Determination of the movement and deformation areas of strata when exploiting longwall of Seam 11 under the open-pit mine at Ha Lam Coal Mine, Vietnam

Exploiting the face under the finished open-pit area always has potential risks of unsafety. It can be the risk of water flowing into the face or the occurrence of water cracking and mud, which is a very dangerous phenomenon in mining. The larger the dimension of the influence area of exploiting the face is, the greater the risk of water problems entering the face is. Through the assessment and analysis of the coal exploitation at the synchronized mechanized face of Seam 11 located under the open pits that have finished exploitation at the Ha Lam coal mine, the article has determined the movement and deformation areas of strata in the surrounding area to assess the influence of water in the open pit into the face. In order to avoid the risk of water cracking, the impact of these risks on the workers’ safety and the exploiting process, the determination of the movement and deformation areas of surrounding rocks when exploiting the longwall face of the coal Seam 11 is very necessary for the Ha Lam coal mine. By collecting and analyzing geological conditions of the rocks around the face, combined with numerical modelling methods, the author has determined the dimension of the collapse and cracking areas when exploiting the face. The results of the research from this article have helped the Ha Lam coal mine take the initiative to choose and organize a rational mining solution. On that basis, it is possible to prevent the influence of water in an open-pit mine to ensure safety during exploiting the face of coal Seam 11.


Introduction
Ha Lam coal mine is one of the largest and most modern underground mines in Vietnam. Because the upper part of some coal seams are exploited by the open-pit mining method, when the mining part ends, open-pit pits will be formed [1]. According to calculations and forecasts, although the open pits have been dumped, they still contain a large amount of water, especially in the rainy season [2]. Therefore, when mining work continues by underground mining method in the area under these open pits, there will be many potential risks and insecurity, especially the risk of water cracking and mud [3,4]. Currently, in a number of mines in the Quang Ninh coalfield, including Ha Lam coal mine, which is facing many difficulties in exploiting the seams, the coal reserves located under the open pit have finished exploitation. This is a problem that is not only concerned by the leaders of the mines, but also by the leaders of the Vietnam National Coal -Mineral industry Corporation [5,6]. The longwall of Seam 11 in the Ha Lam coal mine is located under the area of open pits, including pit -54; -80; -60. The stratigraphic distance to the bottom of the pit is about 100 ÷ 120 m, the stratigraphy consists of a layer of siltstone lying on the coal seam with a thickness of about 5 ÷ 6m, followed by a layer of sandstone with a thickness of about 5 ÷ 18 m, above the sandstone layer is The gravel layer has a thickness of about 15 ÷ 35 m, above the gravel layer is a layer of siltstone mixed with sandstone with a thickness of about 12 ÷ 40 m [7]. The exploitation of this seam leads to unsafe risks such as: subsidence; surface deformation; the mine pressure in the mining area affecting the roadways are very complicated, as well as the phenomenon of the influence of the water contained in the open pit on the underground mine is very large, the risk of the water cracking, mud in the mine. In order to avoid potential water risks, it is necessary to observe, forecast and calculate the height of the collapse area and the fractured area when exploiting Seam 11 of the Ha Lam coal mine [8]. On that basis, it is possible to assess the level of influence of water on mining. In this paper, the author uses the numerical modeling method, by using UDEC software to monitor and determine the height of collapse and cracking of the roof when exploiting the longwall of Seam 11. Input parameters of rock and coal conditions are collected from the actual mine, build a model and conduct model analysis on the basis of the mining plan of the respective longwall.
The analysis and observation results from the numerical model have determined the height of the movement and deformation zones of the roof corresponding to the displacement in the strike direction of the longwall. This research result is also the basis for the Ha Lam coal mine to be proactive in deploying and choosing a reasonable mining plan for the conditions of the coal mine.

Current status of exploited open pits
The waste dump in the Seam 14 at West Side is dumped by Ha Lam and Nui Beo mines, which is currently at +160 m in the southwest, while the design level is +70 m. The waste dump area in the Seam 14 at West wing includes 03 open pits that have been finished and 01 open pits that is being exploited, specifically as follows: • Open-pit mining of Seam 14 at West Side at level -80 has finished mining and dumped waste to the level of +30. Open-pit mining is located on the longwall CGH11-1.16, the stratigraphic distance from the longwall to the bottom of the pit is about 85 ÷ 100 m, the stratigraphy consists of a siltstone layer lying on the coal seam with a thickness of about 5 ÷ 12 m, followed by a sandstone layer, the gravel has a thickness of about 15 ÷ 60 m, above the sandstone layer, the grit is a layer of siltstone with a thickness of about 20 ÷ 60 m. The work of dumping waste at this landfill in recent years has been carried out in the form of a circumference, dumping in the order of exploitation (when the mining is finished, dump the waste there), the height of the waste layer is from 30 ÷ 50m, almost no floor has entered the end position [11]. below the upper pits has been mobilized by Ha Lam coal mine to exploit by the underground method according to the planning of the build mining investment project below -50 Ha Lam coal mine. The coal seam in this area belongs to the type of thick and gentle seams, exploited and using the roof control method by full caving, thus creating collapsed and cracked areas capable of developing from the ground to the topographic surface, if the mining depth is not too large [12][13][14].
Synthesized hydrogeological conditions in the area can be commented that the water source affecting underground mining is mainly water from open pits that have been exploited with a flow of about 500 000 m 3 [15]. Therefore, in the process of preparing and exploiting the longwall  options for depreciation and recovery when the longwall moves in the corresponding direction, specifically as follows: Option 1: the exploitation height of the longwall is 2.6m, recovering 90 percent (equivalent to 9.4 m) When the longwall is cut in the strike direction corresponding to specific lengths, the movement of the longwall is monitored, and the model shows the results of the roof collapse and the corresponding crack formation process. In figure 7   The results from the model show that when the longwall is cut in the 95 m strike direction, the height of collapse and cracking of the roof is from 50 ÷ 55 m.  has the monitoring results on the model indicating that it is safer than the two options above. However, this is the option with the largest coal loss among the three options. The research results of the article can serve as a basis for the Ha Lam coal mine to consider forecasting the movement and deformation height of the roof when exploiting the longwall of Seam 11 for each of the above mining options. Based on the assessment and monitoring of hydrogeological conditions, as well as the calculation of the water flow stored in the open pit, Ha Lam coal mine can choose a reasonable exploitation plan for the longwall of Seam 11. In the process of exploiting the longwall, it is necessary to continue to monitor and evaluate the water appearing in the longwall, and at the same time maintain an appropriate drainage solution, if any abnormality is found, it is necessary to have timely solutions.

Results and discussion
The rational exploitation plan applied at the longwall of Seam 11 in the Ha Lam coal mine will bring high efficiency and meet the requirements of actual production. On that basis, improve IOP Publishing doi:10.1088/1755-1315/1049/1/012009 11 the ability to use the equipment effectively, improve the safety in the mining operation of the longwall, and prevent the occurrence of water and mud problems. Therefore, the exploitation of coal seams under the open pit area is an extremely necessary problem. The combined results of the article can also serve as a document for researchers in the field of coal mining technology in the Quang Ninh coalfield.