Coal mines in the western areas of China experience low mining rates and
induce many geohazards when using the room and pillar mining method. In this
research, we proposed a roadway backfill method during longwall mining to
target these problems. We tested the mechanical properties of the backfill
materials to determine a reasonable ratio of backfill materials for the
driving roadway during longwall mining. We also introduced the roadway layout
and the backfill mining technique required for this method. Based on the
effects of the abutment stress from a single roadway driving task, we
designed the distance between roadways and a driving and filling sequence for
multiple-roadway driving. By doing so, we found the movement characteristics
of the strata with quadratic stabilization for backfill mining during roadway
driving. Based on this research, the driving and filling sequence of the 3101
working face in Changxing coal mine was optimized to avoid the superimposed
influence of mining-induced stress. According to the analysis of the surface
monitoring data, the accumulated maximum subsidence is 15 mm and the maximum
horizontal deformation is 0.8 mm m
Recently, due to the high intensity and large scale of coal mining around the world, problems associated with resources and the environment have become more severe, and an increasing number of mining-induced hazards have been registered (Shi and Singh, 2001; Castellanza et al., 2010; Pasten et al., 2015; An et al., 2016; Zhai et al., 2016). The effects of these problems are especially prominent in the ecologically vulnerable areas of western China (Si et al., 2010; Ding et al., 2014). Many small and medium-sized coal mines in this area use room and pillar mining method and have a low mining rate (Jiang et al., 2015), leading to abandonment of the coal resources in underground. In addition, the burial depth of the coal seams in the region is usually shallower. During mining, geological behavior becomes more important, threatening mine production safety and usually causing unpredictable disasters, such as surface subsidence, landslides, and water inrush caused by karst environments (White, 2002; Vigna et al., 2010; Parise, 2012, 2015; Lollino et al., 2013). In particular, when karst caves are at shallow depth the effects at the ground surface may be extremely severe (Parise, 2008; Parise and Lollino, 2011), and the direct connection between the surface and the underlying karst aquifers usually functions as a channel for water inrush (Gutierrez et al., 2014), thus posing a great threat to coal mines. At present, previous studies proposed strip mining (Chen et al., 2012; Guo et al., 2014a) to resolve these problems. However, strip mining resulted in a low mining rate of coal resources as well as in other problems. To solve these difficulties, this research proposes roadway backfill technique in longwall mining based on the solid backfill mining method (Zhang et al., 2011; Junker and Witthaus, 2013; Guo et al., 2014b). This method has been recently developed and has become popular and applied at a large scale, providing an effective solution to the previously mentioned problems.
Surface subsidence induced by room and pillar mining and working face layout.
Recently, research into the strip filling method has promoted the development
of the roadway filling method and its underlying theory (Zhang et al., 2007;
Yu and Wang, 2011; Chen et al., 2011; Sun and Wang, 2011), but few studies
have been conducted on backfilling the driving roadway during longwall
mining, especially from the perspective of the strata movement
characteristics. Moreover, different types of backfill materials with
different mechanical properties exist, and if the room and pillar mining
method is adopted then strata behavior becomes the most important factor at
shallow burial depths. Consequently, the characteristics of the strata
movement in regions mined using this approach are significantly different
from those in traditionally mined regions. The room and pillar mining method
is normally used for shallow mines in western China to prevent the overlying
strata from caving in and collapsing. A great quantity of coal resources is
abandoned underground due to the use of the room and pillar method, which has
caused a huge waste of coal resources. Additionally, the coal pillars creep
over time and gradually fail (Bell and Bruyn, 1999; Castellanza et al.,
2008). During pillar failure, the roof strata become fractured and the
collapse progresses upwards (Ghasemi et al., 2012; Cui et al., 2014; Parise
et al., 2015). Finally, surface subsidence results in building damage,
environmental destruction, etc. The Changxing coal mine is used as a case
study in this paper, and the roadway backfill method is proposed to solve the
problems discussed above. We tested the mechanical properties of a backfill
material composed of common aeolian sand, loess, and a cementing material. We
also simulated strata movement characteristics with different roadway driving
and filling sequences by FLAC
The Changxing coal mine, located about 15 km north of Yulin, Shaanxi
Province, covers a field area of 4.82 km
The room and pillar mining method was originally adopted in the Changxing coal mine, resulting in a mining rate of only 30 %. In addition, problems such as severe surface subsidence and significant coal loss caused by the instability of the mined rooms and coal pillars have threatened the mining field. Meanwhile, due to coal pillar failure, magnitude 2.5 and 2.8 earthquakes have occurred in the Changxing coal mine, leading to the deterioration of vegetation, water loss, and ecological damage.
At present, the Changxing coal mine only has 3101, 3103, 3015, and 3107 working faces that remain minable, as shown in Fig. 1. The mining area is located at the northeastern part of the mine field. Under these circumstances, the roadway backfill method in longwall mining was used to solve these problems. In this method, backfill materials are used to fill the mined-out area, serving as a permanent stress-bearing body that supports the overburden. Overlying strata may slowly sink as a consequence; therefore it is of critical importance that the surface subsidence is effectively controlled. Additionally, by adopting this method the coal recovery can be increased from 30 to 70 %; therefore, the recoverable coal resource can reach 3 427 000 t.
Since the Changxing coal mine is located in western China, an area where the aeolian sand and loess are widespread, we used these deposits as the principal backfill materials for maximum cost reduction. To improve its resistance to deformation, the backfill material consisted of three kinds of material: aeolian sand, loess, and a cementing material. Specifically, aeolian sand and loess were used as a coarse aggregate and a fine aggregate, respectively. After mixing with the cementing material, test samples of the backfill material were prepared. The testing scheme is shown in Table 1. The mechanical properties of each backfill material were a result of triplicate tests on each sample.
Testing scheme.
Schematic diagram of test system.
Plan and elevation views of the steel chamber.
As shown in Fig. 2, the test system mainly consists of a SANS material testing machine and a self-made compaction device. Equipped with data acquisition software, the system can obtain the mechanical parameters such as load and displacement. The compaction device comprised a steel chamber, a base, a dowel bar, and a loading plate. The internal radius, external radius, height, and wall thickness of the steel chamber were 125, 137, 305, and 12 mm, respectively. The steel chamber and base were connected by flanges. In addition, with a radius and a height of 124 and 40 mm, respectively, the loading plate is able to apply a uniform force to the samples (see Fig. 3).
The test procedure was as follows:
Putting the samples into the compaction device: some samples were weighed and then put into the compaction device in layers,
followed by the smoothing of the sample surface. Calculating the original filling height of the samples before loading: when the samples were put into the compaction device, the loading plate was
put on the upper surface of the samples. The heights of the steel chamber and
the dowel steel and the thickness of the loading plate were Applying axial load to the samples: before loading, the positions of the upper plate and the dowel bar of the
test machine were adjusted to align the dowel bar with the center of the
upper plate and let it make contact with the upper plate. Meanwhile, the
displacement and load were recorded during the compaction of the samples.
Original filling height of the samples before loading.
The stress–strain curves of the backfill materials are shown in Fig. 5, where
the following features can be appreciated.
All the stress–strain curves had two phases:
a rapid deformation phase up to 1 MPa slower deformation thereafter, up to 6 MPa. During
the slow deformation phase, the stress–strain relationship was quasi-linear.
Obviously, Scheme 2 was the stiffest backfill material. If the
compaction pressure applied to the backfill material was 1 MPa or greater in
the preliminary stage, the deformation of the backfill material during the
later period was lower.
The principle of roadway backfill technique is shown in Fig. 6. The length of a backfilled mining roadway is usually 150 to 300 m, with a width of 5 to 10 m. The width of the unexploited coal pillars is usually 2 to 5 m. The equipment for backfilling the driving roadway includes the mining equipment and the backfill equipment. The mining equipment consists primarily of a continuous shearer, a loader, a trackless tired vehicle, etc. while the backfill equipment is primarily a material thrower, a belt conveyor, etc.
Stress–strain curves of backfill materials.
Principle of roadway driving with backfill technique.
Based on the roadway layout, a numerical calculation model was established
using FLAC
Numerical calculation model.
The numerical simulation consisted of two schemes: single roadway driving
and driving on multiple roadways. Specific details are as follows:
Scheme 1: single roadway driving was performed on the coal seam, and the
strata movement characteristics were simulated for roadway driving with
excavation widths of 3, 5, 7, and 9 m, respectively. Meanwhile, the zone
affected by the abutment stresses by the roadway for different widths of the
driving roadway was determined. Scheme 2: the driving and filling sequence for multiple roadways was
determined according to the zone affected by the abutment stress around the
roadway. Meanwhile, the strata movement trends during driving on multiple
roadways and filling were simulated for roadway widths of 3, 5, 7, and 9 m.
According to the simulation scheme for driving on a single roadway, the distribution of the abutment stresses by the roadway (at different widths) was obtained and is shown in Fig. 8.
Distribution of abutment stresses on the roadway.
As the roadway width increased, the maximum stress and the zone affected by the abutment stress on both sides of the roadway would be larger gradually. When the width of the excavation roadway ranged from 3 to 9 m, the zone affected by abutment stress was as high as 2.5 to 3.0 times the width of the excavation roadway. Therefore, the stress concentration factor is the ratio of the peak value of the abutment stress to the initial rock stress, which can be used to show the effects of the abutment stress variation with the different roadway width. If the peak value of the abutment stress changed from 2.8 to 4.3 MPa, the stress concentration factor changed from 1.1 to 1.7.
Physico-mechanical parameters of the coal and rock mass.
Therefore, in order to ensure the stability of the surrounding rock while driving on the roadway, the distance between two adjacent excavation roadways must be at least 3 times longer than the width of the excavated roadway.
Based on the simulation of driving on a single roadway, the distance between two adjacent roadways was designed to be 3 times longer than the width of the excavation roadway during driving. Meanwhile, coal pillars with a width of 3 m were established between every second roadway. Roadway driving and filling was divided into four stages in total (see Fig. 9).
Simulation of driving on multiple roadways.
Roof subsidence with different width of roadway.
Stress distributions with different width of roadway.
Based on the simulation scheme for driving on multiple roadways, the roof subsidence for different widths of roadway driving can be obtained (see Fig. 10).
Figure 10 shows the following:
Roof subsidence gradually increased with the width of the roadway. When the width of excavation roadway ranged from 3 to 9 m, the maximum roof
subsidence in the first, second, third, and fourth stages varied between 3 to
14, 6 to 53, 9 to 147, and 12 to 238 mm, respectively.
Additionally, roof subsidence above the coal pillar was less than that in the
backfill body. In the first, second, and third stages, different stresses on
the coal pillar and backfill body led to a wave-shaped roof subsidence curve.
In the fourth stage, the subsidence curve tended to be smooth after the
roof was stabilized.
In summary, the design of a roadway driving sequence and the roadway length can reduce the effects of mining between two roadways. Meanwhile, the joint support of coal pillars and the backfill body can effectively control strata movement.
Figure 11 shows the stress distribution in the mining field for different roadway widths during driving on multiple roadways. At the same stage, the stress in the mining field gradually increased with the increasing of roadway width. Meanwhile, for roadways of the same width, driving and filling at each stage gradually increased the stress in the mining field. When the width of the roadway changed from 3 to 9 m, the maximum stresses in the first, second, third, and fourth stages in the mining field were between 2.8 to 3.9, 3.1 to 4.9, 3.6 to 6.6, and 4.3 to 7.4 MPa, respectively. As the overlying strata subsided with driving and filling at each stage, the coal pillars were gradually compressed, and the stress by the coal pillars increased, reaching a maximum in the middle of the mining field. The backfill materials as the main supporting body effectively changed the stress state in the surrounding rock during the backfill process of the driving roadway. Meanwhile, the design of the roadway driving sequence and the roadway length avoided the superposition of mine-induced stress and dissipated the effects of the mining.
The stability of the established coal pillars must be evaluated for roadway
backfill method during mining. The safety coefficient is most appropriate
for consideration when designing underground coal pillars – the larger the
safety coefficient is, the lower the failure probability of the coal pillars.
The safety coefficient of a coal pillar (
Plastic zone distribution for a 9 m wide excavation roadway.
The development of the plastic zone in the mining field when the width of excavation roadway was 9 m is shown in Fig. 12.
As shown in Fig. 12, plastic zones with thicknesses of about 0.5 m were generated on both sides of the coal pillars, which were supported by the backfill materials. The width of the elastic zones that developed was 2 m, accounting for 67 % of the coal pillar cross section, which demonstrated that no instability was generated therein.
When backfilling mining is used to mine a coal seam, the overlying strata are disturbed, inducing secondary settlement. The first strata movement occurred when the original reservoir was formed, which is called “first stabilization”. The second was primarily caused by the gradual compression of the overlying strata on the backfill bodies and the coal pillars. During this process, the compressed backfill bodies and coal pillars give rise to the movement of the overlying strata, finally stabilizing it, which is called “quadratic stabilization”. The movement of the strata after quadratic stabilization during backfill mining of a driving roadway is a dynamic process. It includes the mining of coal seams and backfilling the driving roadway during the mining, the compression of the backfill bodies and coal pillars, the gradual subsidence of the overlying strata, and the final stabilization of the overlying strata. The roof subsidence profiles after quadratic stabilization of the backfilled driving roadway during mining are shown for different excavation roadway widths in Fig. 13.
The results in Fig. 13 allow the following conclusions to be drawn:
The
roof subsidence increased with an increase in the excavation roadway width
after quadratic stabilization. When the width of the excavation roadway
changed from 3 to 9 m, the maximum roof subsidence changed from 12 to
238 mm. The backfill body as the supporting body absorbed and
transferred the mining-induced stress. With a continuous compressing force
from the overlying strata, the porosity of backfill body gradually decreased,
leading to better control of the stability of the overlying strata.
Considering the width of the road header and the control of the rock surrounding the roadway, the excavation roadway at the working face was designed to be 7 m wide, with 3 m wide coal pillars. Figure 14 shows the working face layout used in the Changxing coal mine.
According to the distribution of the abutment stress by the roadway in single roadway driving, when the width of the excavation roadway was 7 m, the zone affected by the abutment stress imposed on the roadway was 18 m wide (see Fig. 8). After accounting for the safety factors, we determined that the distance between two adjacent driving roadways should be 21 m, which was beyond the scope of stress influence. In the simulation scheme for driving on multiple roadways, when the width of excavation roadway was 7 m, the maximum roof subsidence in the first, second, third, and fourth stages were 9, 12, 26, and 238 mm, respectively. Moreover, the maximum stresses in the first, second, third, and fourth stages in the mining field were 3.6, 3.9, 4.9, and 6.6 MPa, respectively. According to the safety coefficient calculation of a coal pillar (Eq. 1), the safety coefficient of the coal pillar was 3.5 at a roadway width of 7 m, which meant the failure probability of the coal pillars was approximately equal to 0 %.
Roof subsidence after quadratic stabilization.
Thus, in order to avoid the stress influence and improve the coal recovery ratio, the coal seam must be excavated and filled in stages which could make two adjacent driving roadways beyond the scope of stress influence. The design of the roadway driving and filling sequence in the 3101 working face of the Changxing coal mine can be divided into four stages.
In stage 1, by driving from the headgate to the tailgate
at the working face, roadway (2) was produced. Afterwards, roadway (1) was
excavated while roadway (1) was filled. As shown in Fig. 15a, this process
continued until the all of the driving and filling in stage 1 were finished. In stage 2, the coal pillar was mined, resulting in the
formation of roadway I. Meanwhile, coal pillars with a width of 3 m were
established on both sides of roadway I. Afterwards, as shown in Fig. 15b,
driving on roadway I proceeded while roadway I was filled. This process
continued until the driving and filling in stage 2 was finished. In stage 3, the coal pillar on the right-hand side of
roadway I was mined, resulting in the formation of roadway a. Meanwhile, 3 m
wide coal pillars were established on both sides of roadway a. Afterwards, as
shown in Fig. 15c, driving on roadway b proceeded while roadway a was
filled. This process continued until the driving and filling in stage 3 was
finished. In stage 4, the coal pillar on the left-hand side of
roadway I was mined, resulting in the formation of roadway A. Meanwhile, 3 m
wide coal pillars were established on both sides of roadway A. Afterwards, as
shown in Fig. 15d, driving on roadway B proceeded while roadway A was
filled. This process continued until the driving and filling in stage 4 was
finished, as shown in Fig. 15e.
Layout of the mining system used in the Changxing coal mine.
Design of the driving and filling sequence in the backfill of the driving roadway during mining.
At present, the 3101 working face has been advanced for 170 m. To thoroughly study rock strata movement and deformation rules in the Changxing coal mine, ground movement observation stations were built in corresponding position above the working face. Surface movement observing proceeded from 5 January to 5 December 2014, and the curve of surface subsidence measurements against time can be seen in Fig. 16.
According to the analysis of the surface monitoring data, the accumulated
maximum subsidence at the surface monitoring point is 15 mm and the maximum
horizontal deformation is 0.8 mm m
Curves of surface subsidence measurements against time.
When the roadway backfill method was adopted in the Changxing coal mine, the
coal recovery ratio approached 70 % and the compression ratio was more
than 90 %. Also, the loess and aeolian sand treatment capacity reached
718 000 t a
By studying the strata movement characteristics when backfilling the driving
roadway during mining, we arrived at the following conclusions:
The mining rate of the room and pillar method is less than 40 %.
Meanwhile, the coal pillars creep over time and gradually fail. During pillar
failure, the roof strata become fractured and their collapse progresses
upwards. The method of backfilling the driving roadway during longwall mining
was proposed to solve these problems. In this method, the mined out area is
backfilled to serve as a permanent stress-bearing body that supports the
overburden. The overlying strata may slowly sink as a consequence, and therefore
it is of critical importance that strata movement and surface subsidence are
effectively controlled. Aeolian sand, loess, and cementing materials were used to prepare the
backfill materials in this study. Testing determined the mechanical
properties and compositions of the backfill materials. The strata movement characteristics when driving on a single roadway
were obtained, and the zone affected by abutment stress in the mining field
was determined by simulation and used to optimize the sequence of driving on
multiple roadways accompanied by the acquisition of the strata movement
characteristics when driving on multiple roadways. Roadway backfill technique during longwall mining of the 3101 working
face of the Changxing coal mine was used as an engineering case study in
this work. Based on the strata movement characteristics of driving on single
and multiple roadways, the driving and filling sequence of the 3101 working
face was optimized to avoid the added effects of mining-induced stresses. According to the analysis of the surface monitoring data, the accumulated
maximum subsidence is 15 mm and the maximum horizontal deformation is
0.8 mm m
The underlying research data are available upon request from the corresponding author.
The authors declare that they have no conflict of interest.
This research was supported by the Qing Lan project (Education Department of Jiangsu, 2014(23)), Foundation for Distinguished professor of Jiangsu Province (Education Department of Jiangsu, 2015(29)), and National Natural Science Foundation of China (51504238). Edited by: M. Parise Reviewed by: two anonymous referees