustrated in Fig. 4, the mining effected boundary continuously extended with increasing cave amount, yet the rock strata beyond the boundary remained unaffected; the largest vertical displacement appeared at the center of cave roof, and a vertical downward displacement zone formed between two caves; in the meantime, affected by lateral and bottom stress, vertical upward displacement took place at bottom rock strata; the retained supporting coal pillars bore the gravity shifted from upper rock strata. The center of each cave roof was taken as a monitoring point to record the excavation amount change of each mining cave. These mining caves were spread among permanent coal pillars. And the deformation at monitoring points was illustrated in Fig. 5. The analysis results show that surface deformation caused by HMS mining method was relatively small. Specifically, the vertical displacement at central cave roof among a single unit of supporting coal pillars ranged from 0.007 m to 0.01 m, and caves mining in front and at the back caused even smaller impact on the vertical displacement of current cave’s roof center. HMS application in ultra-thick coal seam excavation in HSCM allows to exploit 2.25 Mt raw coal from one side of the endslope, and the recovery ratio is merely 3.25%. Assuming that the investment for HMS equipment is 200 million yuan per set, and the mining cost is 12 yuan/t, then the economic benefits achieved on one side of end slope would be 178 million yuan. 3.3. Mining scheme of LSSMM When the mining method of inner-dumping with end-wall covered is applied in surface coal mines with nearly horizontal occur-
rence, end slopes are normally buried soon after being excavated. So they are not those permanent slopes that exist in the entire mining process. As a matter of fact, their being and disappearing are periodic. Generally speaking, rock slope stability is inclined to decrease with the passage of time. The traditional slope angle is usually designed under the assumption that the open pit slopes are permanent. Hence, if they only exist for a short time, the originally designed slope angle would seem far too conservative. As mentioned earlier, with the mining method of inner-dumping with end-wall covered, the exposure time of end slopes is shortened to a certain extent. And this feature serves as technical support for improving end slope stability and enlarging end slope angle. According to surface mining engineering features, when establishing the relationship between rock mass strength and existence time of end-slope, the only requirement is to guarantee the end slope stability within its life span [18]. LSSMM method is based on such slope timeliness. By enlarging end slope angle, more coal resources could be excavated without extra overburden stripping. The end slope angle in HSCM is 33°. With lower benches generated by casting blast, the end-slope angle could be changed by adjusting upper benches. The end-slope stability results at different slope angles were analyzed, as listed in Table 2. At present, the end slope angle in HSCM is 33° and the corresponding stability coefficient can reach 1.4, representing relatively high stability. However, the stability coefficient is inclined to decrease as slope angle goes up. For instance, when end slope angle is enlarged to 38°, the stability coefficient would be reduced to 1.21. In this case, recoverable resources on one side of the endslope is 11.6 Mt and the recovery ratio is 17.2%. Meanwhile, 19.95 106 m3 overburden rock need to be stripped. Assuming that the pure stripping cost is 16 yuan/m3, the pure coal mining cost is 13 yuan/t, and the selling price is 180 yuan/t, the directly increased economic benefits would reach almost 1.618 trillion yuan. 4. Optimum mining method of RBES 4.1. Key indicators for selecting RBES mining methods
Fig. 5. Vertical displacement curve of mining cave roof center by HMS.
(1) Resource recovery ratio Resource recovery ratio is an important reflection of sustainable resource development, and it is also a significant index to weigh up resource recovery method. The major objective of end-slope resource recovery is to improve overall coal resource recovery ratio. With differentiated technical methods, processes, equipment, etc., each mining method could achieve varying degrees of
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Table 2 Overburden amount of end-slope angle adjustment per meter. Angle (°)
Overburden (m3)
Coal extraction (t)
Stripping ratio (m3/t)
Stability coefficient
33 34 35 36 37 38
0 418.567 1054.863 1389.990 1925.766 2432.832
0 339.058 606.575 885.512 1149.383 1414.366
1.23 1.74 1.57 1.68 1.72
1.40 1.33 1.29 1.27 1.23 1.21
Table 3 Preferred evaluation indicators for end-slope mining method. Indicator
UMM
HMS
LSSMM
Resource recovery ratio Technical process complexity Technical process adaptability Impacts on surface mining production Production safety Economic benefits
X11 X12 X13 X14 X15 X16
X21 X22 X23 X24 X25 X26
X31 X32 X33 X34 X35 X36
Table 4 Scaling method of judgement matrix. Scale
Meaning
1 3
Means that the two factors are of equal importance Indicates that one factor is slightly more important than the other one Indicates that one factor is obviously more important than the other one Indicates that one factor is significantly more important than the other one Indicates that one factor is extremely more important than the other one The median value of the two adjacent judgments above
5 7 9 2,4,6,8
end slope resource recovery. And a higher recovery ratio translates into an advantage that a certain mining method possesses in this regard.
angle and rock mass strength. Specifically, HMS is only applicable to recover crop coal or end-slope coal resources with flat occurrence and a less-than-8° dip angle, but it is especially effective in thin seam extraction. When it comes to thick coal seam mining, the efficiency of UMM and HMS is restricted; yet the thinner the coal seam, the fewer resources can be excavated by LSSMM. (4) Impact on surface mining production In the aspect of working face layout, both UMM and HMS application require minimum bottom width of open pit coal mine. In addition, working face moving and shifting in end-slope should be temporally consistent with the advancing of open pit mining and dumping. In this sense, the effect that RBES excavation and haulage system exerts on regular open pit haulage system is something that cannot be neglected. As a matter of fact, the implementation of LSSMM is accompanied by the amalgamation, revocation and reconstruction of end-slope haulage berm, hence, the economic rationality of raw coal haulage system and the end slope stability would both be affected to a certain degree. (5) Production safety After the introduction of end-slope mining methods, their own production safety issue along with their impact on normal open pit mine production security would bring about higher probability of accidents. For example, mining disturbance might cause surface deformation and undermine slope stability, and end-slope resources haulage system may be coupled with that of raw coal.
(2) Technical process complexity (6) Economic benefits Different mining methods require different technologies and devices. The production organization and management patterns are quite differentiated in the following aspects such as working face layout, roadway excavation, haulage system arrangement and working face moving and shifting. There is also a remarkable disparity in technical complexity, equipment amount and personnel matching. (3) Technical process adaptability
Economic benefits represent an important indicator to determine whether end-slope resources could be mined. Specifically, UMM cost and investment are closely related to technical mining process. HMS is characteristic of large one-time investment and low production cost, while LSSMM requires no additional equipment investment. 4.2. Establishment of optimum model based on TOPSIS
Mining method and process selection for excavating resources beneath end-slopes are influenced by their occurrence characteristics such as coal seam thickness range and its change, seam dip
By calculating the proximity between evaluated objects and ideal targets, TOPSIS ranks a finite number of evaluated objects based on their pros and cons, thus obtaining the optimal mining
Table 5 Preliminary evaluation indexes for end-slope mining methods in HSCM. Indicator
UMM
HMS
LSSMM
Resource recovery ratio Technical process complexity Technical process adaptability Impacts on surface mining production Production safety Economic benefits
17.36% Medium Medium Medium Relatively poorer 1.56 trillion yuan
3.25% Medium Difficult Relatively greater Relatively higher 0.178 trillion yuan
17.2% Simple Simple Relatively smaller Safe 1.618 trillion yuan
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
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Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
Table 6 Evaluation indicators for end-slope mining schemes of HSCM. Indicator
Weight
UMM
HMS
LSSMM
Recovery ratio Technical process complexity Technical process adaptability Impact on surface mining production Production safety Economic benefits
0.248 0.124 0.057 0.181 0.210 0.181
0.1736 3 5 5 3 15.60
0.0325 3 3 3 7 1.78
0.172 5 7 7 5 16.18
Table 7 Relative proximity of index value and optimum value and the sorted result. Method
Di+
D i
Ci
Result
UMM HMS LSSMM
0.302 0.467 0.101
0.392 0.207 0.490
0.565 0.307 0.830
2 3 1
method. According to TOPSIS calculation principle, firstly, normalization treatment is carried out on original data matrix to obtain the optimal vector and the worst vector. Secondly, the distance between every single evaluated object and the optimal/worst vector is calculated, respectively. The calculation results are used to determine the proximity between them and the optimal method [19,20]. (1) Table 3 is a list of evaluation indicators mentioned above. High merit indicators are usually converted using reciprocal method, e.g. Xij0 = 1/Xij, which is assumed as a high merit indicator converted from a low merit one. And evaluation indicators list with same trending were obtained below. Qualitative indicators are quantified using the scaling method of the constructive judgment matrices in Table 4. (2) Normalization treatment was conducted on original data matrix and the corresponding matrix was established.
8 .qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 > > < aij ¼ X ij i¼1 X ij ðOriginal superior indicatorsÞ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P n > 0 0 2 > : aij ¼ X ij i¼1 ðX ij Þ ðOriginal inferior indicatorsÞ
ð5Þ
where aij denotes the value of a certain evaluated object i in the jth indicator; Xij the value of ith evaluated object in the jth indicator; and Xij0 the value of the ith evaluated object after conversion in the jth indicator. The normalized matrix is shown as follows:
2
a11
6a 6 21 A¼6 4 an1
(
a12
a1m
3
a22
a2m 7 7 7 5
an2
anm
A ¼ ai1 ; ai2 ; ; aim
ð8Þ
where Dþ i and Di denote the distance from the optimum method and the worst method to the ith evaluated object, respectively; and xj the weight of the jth indicator. (5) The proximity Ci between each evaluated object and the optimum method was given by Eq. (9):
Ci ¼
Dþi
Di þ Di
ð9Þ
(6) The evaluated objects ranked according to their respective proximity. The higher value of Ci, the better assessment results. 4.3. Optimal mining method selection for RBES To identify the optimal mining method for extracting ultrathick coal seam in HSCM, factors such as technical process adaptability, mining parameters, resource recovery ratio and economic benefits of different mining methods were analyzed, respectively. And the preliminary evaluation indicators are as shown in Table 5. In addition, those qualitative indicators were further treated according to the scaling method of constructing judgment matrix in Table 4, thus the final evaluation indicators were obtained in Table 6. All indicators but recovery ratio and economic benefits in Table 4 are qualitative. Hence, they need to be converted into quantitative ones by the scaling approach in Table 4 before getting involved in solution optimization. Based on calculation principle of TOPSIS, the results were obtained as shown in Table 7. Rank the evaluated objects based on the value of Ci. The closer to 1 the value of Ci, the closer to the optimum level the method. Of course, if the value of Ci is close to 0, the corresponding method is approaching the worst level. According to the calculation results, LSSMM was assessed as the best method, which indicates that under the current technical process conditions, LSSMM is the optimum method for mining HSCM ultra-thick coal seam along with the process of mining and stripping engineering. 5. Conclusions
ð6Þ
(3) The maximum value of each column is the optimum vector A+ and the minimum value is the worst vector A:
Aþ ¼ aþi1 ; aþi2 ; ; aþim
8 ffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pm 2 > þ < Dþi ¼ j¼1 xj ðaij aij Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi > : D ¼ Pm xj ða aij Þ2 i j¼1 ij
ð7Þ
(4) Indicator weight was determined using set-valued iteration method. And the distance between indicator value of every single evaluated object and its optimum method Dþ i , and the distance between indicator value and its worst method D i were calculated respectively, as shown in Eq. (8):
(1) Based on advancing relations of open-pit coal mining and dumping engineering, the temporal and spatial characteristics of open pit end-slope were analyzed. The stability coefficient of end-slope decreases dramatically with increasing exposed area, and a smaller exposed free face is beneficial to the improvement of slope stability coefficient. The free face area of end-slope is closely related to pit bottom width, the advancing speed of operating lines at working slope and inner dumping site. (2) This paper analyzed and summarized three mining methods, namely UMM, HMS and LSSMM. Mining parameters regarding UMM and HMS were identified and the impacts that mining activities exert on surface deformation were also
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002
Z. Zha et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
obtained through analysis and simulation. Specifically, in the case of UMM application, the subsidence of monitoring points at central strip roof was around 24 cm and the maximum vertical displacement of surface monitoring points was 20 cm. In comparison, the application of HMS method had a smaller influence on surface deformation. Besides, with the aid of analysis software, the critical end-slope stability angle in HSCM was identified as 38° and the recoverable resources in unilateral end-slope was 11.6 Mt. (3) On the basis of TOPSIS method, a model was established for the sake of selecting the optimal mining method for mining resources beneath end-slope. Resource recovery ratio, technical process complexity, technical process adaptability, production safety and economic benefits were taken as evaluation indicators in the model. The final results indicate that LSSMM was superior to the other two methods except that its resource recovery ratio was merely 17.2%.
Acknowledgments Financial supports for this work, provided by the National Natural Science Foundation of China (No. 90510002) and the Science and Technology Research of the Ministry of Education of China (No. 306008), are gratefully acknowledged. References [1] Che ZX, Yang H. Application of open-pit and underground mining technology for residual coal of end-slopes. Mining Sci Technol 2010;20(2):266–70. [2] Liu ZM, Wang SL, Cai QX. Application of top-coal caving method in Anjialing surface mine. J China Univ Mining Technol 2001;30(5):515–7. [3] Xu ZY, Cai QX, Shang T. Study of end-slope coal mining and its system layout. J Mining Saf Eng 2006;23(4):406–10.
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[4] Shang T, Cai QX, Che ZX. End-slope mining method and system arrangement of Anjialing surface coal mine. Opencast Mining Technol 2007;22(5):4–6. [5] Shang T, Shu JS, Cai QX, Che ZX. Space-time relationship between end-slope coal extraction and dumping and mining of open pits. J China Univ Mining Technol 2001;30(1):27–9. [6] Cai L, Sun JB, Ma ZY. Application of HMS in surface coal mine. Modern Mining 2011; 27(11): 63–64, 74. [7] Liu WG, Wang LS, Fu Q. SHM highwall mining technology and key issues of application. Coal Eng 2012;44(6):1–4. [8] Shimada H, Chen YL, Hamanaka A, Sasaoka T, Shimada H, Matsui K. Application of highwall mining system to recover residual coal in end-walls. Procedia Earth Planetary Sci 2013;6:311–8. [9] Cai QX, Zhou W, Che ZX, Liu Y, Peng HG. Research on the mining method and stripping ratio of steep end-slope mining in surface coal mines with flat coal deposit. J China Univ Mining Technol 2007;36(6):743–7. [10] Liu Y, Che ZX, Li ZQ, Cai QX. Analyses of end slope remnant coal mining and slope exposure time. J China Univ Mining Technol 2006;35(6):727–31. [11] Cai QX, Zhou W, Shu JS, Liu Y, Peng HG. Analysis and application on end-slope timeliness of internal dumping under flat dipping ore body in large surface coal mine. J China Univ Mining Technol 2008;37(6):740–4. [12] Liu KG, Wang JC, Xu JH. Study on mining system and stability of coal pillar in short wall mechanized mining. J China Univ Mining Technol 2005;34(1):24–9. [13] Du JP. Mining Science. Xuzhou: China University of Mining & Technology Press; 2009. [14] Chen YL, Cai QX, Shang T, Peng HG, Zhou W, Chen SZ. Zonal extraction technology and numerical simulation analysis in open pit coal mine. Int J Mining Sci Technol 2012;22(4):487–91. [15] Liu L. End slope mining technologies introduction. Opencast Mining Technol 2012;27(1):36–7. [16] Wang SG, Yao GL, Bao H. Application analysis on end slope shearer in open-pit mine. Opencast Mining Technol 2014;29(10):28–30. [17] Zipf JRK, Mark C. Ground control for highwall mining in the United States. Int J Mining, Reclamation Environ 2005; (19)3: 188–217. [18] Afrapoli AM, Osanloo M. Determination and stability analysis of ultimate open-pit slope under geomechanical uncertainty. Int J Mining Sci Technol 2014;24(1):105–10. [19] Zhao KN. The evaluation about validity of regional innovation system based on Topsis method. World Regional Stud 2009;18(2):71–5. [20] Chen X, Wu YH. The evaluation and comparison of corporation’s independent innovation ability in different regions based on TOPSIS method. Sci Technol Prog Policy 2009;26(15):131–4.
Please cite this article in press as: Zha Z et al. Comparative study of mining methods for reserves beneath end slope in flat surface mines with ultra-thick coal seams. Int J Min Sci Technol (2017), https://doi.org/10.1016/j.ijmst.2017.10.002