optimization of concurrent mining and reclamation plans for single coal seam: a case study in...
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O R I G I N A L A R T I C L E
Optimization of concurrent mining and reclamation plansfor single coal seam: a case study in northern Anhui, China
Zhenqi Hu • Wu Xiao
Received: 1 September 2011 / Accepted: 2 July 2012 / Published online: 19 July 2012
Springer-Verlag 2012
Abstract The extraction of underground ore body inev-
itably causes a large amount of land subsidence. Currentreclamation technologies in China mainly focus on stable
subsided land, which means most of the affected lands are
submerged into water because of the high groundwater
table in some areas, leading to the loss of soils and inef-
ficient reclamation. Therefore, a new technology for
reclaiming unstable subsiding land is being studied for
restoring farmland as much as possible, based on a case
study in northern Anhui, China. In consideration of the
mining plan, subsidence processes in various stages were
analyzed and some related factors such as vertical subsi-
dence, post-mining slope, water area, and land use condi-
tion were also simulated. Due to mining activities, useful
farmland has gradually decreased to merely 14.4 % of the
pre-mining area. In this study, the following stages were
modeled from pre-mining to post-mining: (1) percentage of
farmland was 100 % in stage (a) (pre-mining), (2) 72.5 %
in stage (b), (3) 67.3 % in stage (c), and (4) 14.4 % in stage
(d) (post-mining). The results show that 86.6 % of culti-
vated land was submerged into water and lost its capacity
for cultivation after coal mining. Reclamation plans for
stages (b), (c), and (d) were made by a traditional recla-
mation method called ‘‘Digging Deep to Fill Shallow’’.
Based on scenario simulation of reclamation, the farmland
reclamation percentages were improved to 78.3, 73.3, and
40.70 %, respectively. Taking the percentage of reclaimed
farmland as the preferred standard, concurrent mining and
reclamation for stage (b) and (c) could increase farmland
reclamation percentages to 37.6 and 32.6 %, respectively,
compared with the farmland reclamation percentage of post-mining [reclaiming the land in stage (d)]. The results
reveal that optimum reclamation time should be at stage
(b). Therefore, under current technical conditions, con-
current mining and reclamation could enhance the quantity
of cultivated land and provide better land protection and
food security in the mined areas with high groundwater
table.
Keywords Mining subsidence Concurrent mining and reclamation Land protection Subsidence prediction Planning scheme Optimization
Introduction
China is the number one coal production and consumption
country. The coal outputs exceeded 3 billion tons in 2010,
accounting for 42 % of the global production (Wang 2009).
However, mining and utilization of coal may cause serious
environmental impacts such as coal waste disposal, poi-
sonous gas emissions, land subsidence, landscape change,
etc. Comparatively, land subsidence seems to be one of the
most prominent problems in China, because 92 % of the
coal output comes from underground mining, with thou-
sands of underground long wall panels. Underground
mining activities disturb the surface severely, and this sit-
uation is even more prominent in east and northeast China
(Xiao et al. 2009), which are both main coal and agricul-
tural production regions (called ‘‘Overlap Region’’). The
Overlap Region covers 40 % of the total farmland in China
and contains 58 % of national coal production and 45 %
of food production (Hu and Luo 2006). High-intensity
extraction of underground coal results in large-scale
Z. Hu (&) W. XiaoInstitute of Land Reclamation and Ecological Restoration,
China University of Mining and Technology (Beijing),
Beijing 100083, China
e-mail: [email protected]
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DOI 10.1007/s12665-012-1822-9
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ground settlement, barren farmland, and a decrease in
cultivated land. It is estimated that the subsidence area
extends from 0.2 to 0.33 ha, when 10,000 tons of coal are
extracted underground. Subsidence is expected to expand
2 9 104 ha annually throughout China (Hu 1996). Thou-
sands of hectares of affected land lost the capacity of
cultivation because of extraction of a thick coal seam and
high groundwater table. The depth of ponding water on the
ground surface could reach 13 m (Wang et al. 2009). Thus,
farmland loss and degradation caused by underground
mining activities is serious in China. In fact, coal mine
production increased over the past 10 years and reached its
peak with 18.35 % growth rate in 2003 (Fig. 1). It is pre-
dictable that mining production will continue to expand in
the foreseeable future, because of rapid economic growth.
Thus, the quantity of subsided agricultural land will con-
tinue to rise.
According to the Annual Bulletin issued by the Ministry
of Land and Resource (MLR) in 2008, cultivated land was
decreased to 1.22 9 108 ha at the end of December 2008,
with merely 0.09 ha farmland per capita (Fig. 2). Conse-
quently, the Chinese government also retains the amount of
1.2 9 108 ha farmland as a base red line to ensure food
security in 2020. How the limited cultivated land resources
are protected is particularly important at this stage of rapid
economic growth.
Abandoned mine site hazards assessment (Kim et al.
2006, 2009) and mine land reclamation have been exten-
sively studied by many scholars (Bascetin 2007; Wu et al.
2009; Xiao et al. 2011). Approximately, 60 % of theworld’s coal production comes from underground mines,
and China accounts for much of the worldwide under-
ground operations (Table 1). Comparatively, the other four
main coal production countries such as USA, India,
Australia, and Russia adopt less underground mining.
Therefore, farmland subsidence and related reclamation are
relatively less of a concern in these countries (Bell et al.
2005). The impact of subsidence may vary from site to site
due to differences in geology and soil conditions. For
example, Illinois, USA, also a typical coal and agricultural
production overlap region, has maximum subsidence of
about 3 m (Darmody et al. 1992; Darmody 1995), thus,making reclamation work much easier as compared to
China. In addition, reclamation strategies are much more
diverse outside of China where agricultural land resources
and population pressures are not as pressing.
Since the 1980s, China’s land reclamation has made
considerable progress in restoring subsidence land with
different methods (Hu 1994a, b); they are:
(a) ‘‘Digging Deep to Fill Shallow’’: The technology
divides the subsidence-prone area into two parts: deep
and shallow, then makes the deep area deeper by an
excavator for digging a fish pond, and the excavated
soil is moved to the shallow part for reclamation of
cultivate land. Hydraulic dredge pumping is one of
the ‘‘Digging Deep to Fill Shallow’’ methods.
(b) Directly reconditioning: If there is no water in some
shallow subsidence areas and the subsurface water
level is not very high, the method of directly
reconditioning the subsided land can be used. Usu-
ally, the subsided land is leveled by bulldozers or
often by manual work. If the slope of the subsided
land is large, terraces should be used.
Fig. 1 Coal yield and growth rate from 1990 to 2010 in China
Fig. 2 Cultivated land quantity variation from 2001 to 2008 in China
Table 1 Percentage of coal production by mining method in 2006
Country Underground (%) Surface (%) Total (Mt)
China 95 5 2,380
USA 31 69 1,054
India 19 81 447
Australia 22 78 405
Russia 309
South Africa 257
Germany 197
Indonesia 195
Poland 156
Total world 60 40 6,195
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(c) Filling: A type of landfill method, filling subsidence
land with some filling materials such as coal wastes
and fly ash.
(d) Drainage: Establishment of a system of drains such as
a trench can make the impounded water drain away
and lower the subsurface water level so that the
subsidence land is relatively raised.
Both filling and non-filling methods are good earthwork
measures and well applied in China. However, all recla-
mation works were focused on stable subsided land, which
means subsidence reaches the final settlement condition
after the excavation of coal is completed. In this situation,
more than half of the subsided land was already submerged
in water and the fertilized topsoil was lost permanently. The
manner of reclamation is a kind of post-mining retreatment;
it leads to a low percentage of farmland reclamation and
inevitably increases the reclamation cost. Therefore, it is
necessary to develop a new technology that could take
measures for advance planning, conservation, and man-agement before land subsidence falls below the water table.
The improved plan and technology could raise the farmland
percentage and lower reclamation costs. This research takes
a colliery located on the eastern plain in China as an
example, to analyze and simulate subsidence processes
according to the geological condition and mining system,
and optimize concurrent mining and reclamation planning.
Basic information in the study area and research
methods
Basic information in the study area
The study area is located in the northern part of Anhui
Province in eastern China, covering about 24.1 ha. It is a
typical plain mining area in eastern China, with flooded
subsidence lands and densely populated villages. The nat-
ural elevation is ?30 to ?32 m above mean sea level, with
an average of ?31.0 m. The underground water table is
about 2 m below the soil surface, and the ground is flat
with a slope of 0–2. The climate is humid continental,
typical of that of eastern China, with great inter-annual
variability of precipitation. Most precipitation falls
between June and September. Because of its humid cli-
mate, abundant sunshine, and four distinct seasons, it is one
of the most important grain production areas in China.
The main coal seams in this area are named nos. 4 and 6,
with an average thickness of about 2.2 and 2.8 m, res-
pectively. The buried depths of coal seams are about
350–450 m and 380–550 m, respectively. The dip angle of
coal seams varies from 3 to 15. The overburden has
a thick alluvial strata reaching 120–160 m, and the
compaction of soil caused by the loss of water makes the
subsidence factor reach 0.96 (Table 2). A schematic of a
typical geological section is shown in Fig. 3.
The eight panels referring to no. 6 coal seam were
extracted from March 1997 to the end of 2004 leading to a
maximum subsidence depth of 2.5 m. The subsided land
was reclaimed to some fish ponds and farmlands with
original elevation in 2005. Excavation of the no. 4 coal
seam commenced in January 2007; four panels were
included and will last till September 2012. The layout of
panels and mining schedule are illustrated in Fig. 4 and
Table 3. Thus, the influence of the mining activity in the
study area could be treated as a single coal seam (no. 4)
extraction. This paper analyzed the dynamic evolution of
ground condition and land use to optimize the concurrent
mining and reclamation.
Research methods
Dynamic mining subsidence prediction and geological
information system (GIS) were used in this paper; the
process of dynamic scenario simulation was derived
according to the specific geological condition and mining
schedule in different stages. Mining subsidence prediction
Table 2 Basic parameters for surface movement and deformation
prediction
Parameters Value
Coefficient of the initial mining (q) 0.96
Coefficient of the repeated mining (q0) 1.20
Horizontal movement coefficient (b) 0.3
Influence propagation angle tangent (tgb) 1.80The displacement distance (S ) 0.03H0
Subsidence velocity factor (c) 11.6
Influence angle (h) 89
Fig. 3 Schematic of typical geological section in the study area
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system (MSPS), developed by China University of Mining
and Technology, was chosen to analyze the mining impact,
and Arcinfo, released by Environmental Systems Research
Institute (ESRI), was chosen for ground analysis and sim-
ulation. The procedures are as follows:
1. Acquisition of original terrain data
According to the information provided by Hengyuan
colliery, the topographic map surveyed in 2005 by the total
station was adopted, which contains elevation points and
isograms.
2. Acquisition of subsidence information in various
stages
After decades of research, some useful theories and
methods of mining subsidence prediction (Konthe 1959;
Peng and Luo 1991) such as profile function, influence
function, and probability integration method have been
produced. In the recent 30 years, deterministic (mecha-
nistic) methods including finite element method and
boundary element method based on rock mechanics were
presented (Kratzsch 1983). In deterministic modeling, the
in situ mechanical properties of rocks are of particular
importance. Probability integration method, developed
from random medium theory, is the most mature andapplied prediction model in China; the software (MSPS)
used in this paper was programmed based on this theory.
Dynamic subsidence prediction was also applied, utilizing
one of the most widely used time functions proposed by
Knothe (1959):
f ð xÞ ¼ 1 ect
where t is the time that elapsed from the opening of the
cavity and c is the subsidence velocity factor, whose value
depends on the characteristics of the ground and on the
units in which the time is expressed.
3. Construction of dynamic ground subsidence model
Dynamic ground subsidence model can be constructed
in two phases: raster interpolation and superimposition.
(a) Raster interpolation
Interpolation predicts values for cells in a raster from a
limited number of sample data points. It can be used to
predict unknown values for any geographic point data:
elevation, rainfall, chemical concentrations, noise levels,
and so on. Since the obtained original terrain data and
dynamic subsidence information in step (1) and step (2)
were presented as points and isograms, interpolation was
then adopted by Arcinfo to achieve comprehensive raster
information. The vector information (points and isograms)
was converted to raster data format automatically by
Arcinfo.
(b) Superimposition
Superimposition between interpolated raster data of step
(1) and step (2) could generate a state of ground subsi-
dence. By using this method, dynamic ground subsidence
could be produced for different mining stages.
4. Analysis and monitoring
The dynamic ground subsidence model for each panel
and various stages could be determined and analyzed based
on mining plan. Temporal and spatial dynamic changes of
the ground could be monitored, including vertical subsi-
dence, extent of water area, land use change, slopes, and
aspects of surface in each mining stage. Virtual reclama-
tion schemes at different stages were produced. Finally, the
percentage of restored farmland was considered as the
preferred standard to optimize the schemes.
Fig. 4 Layout of no. 4 coal seam workface
Table 3 Mining schedule of no. 4 coal seam
Number of
panel
Mining time Average
depth (m)
Average seam
thickness (m)
452 2007.1–2007.12 280 2.2
453-1 2009.12–2010.6 280 2.3
453-s2 2010.11–2011.3 280 2.3
454 2011.9–2012.9 300 2.0
455 2008.4–2009.7 250 2.1
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Results and analysis
Scenario simulation
1. Original terrain data
The study area is 24.1 ha, with a north–south length of
about 1,100 m and east–west width of about 220 m
(Fig. 5). The surface was roughly flat before extraction of no. 4, with the elevation between 30 and 32 m and slope
between 0 and 2. The study area was good farmland with
aquaculture ponds on both sides.
2. Mining subsidence prediction at various stages
Prediction of ground subsidence associated with under-
ground mining based on the shape of the final settlement
trough of the ground surface could be insufficient. Surface
subsidence due to mining is a dynamic process which
obeys mechanical principles (Saids 1995). Particularly,
dynamic processes of land use change and water ponds
distribution in temporal and spatial domain could provide amore intuitive and rational vision for the manager and
engineer in the coal and grain overlap region. Character-
istics and timing of mining subsidence could be monitored
through dynamic prediction. The prediction parameters are
indicated in Table 2.
In consideration of the mining schedule and layout, the
study area was divided into four stages as shown below:
(A) two panels 452 and 455 were mined completely in
July 2009,
(B) panels 452, 455, and 453-1 were mined completely in
September 2010,
(C) panels 452, 455, 453-1, and 453-2, were mined
completely in March 2011,
(D) panels such as 452, 455, 453, and 454 were all mined
completely in September 2012.
Movement and deformation at various stages were rep-resented as subsidence isograms and deformation isograms.
The study area is located in eastern China, a region with
thick alluvium on the near surface and a high underground
water table (the groundwater table is 1–3 m below the soil
surface). According to previous studies, cracks derived
from deformation have a small influence on the utility of
the cultivated land surface. The most significant impacts to
cultivated land were submergence into water caused by
vertical subsidence, and soil erosion and deterioration
caused by additional slope at the edges of the subsidence
basin (Hu et al. 1997). Therefore, dynamic vertical subsi-
dence based on temporal and spatial distributions wastreated as a major consideration for scenario simulation.
3. Scenario simulation at various stages
Based on the local conditions, the local government set
the criteria for subsidence damage: the land becomes a
permanent water area when the vertical subsidence reaches
2.0 m, and the land becomes a seasonal water area when
the vertical subsidence reaches 1.5 m. The criteria were
validated through field measurement in this research, and
ground elevations of ?29.0 and ?29.5 m were defined as
permanent and seasonal water criteria. These validated
criteria would benefit temporal and spatial analyses. Thedevelopment of ponding areas at various mining stages is
subdivided into four stages, as shown in Fig. 6.
Stage (A) (2007.1–2009.7): Excavation of coal at panels
452 and 455 were completed. Mining activities only
reached a super-critical level in strike orientation and were
sub-critical in inclination orientation because two panels,
Fig. 5 Pre-mining TIN in the study area
Fig. 6 Changes of water area at various stages
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453 and 454, were not mined. Thus, vertical subsidence on
the ground was not serious and created no ponding area.
Stage (B) (2009.8–2010.9): Excavation of panel 453-1.
In this stage, water rose in the ground and expanded
gradually until September 2010.
Stage (C) (2010.9–2011.3): After a slight break, the
extraction of panel 453-2 made the situation even worse.
However, the water area, including both perennial and
seasonal, changed slowly.
Stage (D) (2011.4–2012.10): Panel 454 was the final
panel in this mining area. Its excavation led to a severe water
expansion in the ground, because the mining reached a super-
critical level both in inclination and strike orientation.
According to dynamic ground subsidence simulation,
land use change was analyzed (Fig. 7). (a), (b), (c), and
(d) correspond to the four mining stages of (A), (B), (C),
and (D). Although the excavation of panel 452 and 455
impacted the ground, it did not change the land use because
there was no water accumulation on the surface (Fig. 8).
Along with the excavation of panel 453-1, water gradually
rose in the ground and finally reached 20.6 ha, accounting
for 85.6 % of the study area; the area of cultivated land
Fig. 7 Land use changes at various stages on the ground
Fig. 8 Reclamation layout and
elevation at various stages
exception of the stage A without
reclamation work
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decreased to 3.5 ha, only accounting for 14.4 % of the
study area. The proportions of cultivated land were 100,
72.5, 67.3, and 14.4 %, respectively, corresponding to the
four mining stages. This shows that the excavation of no. 4
coal seam led to a lot of farmland loss because of water
accumulation due to mining subsidence.
Optimization of reclamation plans and analysis
Dynamic ground scenario simulation provides a vision to
understand the development of subsidence in the ground.
If reclamation work was launched after all the mining
activities had ended, a lower percentage of farmland res-
toration would be produced because most of the land would
subside into water and lack of filling materials. Further-
more, the existence of large areas of water may make the
reclamation work more difficult and increase reclamation
costs. Because of submergence of important topsoil into
water, the reclaimed land might have poor soil condition
due to lack of topsoil. Therefore, reasonable reclamationtime and the best reclamation plan seem to be particularly
important in a high underground water table area.
1. Optimization of reclamation plan
Following are the basic principles for making a con-
current mining and reclamation plan:
(a) Reclamation earthwork adopted ‘‘Digging Deep to
Fill Shallow’’ technology.
(b) Filling materials from outside of the study area for
reclaiming the subsided land were not used.
(c) Dynamic subsidence prediction for every stages wasadopted and the elevation of reclaimed land at every
reclamation stage should be rational to ensure that the
reclaimed land could withstand the impact of follow-
ing settlements.
According to the considerations above, different stages
of mining reclamation measures were taken into account.
The extraction level of panel 452 and 455 had less impact
on agriculture in stage (a); therefore, only reclamation in
stages (b), (c), and (d) was considered. The layout anddesigned elevation are shown in Fig. 8. The percentage of
reclaimed farmland reaches 78.3, 73.3 and 40.7 % at the
stage (b), (c), and (d), respectively.
2. Result analysis
Extracting a horizontal layered ore body is a continuous,
slow, and gradual subsidence process. The manner and
extent were different in various stages. Table 4 shows the
advantages, disadvantages, and the reclamation efficiency
at various stages.
If reclamation measures were taken too early (stage a),
the influence of underground extraction would not betransmitted to the ground completely or the effect may not
be sufficient to interrupt normal agricultural activities. This
means increased costs and more interruption of agricultural
activities. The reclamation project needs to accommodate
subsequent settlement, including vertical subsidence, tilts,
horizontal deformation and curvatures. Conversely, if rec-
lamation measures are taken too late (stage d), most of the
topsoil will sink below the water table and soil resources
cannot be guaranteed for reclamation. This increases both
reclamation costs and construction difficulties, and may
lead to a very low percentage of farmland reclamation and
poor farmland productivity. Therefore, the most appropri-ate stages would be stage (b) and stage (c), when panel 453
was under mining. At this stage, the land began to
Table 4 Optimization and comparison of reclamation plans in various stages
Stages Mining condition Condition before
reclamation
Advantages Disadvantages Percentage
of reclaimed
farmland
(%)
(a) 452 and 455 Start of subsidence,
no water area
Most of the topsoil resources
available, high percentage
of reclaimed farmland
High reclamation costs, occupying
the land for so long a period
influences normal agricultural
work, more residual subsidence
–
(b) 452, 455, and 453–1 Water area rise, with
less area
More topsoil could be
protected, high percentage of
reclaimed farmland
High reclamation costs, partial
residual subsidence influence
78.3
(c) 452, 455, and 453 Subsidence increases,
water area expands,
but with a low
proportion
More topsoil could be
protected, high percentage of
reclaimed farmland
High reclamation costs, partial
residual subsidence influence
73.3
(d) 452, 455, 453, and
454
Most of the area
submerged into water,
little farmland exists
Stable ground, less residual
settlement exists
No topsoil resource, low percentage
of reclaimed land
40.7
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submerge into water and the land reclaimed at this stage
could save a lot of soil and guarantee high percentage of
farmland reclamation.
Conclusions
1. Based on mining schedule and geological condition,dynamic scenarios of land subsidence due to coal
mining are simulated by dynamic subsidence predic-
tion and spatial analysis for a horizontal layered single
coal seam. This paper simulated three stages of land
subsidence caused by underground mining. The failure
mode, damage area, and degree of damage were ana-
lyzed quantitatively. The results revealed that: the
percentage of farmland decreased gradually from 100
to merely 14.4 % from pre-mining to post-mining due
to mining subsidence (the four stages correspond to
100, 72.5, 67.3, and 14.4 %, respectively). If recla-
mation measures were adopted until the ground sta-bilized, 86.6 % of cultivated land would submerge into
water and lead to a low percentage of reclaimed
farmland (40.7 %).
2. Based on the scenario simulation, a virtual reclamation
plan was under consideration for three mining stages.
Farmland reclamation percentages were 78.3, 73.3,
and 40.7 % corresponding to stage (b), stage (c), and
(d).
3. Taking the percentages of reclaimed farmland as the
preferred standard, the farmland reclamation percent-
ages at stages (b) and (c) could be increased to 37.6
and 32.6 %, respectively, compared against delaying
reclamation until the land stabilizes [stage (d)].
Consequently, restoring subsided land in stage (b) or
stage (c) would decrease construction difficulty, pro-
tect the topsoil maximally, and enhance farmland
reclamation percentage. The optimum reclamation
time was at stage (b).
In summary, research shows that using dynamic subsidence
prediction with reclamation scenario simulation and taking
the percentage of reclaimed farmland as the preferred
standard, the best concurrent mining and reclamation time
and plan can be obtained.
Acknowledgments The research has been supported by nonprofit
industry research and special funds provided by the Ministry of Land
and Resources in China, approval no. 200911015-03. The authors
offer their special thanks for the research and valuable assistance
provided by Anhui Wanbei Coal Group and Anhui Hengda Ecological
Construction Engineer Co., Ltd. Special thanks also go to Dr. R.G.
Darmody (University of Illinois at Urbana-Champaign), Dr. Richard
Barnhisel (Executive Secretary of American Society of Mining and
Reclamation), Dr. S.K. Chong (Southern Illinois University at Car-
bondale) and Mr. Steve Hewitt for their comments and editing.
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