Research on stability of rock mass and +30 level surface construction works when re-exploiting the seam H10 at Mong Duong Coal Mine, Vietnam

The re-exploitation of coal seams located near the ground is one of the solutions to increase output, reduce mining investment costs, and avoid wasting coal resources. The re-exploitation of coal seams will also cause instability of the surrounding rock mass and may affect surface construction works. Through the process of re-exploiting the longwall in seam H10 at Mong Duong Coal Mine, the authors have studied and evaluated the stability of the rock mass and +30 level surface works (including fan station and gateroad). To achieve the results in this study, the numerical simulation model method and the analytical method were applied. The model analysis results have determined that the displacement and deformation areas of the rock mass around the mining area correspond to the length of the cut in the strike direction of the longwall H10. The analysis and calculation results from the model show that the longwall in seam H10 can be re-exploited when leaving a protective coal pillar about 50 m from the center of the +30 fan station at the east side; this distance ensures the stability of the rock mass that located near the ground and the surface works at +30.


Introduction
C urrently, underground mines in Vietnam are more and more deeply exploited, so the mining conditions are more difficult and complicated. Coal mining is still predominant in Vietnam. The country will have to rely on coal as one of the key energy sources for a long time in order to meet its energy needs. Since coal has been mined in Vietnam for a long time, the country is now facing such challenges as the depletion of readily available reserves and an increase in the share of coal mined underground in difficult geological conditions [1]. Therefore, the re-exploiting of coal seams located near the ground has been interested in recent times by some underground mines in Quang Ninh coalfield. However, the re-exploiting seams are directly related to the stability of the topographic surface works, directly related to surface subsidence, displacement, and deformation affecting the surface works [2e6]. To be able to re-exploit these shallow seams, it is necessary to base on specific conditions at the actual site and at the same time, use a calculation method that evaluates many factors to forecast the level of impact on the stability of surface works and ground subsidence when re-exploiting shallow seams [7e10].
In the underground coal mines in Quang Ninh coalfield, although now they have been exploited to a deep level (À300 or À450 m), some mines are still interested in re-exploiting the shallow coal seams (near the surface) to recover minerals, reuse existing mine works to reduce investment costs and increase output. If exploiting seams located near the ground, it may affect the stability of surface works (Residential areas, national works, roads, works for mining of mines, lakes, streams, etc.). The longwall in seam H10 at Mong Duong Coal Mine is located below the fan station and gateroad at þ30 level. So, when re-exploiting the longwall, it is necessary to have a reasonable solution to ensure the stability of the topographic surface works; at the same time, the size of the protective coal pillar needs to be kept as small as possible. It is necessary to study the stability of rock mass around the longwall area and near the topographic surface for the condition of the longwall in seam H10 at Mong Duong Coal Mine and some mines with similar conditions in the Quang Ninh coalfield.
The size of the protective coal pillar has also been calculated by underground mines based on regulations, experiences, and theories. However, high safety coefficients are still used in the calculation, or some regulations may not be suitable for current mining technology and pressure control method. Therefore, it is necessary to re-evaluate and recalculate the protection limits when exploiting the shallow coal seams to recover useful minerals. Up to now, there have been lots of research and evaluations of the effects of exploiting these coal seams on topographic surface works. Case studies are used, such as: similar material modeling methods [11e14]; numerical simulation methods [15e21]; field monitoring methods [22e25]. These studies were conducted under specific field conditions. A similar material modeling method requires high cost, long research time, a limited number of research options, not to mention many influencing factors. The numerical simulation method can study many options, including many influencing factors, low cost, and fast research time. The article is based on the field conditions of Mong Duong Coal Mine, using numerical simulation methods to simulate the process of re-exploiting the seam H10, leaving protective coal pillars to ensure safety and not cause surface subsidence and damaged fan station, gateroad at þ30 level at Mong Duong Coal Mine.

Current status of þ30 level surface works in Mong Duong Coal Mine
On the topographic surface of the level þ30 east side in Mong Duong Coal Mine, there is a fan station and a gateroad to serve the work of ventilation, travel, and transportation for the exploitation longwall in seams H10 and G9. Coal seams H10 and G9 are located at a depth of 100e130 m above the ground where the þ30 level fan station and gateroad are built.
The þ30 level gateway is supported by reinforced concrete, while the fan station is built with brick walls. At this fan station, two fans of type 2K56eN02.4 are arranged, location diagram of the fan station and gateroad at þ30 level is shown in Fig. 1 [26].
The longwall from À100 to À170 level at west side in coal seam H10 has been exploited by drilling and blasting technology; the exploitation direction of the longwall is from the border to the center, combining the ZRY soft support, leaving the coal pillar far from the center of the fan station was about 130 m. During the mining process of this longwall, the fan stations and gateroad have not been affected; the topographic surface has not appeared crack.

Geological cross-section
Two geological cross-sections of the þ30 fan station and gateroad are shown in Fig. 2 [27]. Two geological cross-sections show the thickness of the seam, the position of the seam H10 and rock layers, other coal seams in the area, as well as the fan station and the gateroad at þ30 level. These crosssections are used to build a simulated numerical model.

Model input parameter
Based on the geological characteristics of the study side, the parameters of the coal mass and the

Model size
In this study, RS 2 software was used to simulate the mining process of the longwall in coal seam H10 at Mong Duong Coal Mine. This is a software commonly used in the field of mining. Its characteristics are easy to use, and it simulates the thickness and the actual position of the rock layers, rock layer characteristics such as faults and groundwater levels. The numerical simulation model is built on the basis of geological conditions and geological cross-section in the strike direction passing through the area of the þ30 level fan station and the gateroad of with the dimensions of 326.8 m height, 450 m width. The analysis results from the model will evaluate and forecast the effect of re-exploiting the coal seam H10 on the stability of the rock mass near the ground above the fan station and the þ30-furnace door. The simulation model is shown in Fig. 3.

Simulation of the mining process of the seam H10
The total length in the strike direction of the mining area of the longwall in seam H10 is 220 m. The numerical simulation model will simulate the moving process of the longwall in seam H10 with the corresponding length in the strike direction. Thus, the sequence of exploitation in the model from the border to the center follows the following steps: step 1, the length of exploitation is 10 m; step 2, the next exploitation length is 10 m (corresponding to 20 m in the strike direction); step 3, the next exploitation length is 30 m (corresponding to 50 m in the strike direction); step 4, the next exploitation length is 50 m (corresponding to 100 m in the strike direction); step 5, the next exploitation length is 70 m (corresponding to 170 m in the strike direction). The length of the protective coal pillar left is about 50 m. The simulation steps are shown in Fig. 4.

Results and discussion
To evaluate the influence of the rock mass near the ground and the þ30 level fan station, gateroad, the results of the calculation of stratigraphic stress sigma 1, sigma 3, plastic deformation area, and subsidence area in the rock mass will be presented below:

Calculation results of stratigraphic stress
Calculation results of sigma 1 and sigma 3 stratigraphic stresses in the rock mass of the geologic section in the strike direction at the location of the þ30 level fan station and gateroad are shown in Figs. 5 and 6.
From the results of the model analysis, it is shown that the sigma 1 and sigma 3 stresses distributed in the model have results corresponding to the hypothesis that the stressincreases with depth (stratigraphic stress gH, where g is the specific weight of rock mass, H is the depth of stress calculation point). With the above stress simulation results, we see that the model built to simulate shows the stress value increasing with depth, so it is consistent with the theory of vertical stress in the rock mass. Also, from the results of stratigraphic stress distribution in Figs. 5 and 6, we can see that the stresses of sigma 1 and sigma 3 increase gradually with the depth of the rock mass. At the surface of the model, the stress is equal to zero and gradually increases to the bottom of the model to a depth of À350 m; the stress sigma 1 is about 8.1 MPa, and the stress sigma 3 is about 4 MPa. Thus, the sigma 1 stress in the rock mass is about 2 times the sigma 3 stress.  Figure 7 shows that when the longwall cut 10 m in the strike direction, the plastic deformation process appeared in the rock mass around the longwall. But due to the small cutting length, the affected area is limited to the claystone layer, where adjacent to the The simulation results in Fig. 9 show that when continuing to exploit 30 m of the seam H10 (i.e., the longwall cut 50 m in the strike direction), the plastic deformation area keeps developing and expanding vertically and horizontally in the rock mass, but the plastic deformation zone in the vertical rock mass develops more strongly than in the horizontal direction. This problem is explained by the fact that at this time, the mining distance is large enough that the rock layers on the roof of the longwall have collapsed, so the plastic deformation zone develops deeper into the rock mass above the roof. In this case, the extent of the plastic deformation zone is about 15 m. The simulation results in Fig. 10 show   that when continuing to exploit 50 m of the seam H10 (i.e., the longwall cut 100 m in the strike direction), the plastic deformation area continues to develop and expand. At this time, the plastic deformation area continued to expand into the siltstone layer on the roof of the longwall and began to develop into the claystone, coal layer of seam II.11 (seam II.11 is located above seam H10). In this case, the extent of the plastic deformation zone is about 50 m.

The longwall cut 170 m in the strike direction
Simulation results of the plastic deformation area in the rock mass when exploiting the longwall in seam H10 cut 170 m in the strike direction are shown in Fig. 11.
The simulation results in Fig. 11 show that when continuing to exploit 70 m of the seam H10 (i.e., the longwall cut 170 m in the strike direction), the plastic deformation area continues to develop and expand. At this time, the longwall is still about 50 m from the center of the fan station (in the vertical direction). This distance is the length of the protective coal pillar to ensure the safety of the fan station and the gateroad on the þ30 level ground surface.     The analysis results from Figs. 7e11 are summarized in Table 2. Simulation results of the subsidence area in the rock mass when exploiting the longwall in seam H10 cut 10 m in the strike direction are shown in Fig. 12.
The simulation results show that after exploiting the longwall in seam H10 cut 10 m in the strike direction, there is small subsidence around the longwall area. At this time, the subsidence area is concentrated only on the roof of the longwall in the claystone layer, while the layers deep inside the rock mass have not been affected. The simulation results also show that the total value of subsidence in the rock mass at all locations near the ground and the fan station and gateroad at þ30 level is zero.

The longwall cut 20 m in the strike direction
Simulation results of the subsidence area in the rock mass when exploiting the longwall in seam H10 cut 20 m in the strike direction are shown in Fig. 13.
The simulation results show that after exploiting the longwall in seam H10 cut 20 m in the strike direction, the subsidence area of the longwall developed and expanded into the rock mass on the roof, the subsidence value on the roof of the longwall has increased, but also only appear mainly in the rock layers near the longwall, and there has been no subsidence at the location near the ground. The simulation results also show that the total value of subsidence in the rock mass at all locations near the ground and the fan station and gateroad at þ30 level is zero.

The longwall cut 50 m in the strike direction
Simulation results of the subsidence area in the rock mass when exploiting the longwall in seam H10 cut 50 m in the strike direction are shown in Fig. 14.
The simulation results show that after exploiting the longwall in seam H10 cut 50 m in the strike direction, the subsidence area continues to expand deep into the rock mass and spread out over the rock mass near the ground. The rock mass near the ground has small subsidence, and its value at different locations is different. At the east side location near the mining area, the largest subsidence value is 1 mm, while at other locations, it is about 0.075 mm. Thus, when the longwall cut 50 m in the strike direction, the rock mass near the ground begins to be affected but not significantly, so it does not affect the fan station and the gateroad at the þ30 level.

The longwall cut 100 m in the strike direction
Simulation results of the subsidence area in the rock mass when exploiting the longwall in seam H10 cut 100 m in the strike direction are shown in Fig. 15.
The simulation results show that after exploiting the longwall in seam H10 cut 100 m in the strike direction, the subsidence area continues to expand above the rock mass near the ground. The rock mass near the ground continues to subside; the subsidence value is also different at different locations. At the location on the east side near the mining area, the largest subsidence value is 3.85 mm. At the fan station and the gateroad, the subsidence value is 1.4 mm, and at other locations, the subsidence value is about 0.035e0.07 mm. Thus, when the longwall is cut 100 m in the strike direction, the rock mass near the ground will continue to be affected but not significantly, so it does not have an impact on the fan station and the gateroad at the þ30 level.

The longwall cut 170 m in the strike direction
Simulation results of the subsidence area in the rock mass when exploiting the longwall in seam H10 cut 170 m in the strike direction are shown in Fig. 16.
The simulation results show that after exploiting the longwall in seam H10 cut 170 m in the strike direction, leaving a protective coal pillar far from the fan station and the gateroad at þ30 level is about 50 m (in the vertical direction), the rock mass near the ground continue to subside. The subsidence value at different locations is also different; at the east side location near the mining area, the largest subsidence value is 1.05 cm, and at the fan station and the gateroad location, the subsidence value is 6 mm. Thus, it can be seen that when the longwall cut 170 m in the   strike direction, the rock mass near the ground continues to be displaced, but the displacement value is also insignificant. The above simulation results indicate that when exploiting the longwall in seam H10 cut 170 m in the strike direction and leaving a protective coal pillar of 50 m, it will not affect the fan station and the gateroad at þ30 level. The analysis results from Figs. 12e16 are summarized in Table 3.

Conclusions
On the basis of RS 2 software, geological conditions, the geological cross-section of the fan station area, and the gateroad at þ30 level, the authors have established a numerical simulation model of the reexploiting process of the longwall in coal seam H10 at Mong Duong Coal Mine. Based on those simulation models, the authors analyzed and evaluated the stability of the rock mass near the ground and the surface construction works at þ30 level (including fan station and gateroad) at Mong Duong coal mine.
The results of numerical simulation model analysis show that when the longwall cut is 50 m in the strike direction, the stability of the rock mass near the ground begins to be affected. When the longwall cut is 170 m in the strike direction, at this time the length of the protective coal pillar left is about 50 m (to the center of the fan station), then the rock mass on the ground where the fan station and the gateroad at þ30 level has a maximum subsidence depth of about 6 mm. However, because the fan station is solidly built with reinforced concrete, the gateroad is also built and supported by concrete, so this subsidence value of the rock mass near the ground will not affect the fan station and gateroad at þ30 level.
The research results of the article are a reliable basis for Mong Duong Coal Mine to consider and apply in actual production. At the same time, it is also used as a basis for Mong Duong Coal Mine to evaluate and adjust the solution to re-exploit the longwall in seam H10 to ensure safety and achieve the best efficiency.

Ethical statement
The authors state that the research was conducted according to ethical standards.

Funding body
None.

Conflicts of interest
None declared.