One of the traditional difficulties that many underground mines of the Vietnam National Coal and Mineral Industries Group (TKV) are having to solve is organizing safe production under open-pit mining areas. Specifically, many mines are currently having to maintain technical measures to control and handle the situation of water seeping from the surface of old open-pit mines into the underground mines, ensuring safety, especially during the rainy and stormy season.
Seam 11 of the open-pit mine of Nui Beo Coal Joint Stock Company is an area that has ended exploitation and is being restored according to the coal mine closure project at seams 11, 13 and 14 - Nui Beo mine in Ha Tu, Ha Trung and Ha Phong wards, Ha Long city, Quang Ninh province. Previously, the deepest water level measured at this seam was -135m above sea level and at present, the water level is recorded at about -94m. The amount of water in the seam is determined to be at risk of affecting the underground production area being organized below this location.
According to the approved mine closure plan, Ha Tu Coal Joint Stock Company and Nui Beo Coal Joint Stock Company will jointly implement a plan to fill up seam 11 from 2023. It is expected that by 2028, seam 11 will be filled and trees will be planted to restore the environment. While waiting for the restoration of this area, Nui Beo Coal has proactively taken measures to monitor and control the seepage of seam water into the area where underground coal mining is being organized.
Mr. Nguyen Danh Hai, Deputy Head of Geodesy - Geology Department, Nui Beo Coal Joint Stock Company - Vinacomin said: We regularly maintain 4 hydrological monitoring boreholes and periodic drillings every week. On stormy days, we increase the number of monitoring drills to once every 2 days to assess the relationship between surface water and groundwater; at the same time, we have a plan to strictly control and minimize the amount of water flowing into the underground mining area.
With the amount of water remaining in the old open pit, Nui Beo Coal used a pumping system with a capacity of 630m3 /h to pump out and treat the surface water, in order to limit water seepage into the tunnels below. In addition, Nui Beo Coal monitored the entire surface of the terrain, identified and assessed the risk of cracking and subsidence to propose measures to level and compact the terrain to avoid the risk of water seepage. For the terrain above the natural drainage level, the Company built drainage belts to divide water and prevent water from flowing into the open pit.
According to Mr. Pham Quoc Toan, Deputy Head of Technical - Mining Technology Department - TKV: Waste rock with loose structure has large pores, so the bottom of open-pit mines, open-pit mines that have been filled with waste rock are places with the ability to store water and can be considered as water storage objects when exploiting underground mines.
On the other hand, when exploiting the open pit using total fire technology, the equilibrium of the original rock mass will change, creating chaotic collapse areas, areas of crack development, sagging areas, etc. The process of exploiting coal with total fire will also cause the permeability of the partition layer between the open pit and the underground mine to increase, causing unsafety to the underground mine production area.
To limit this risk, underground mines that are producing under open pits have implemented advance exploration drilling and water extraction drilling measures, and added measures such as betting the face, installing reinforcement bars or blasting to dig the mine to advance the face in parts...
"With the open pit mine areas, before starting the exploitation, Nui Beo Coal Joint Stock Company conducted exploratory drilling, groundwater monitoring and maintained it continuously during the exploitation process. According to the technical plan, the minimum drilling length must pass through the old pit bottom that has been leveled to ensure safety for underground mining" - Mr. Nguyen Van Dinh, Head of Technical - Mining Technology Department, Nui Beo Coal Joint Stock Company - Vinacomin said.
Basically, these are also the techniques that many underground mines are applying to ensure safety in conditions of coal mining in open pits using the total fire method. In the future, TKV will research and apply new technical methods and technologies as well as use advanced materials and equipment to limit the impact on the water barrier layer between open pits and underground mines.
In addition, the Group has also studied appropriate disposal options for open-pit mines to minimize the possibility of water accumulation in the dump and prevent water seepage at the bottom of the dump. Appropriate disposal options also help minimize the amount of water flowing directly into the dump and drain the dump; prevent water from seeping into the mine, thereby ensuring safety in production.
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