Study on the Dynamic Development of Ground Fissures in Shallow Coal Seam Mining Under Ditch


 Abstract
In order to study the dynamic evolution law of ground fissures in shallow coal seam mining under ditch，taking the 14210 working face of a mine in northern Shaanxi as an example, this paper analyzes the dynamic development law of ground fissures in the process of mining under ditch through similar material simulation combined with the VIC-3D monitoring system. The results show that there are 20 ground fissures in the 14210 working face. The development width of ground fissures increases first, then decreases rapidly, and finally closes at the bottom of the ditch. The ground uplifts with a height of 1.5 m. The development width of ground fissures increases first, then decreases slightly, and finally tends to stabilize on the left and the right slopes, resulting in staggered steps with a height of 1.6 m and 0.7 m respectively. This change process is consistent with that of the strain of the underlying overburden obtained by VIC-3D strain analysis. According to their development positions and strain changes, the ground fissures are divided into three categories: tensile ground fissures on the left slope T1-T8, squeeze ground fissures at the bottom of the ditch T9-T13, and tensile ground fissures on the right slope T14-T20. Finally the safety analysis of mining under ditch in the 14210 working face is carried out.


Introduction
Ground fissures caused by coal resource mining are the most common geological disasters in mining areas (Howard and Zhou, 2019;Kalogirou et al., 2014), which will cause the lack of water resources and the destruction of surface vegetation and buildings, thus seriously affecting people's safety and living environment (Bi et al., 2019;Can et al., 2012;Deck et al., 2003;Lamich et al., 2015). The Shenfu mining area is one of the typical shallow coal mining areas in China (Fan et al., 2015). With crisscrossing loess gullies, the ground fissures caused by mining have become a prominent problem (Huang et al., 2019a;Xufeng et al., 2011;Yang et al., 2018b).
Many scholars have carried out research on this problem from different angles. In terms of on-site measurement, Yang et al. (2019) took the Burenta Coal Mine as an example and established an observation station to measure and record the periodic changes of related parameters of ground fissures as the working face advanced, and to explore the topographic deformation laws of shallow coal mining.
Through the analysis of the classification of goafs and the distribution characteristics and formation mechanism of ground fissures in Pingyao mining area, Zhou et al. (2011) found that the fissures were non-tectonic, and put forward corresponding prevention and control measures. In terms of physics and computer numerical simulation, through indoor similar simulation experiments, LI et al. (2014) analyzed the spatio-temporal evolution of overburden fissures during coal mining from qualitative and quantitative perspectives. Yang et al. (2018a) presented the mechanical model of "cantilever beam and elastic foundation beam" through two physical simulation experiments, calculated the stress distribution and crack initiation angle of the overlying strata ， and well explained the mechanism of ground fissure generation and expansion. Zhao et al. (2016) used FLAC3D software to simulate coal seam mining, calculated the displacement and tensile strain of the ground and the stratum, and predicted the distribution of ground fissures. In addition, some scholars conducted related theoretical analysis and numerical calculations. Liu et al. (2019), taking the Daliuta coal mine in the Shendong mining area of China as an example, introduced a new comprehensive influencing parameter K of geology and mining on the development of ground fissures to explain the impact of mining speed on the development characteristics of ground fissures. Huang et al. (2019) proposed the relationship between the development speed of ground fissures caused by mining and the scope of the damaged area, and put forward effective measures to control the development of boundary fissures. Li and Liu (2019) explored the dynamic changes of ground fissures in shallow coal seam mining by studying the spatial-temporal distribution of overlying rock fissures and gas conductivity. Álvarez-Fernández et al.

Mining situation of the working face
The working face, with a strike length of 3400 m and a trend of about 300 m, contains three channels： the rubber transport channel, the auxiliary transport channel and the return air channel. Adopting fully-mechanized method and mining 4-2 coal, the working face has a designed production capacity of 5.07 Mt/a. Natural caving method is adopted in roof management. The elevation of the working face floor ranges from 1100 m to 1130 m. To the east is the 14209 working face, to the west the 14214 working face, and to the southeast the coal seam outcrop line and the fire boundary line.

Similar material simulation experiment
In this experiment, the 14210 working face of the mine is taken as the research object, and mining 4-2 coal is simulated. The average thickness of the 4-2 coal under the ditch is 3 m, covered by 16.65-m-thick fresh bedrock and 5.1-m-thick weathered bedrock. On the weathered bedrock is a loose sandy soil layer with a thickness of 1.2-2.5 m. The thickness and physical and mechanical parameters of coal and rock are shown in Table 1. In order to meet the similarity requirements of similar materials and natural rocks, river sand is selected as the aggregate for the model, gypsum and white powder in different proportions as the cementing material, to simulate soft and hard rock formations. In addition, mica powder is used to simulate the layer and interlayer bedding surface. This experiment uses the ratio of soil: sand: oil=4.5:4.5:1 to better simulate the mechanical properties of the overlying red soil layer (Cheng et al., 2016;Zhang et al., 2020). The geometric size of the experimental model is 4000 mm×1010 mm×200 mm (length×height×width), and the geometric similarity ratio is 1:100, as shown in Fig. 1. With a step of 10 m and a mining height of 3.7 m, the simulated working face is excavated from left to right for 34 times and advances 340 m, in order to observe the dynamic development of ground fissures during ditch mining.
The experiment uses the VIC-3D monitoring system (non-contact full-field strain measurement system) to measure the surface deformation of the model (Strungar et al., 2019). By capturing the movement of speckle features at the pixel level, it provides the morphological appearance, displacement and strain data measurement in two-dimensional and three-dimensional space (Pazmino et al., 2014;Tang et al., 2010). The VIC-3D monitoring system is widely used in the fracture process of   With the continuous advancement of the working face, the ground fissures will produce different dynamic changes. At the bottom of the ditch, the development width of T10 fissures first increases, then rapidly decreases, and finally closes (Fig. 2a). Moreover, the ground is uplifted by the compressive stress with a height of 1.5 m (Fig. 2b). The development width of T4 ground fissures on the left slope increases first, then decreases, and finally stabilizes at 0.8 m. Meanwhile, a step-shaped ground fissure is formed with a height of 1.6 m. (Fig. 2c). The development width of T17 ground fissure first increases and then decreases on the right slope, and stabilizes at 0.2 m after the mining is completed. A staggered ground fissure opposite to the left slope is formed on the ground surface, with a height of 0.7m (Fig.   2d).
Through the longitudinal comparison of the fissures on the left slope, at the bottom, and on the right slope, it is found that mining downhill generates forward step-shaped fissures, and mining downhill generates reverse step-shaped fissures, the latter distinctly outnumbering the former. There are fewer ground fissures at the bottom of the trench than on the slope, which have smaller spacing.  (Fig. 3b). When the coal seam is excavated to 220 m, the coal seam mining position is on the right side of the bottom of the ditch. After the working face passes the ditch, the tensile stress disappears and the compressive stress increases, and the strain value of the underlying overburden decreases to 0.056 (Fig. 3c). Then as the working face continues to advance, the coal seam excavation position is further away from T10, the compressive stress further increases, and the strain value of the underlying overburden decreases.
It can be obtained that before the working face passes the ditch, the ground fissure is subjected to tensile stress. As the strain of the underlying overburden increases, the width of the ground fissure also increases, and reaches the maximum when the working face passes through the bottom of the ditch.
After the working face passes the ditch, the fissure is under compressive stress, the strain of the overburden decreases, and the ground fissure first closes and then uplifts (Fig. 3 d). overburden under the T4 ground fissure begins to deform due to tensile stress, with a strain value of 0.014 (Fig. 4a), forming positive staggered steps. When the working face advances to 180 m, the reduction of coal wall support stress leads to a continuous increase in tensile stress, and the strain value increases to 0.064 (Fig. 4b). When the working face advances to 200 m, the goaf is compacted and the strain stabilizes at 0.012 (Fig. 4c).
It can be concluded that the strain of the overburden under the ground fissure on the left slope first increases and then decreases. The corresponding development process of ground fissures is that the width first increases and then decreases, forming larger scattered steps, and finally stabilizes (Fig. 4d).  (Fig. 5a). When the working face advances to 290 m, the coal seam is mined through the T17 ground fissure, the tensile stress of the overburden under the T17 ground fissure decreases, and the strain begins to decrease to 0.042 (Fig. 5b). When the working face advances to 310 m, the coal seam has been fully mined at this point, and the strain value of the overburden under the fissure stabilizes at 0.040 (Fig. 5c).
It can be concluded that the strain of the overburden under the ground fissure on the right slope first increases, then decreases, and finally reaches a stable state. The corresponding development process is that the width of ground fissures first increases, then decreases and finally stabilizes, forming scattered steps (Fig. 5d). However, the ground fissures on the slope are under tensile stress, which widens the ground fissures, and water is more easily conducted on the slope. The water level in the ditch will also directly affect the safety of the working face. In the dry season, the water level does not rise to the slope surface and will not have a major impact on the mining of the working face. However, in the wet season, the water level will rise, and water will enter the working face along the tensile ground fissures on the slope, affecting the safety of the mine. In addition, dynamic ground fissures always develop before the working face is mined, so there is a certain linear relationship between the advancing speed of the working face and the development of ground fissures.
In summary, during the ditch mining period, there are four factors for the influx of water in Ulanbula Ditch into the working face, which are the width of fissures, the distribution density of fissures, the water level in the ditch, and the advancing speed of the working face.
In view of the above factors, with the precondition of guaranteed quality of the project, detraction mining can be used in the advancement of the working face. By setting up coal pillars, the high stress of the overlying rock under mining can be relieved, and the coupled control of uniform surface settlement and the development of ground fissures can be realized (Huang et al., 2019b). At the same time, backfill materials such as coal stone, fly ash, ordinary Portland cement and ultra-high water materials can be used for backfilling, which effectively controls the formation and development of ground fissures (LEI et al., 2014;Zhu et al., 2018b). At the opening cut and the stop mining line, the permanent ground fissures can be repaired to prevent surface water from entering (Zhu et al., 2018a).
The advancing speed of the working face should be accelerated in the dry season and reduced in the wet season. Since there is water all year round at the bottom of the ditch，the advancement should slow down (Zhang et al., 2011). Meanwhile, grouting can be used to consolidate the overlying loose sand, increase the thickness of the bedrock, and weaken the development of the water channel.

Conclusion
This paper takes the 14210 working face of a mine in northern Shaanxi as the research object and conducts a similar material simulation by using VIC-3D detection method. With the advancement of the working face, the dynamic evolution law of ground fissures is obtained, and the safety analysis of ground fissures is carried out. The main conclusions are as follows: 1. The dynamic development process of ground fissures in shallow coal seam mining under ditch is obtained by means of similar material simulation. At the bottom of the ditch, the development width of the fissures first increases and then decreases rapidly, and finally closes and produces a ground uplift.
On the left and the right slopes, the development width of ground fissures increases first and then stabilizes, resulting in staggered steps.
2. From the analysis of the overlying rock strain of the model through the VIC-3D system, it can be seen that at the bottom of the ditch, the strain first increases and then decreases, the stress changes from tension to compression, and the ground fissures uplift. On both the left and the right slopes, the strain first increases and then decreases slightly, which is caused by tension. The fissures display a form of steps. All this is consistent with the phenomenon observed by simulation of similar materials.
3. 4-2 coal mining results in a total of 20 ground fissures. According to their locations and strain changes, they can be roughly divided into three categories: tensile ground fissures on the left slope, squeeze ground fissures at the bottom of the ditch, and tensile ground fissures on right slope.