In order to analyze the relationships of inner dumping with covered end wall in adjacent surface coal mining districts, a trough-shaped uncovered end wall model was put forward. A mathematical model concerning uncovered end wall height (UEWH) was established based on minimum cost method. The inner dump capacity was considered regarding its effect on UEWH. Besides, a comprehensive model was established for calculating optimal UEWH in inner dumping with partially covered end wall mode and a shifting distance optimization model based on cost compensation method for building a provisional haulage system to connect inner dump. As for case study in Huolinhe number 1 surface coal mine, research results show that optimal UEWH was 225 m between North Pit and South Pit whereas in number 3 mining district of North Open-Pit Mine it was 23.50 m when making its transition from cross mining to mining along strike. Nevertheless, due to limited inner dump capacity, inner dump height in South and North mining districts shall be appropriately increased and closed pits from earlier period shall be fully utilized to make room for inner dumping with partially uncovered end walls.
In order to enlarge the economic benefits in early stage and during the entire service period of mining, nearly horizontal and slowly inclined coalfields suitable for surface mining are normally divided into several mining districts. These mining districts are then excavated one by one in accordance with deposit occurrence and the prevailing technical and economic conditions. But in normal cases, inner dumping with the covered end wall in the previous mining district would bring about increased amounts of restripping, resulting in secondary stripping cost when excavating the next mining district [
Basic modes of inner dump covering in surface coal mines.
Entirely covered end wall
Uncovered end wall entirely
Partially covered end wall
By definition, if an end wall is entirely covered, the stripped materials from the previous mining area would be dumped into the mined-out area with the entire end wall covered. Therefore, when the next mining area is excavated, those materials need to be restripped and dumped again which brings about increased stripping costs [
Regarding UEWH in this mode, Gu and Zha (1996) conducted an analysis about the correlation of covering height of the end wall (EWCH) with working face length and the end wall angle of repetitive stripping and restripping time. Based on these analyses, they obtained an optimum EWCH range under different conditions using minimum relevant cost method [
Trough-shaped uncovered section.
According to the geometric relationships between the partially covered end wall and the entirely covered end wall shown in Figure
From (
As a result of the uncovered end wall, the reduced restripping cost per year and the annually increased cost as a result of purchasing external dumping sites and transporting the stripped materials would be expressed as
Due to the influence of inner dumping with partially covered end wall, round transport corridor (using end wall berms on both sides for overburden haulage and dumping) may continue to be used for dumping stripped overburdens for rock stripping benches below bottom level elevation, whereas, for those end wall rock stripping benches above bottom level elevation and adjacent to the next mining district, the only option would be a one-way transport corridor (namely, only one-sided end wall berm could be used for overburden haulage and dumping), since they are not connected to the inner dump as shown in Figure
Round inner dumping corridor versus one-way inner dumping corridor.
Round inner dumping corridor
One-way inner dumping corridor
Generally speaking, in the case of using round inner dumping corridor, the stripped-out overburdens on the left side of working face would be delivered to the corresponding dumping site via the left end wall and it is likewise for the stripped-out overburdens on the right side of working face. When it comes to a one-way inner dumping, haulage system of overburden removal would be influenced on either left or right side of a working face. Hence, the respective one-way and round overburden haulage distance would be expressed as
According to follow-up working characteristics in the dumping with excavating mode, the stripped overburden would be internally dumped onto the bench on the same level. Assume the stripping working line is as long as the dumping working face; namely,
Taking half of the UEWH as the centre of stripped overburdens, then the length of the stripping working face of this level is
Theoretically speaking, the haulage distance from the loading point on the stripping bench to the nearest end wall should not be more than half the length of stripping working face and also not less than the uncovered width on the same levelled dumping bench. As shown in Figure
One-way inner dumping diagram with partially covered end wall.
The corresponding annually increased haulage distance and increased costs resulting from a one-way haulage would be expressed as
Similarly, when the next mining district is under exploitation, a one-way inner dumping corridor would also be applied to working benches above the uncovered zone. The corresponding increased cost is
There exist time differences regarding coal mining and inner dumping engineering in adjacent mining districts. Therefore, according to the time value of capital theory,
Inserting (
On the basis of (
Assuming that
The above roots could help identify the monotonicity of three intervals, namely,
When part of the end wall remains uncovered in the inner dump, the capacity of inner dump would be reduced. However, the situation could be modified by increasing the height of overburden pilings up in the covered area of the inner dump. Therefore, when considering the UEWH, it has to be guaranteed that the inner dumping space that has been extended to its extreme is able to accommodate the remaining overburden amounts resulting from the partially uncovered end wall.
The cross-sectional area
Schematic diagram of inner dump capacity versus uncovered end wall height.
When considering the extended inner dumping space and capacity, the surplus inner dumping space in the case of entirely covered end wall has to be taken into account as well; namely,
To sum up, two factors have been taken into account in determining the partially uncovered end wall height in the case of inner dumping. One is the influence that inner dumping and end wall covering practices exert on the comprehensive economic benefits in adjacent mining districts. The other factor is the inner dump capacity constraint. Here in this article, increased non-dumpable amount problem resulting from partially covered end wall is alleviated solely by increasing the height of inner dump instead of using any external dumping site. In this case, the UEWH shall be given by
To reduce the effects of one-way haulage method on transport costs, a PHP could be built using the stripped overburden as a transport pathway to connect the uncovered end wall and the inner dump working face, as shown in Figure
Schematic diagram of building PHP in the mode of inner dumping with partially uncovered end wall.
PHP shifting distance is one of the important factors that affect the economic benefits of the inner dumping practice. Specifically, the larger the shifting distance, the lower the expenses resulting from PHP building and repeated overburden stripping. On the other hand, trucks have to cover greater haulage distance between the end wall and dumping site to deliver the stripped materials, bringing about increased transportation costs. Therefore, the optimum PHP shifting distance has to be determined by comparing with a one-way haulage distance. The equivalent one-way haulage distance is defined as the sum of the increased haulage distance resulting from vehicles’ climbing up and down the slope as well as increased PHP shifting distance, which should be expressed by
A basic principle is that the equivalent one-way haulage distance should not be greater than the increased distance from truck’s one-way transport corridor; namely,
And the upper limit of the shifting distance would be further given by
Since the transportation cost saved as a consequence of PHP building should not be lower than the restripping cost for the back filling materials used for PHP building, the following expression is obtained:
In this case, the lower limit of shifting distance will be given by
The reasonable scope of PHP shifting distance would hence be given by combining (
Huolinhe number 1 surface coal mine is 10 km long from north to south and 3.4 km wide from east to west. It has a surface area of 34 km2 and mining depth ranging from 864 m to 432 m elevation. Its coal seam runs in the direction of northeast-southwest and it is reclining towards northwest. Coal seam dip angle is no more than 10 degrees [
Uncovered end wall position plan between open pits and mining districts.
At present, SOPM has five external dumps and two inner dumps. Among the external dumps, the eastern, the southern, the western, and the one along the end wall have reached their upper limit of dumping capacity and the remaining capacity in the western 4th dumping site is 75 million m3. The dumping capacity in South Inner Dump and North Inner Dump is, respectively, 707.227 million m3 and 999.015 million m3. As to NOPM, it contains one external dump and one inner dump and their respective capacity is 17.006 million m3 and 25.975 million m3.
When the South Pit and North Pit advance in a parallel direction, their common end wall would be repeatedly covered. And there would also be interaction problem when number 3 mining district of NOPM makes a transition from cross mining to mining along strike. In this case, two modes of uncovered end walls have been put forward to solve these two problems. The first mode and solution is inner dumping with partially covered end wall between SOPM and NOPM. The other one is slow slope and partially uncovered end wall transition mode in adjacent mining districts. Figure
According to the uncovered end wall position in between open pits and mining districts, the relevant parameters are shown in Table
Relevant parameters in between open pits and mining districts.
Symbols | ① Between South Pit and North Pit | ② Transition from cross mining to mining along strike in number 3 mining district in NOPM |
---|---|---|
|
25° | 25° |
|
20° | 20° |
|
258 m~350 m, 300 m on average | 131 m~211 m, 155 m on average |
|
160 m | 100 m |
|
13.23 | 19.03 |
|
10.35 | 4.9 |
|
1250 (PHP building in the middle part) | 1150 |
|
1.91 | 1.91 |
|
4.1422 | 4.5811 |
|
1.5% | 1.5% |
|
17 | 17 |
|
−1.0938 million m3 | −0.4351 million m3 |
According to (
(1) Between South Pit and North Pit, in the case of
(2) Number 3 mining district of NOPM makes a transition from cross mining to mining along strike, and they are −168.23 and 23.50.
In the case of
Therefore, (1) the optimum UEWH between North Pit and South Pit is 225 m, and (2) the optimum UEWH is 23.50 m for number 3 mining district of NOPM when making its transition from cross mining to mining along strike. Meanwhile, considering the restricting influence that inner dumping practice exerts on UEWH, the inner dumps need to be heightened, respectively, by over 140 m and 120 m. However, the fact is that the heightening limit for inner dump is approximately the height of four dumping benches (96 m), which obviously cannot meet the uncovering requirements. According to the uncovered end wall height and length, the stripped overburden amount that cannot be internally dumped because of uncovering the end wall in the two positions mentioned above is 384.66 million m3 and 90.4 million m3, respectively. Therefore, the inner dump height in South and North mining districts should be properly enhanced. In addition, there is extra inner dumping space when the open pits in North and South districts are closed in early stage; thus overburden that needs to be transferred and dumped because of partially uncovered end wall could be properly accommodated. Results from calculations indicate that the PHP in number 3 mining district of NOPM is 48 metres high, 35 metres wide, and 792 metres long. The PHP volume is 3.357 million m3 and its shifting distance is 535 metres. Within the service scope of a single PHP, the transportation costs could be reduced by 3.55 million RMB.
(1) A basic principle in determining UEWH is to reduce restripping amount and the number of affected level transportation systems. It is in this principle that the trough-shaped inner dumping mode with partially covered end wall has been put forward. When the restripping amounts resulting from the effect of uncovered end wall are the same, trough-shaped level uncovering mode would affect the transport systems levels on the end wall less compared with the inverted triangular mode.
(2) A mathematical model for UEWH optimization was established on the basis of minimum cost method. The model takes three factors into comprehensive consideration, namely, restripping and transferring expenses resulting from uncovered zone capacity constraint, haulage distance affected by the uncovered end wall, and transportation cost. In addition, the influence of inner dump capacity constraint on UEWH is also taken into account.
(3) Inner dumping with PHP building to connect inner dump has been put forward to reduce the influence that partially uncovered end wall practice exerts on transportation system. Based on cost compensation method and principle, equivalent one-way haulage distance should not be longer than the increased distance resulting from a truck’s one-way haulage. Besides, the transportation costs saved through PHP should not be less than the restripping expenses of PHP building and backfilling. Under this circumstance, the optimization model of the PHP shifting distance is established.
(4) The paper probes into the production conditions in Huolinhe number 1 open-pit coal mine and practical situations regarding the mining district transition. An optimization analysis was made about inner dumping and UEWH. The analysis results show that optimum UEWH between South and North Pits is 225 m and 23.50 m for number 3 mining district in NOPM when making a transition from cross mining to mining along strike. However, due to limited inner dumping capacity, the height of dumping sites within South and North miming districts shall be appropriately increased. In addition, when open pits in South and North districts are closed in the early stage, it makes further room for inner dumping and would help meet the transferring demand when end wall is partially uncovered.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This study was supported by the Doctoral Starting Funds of Xi’an University of Science and Technology (2017QDJ060).