Impact On The Groundwater Environment And Water Inflow Dynamic Prediction During Underground Mining Of Typical Mines

AbstractAs an important raw material and a main energy source respectively, iron ore and coal play a key supporting role in China's national economic development. Groundwater system is an important part of the ecological environment system, due to characteristics such as extensive distribution, convenient exploitation, good water quality etc., on the one hand, groundwater resources provide the high quality water resource for human being, and on the other hand, plays a vital role in maintaining ecological balances. Groundwater usually associated with mineral resources, thus cause a series of hydrological geological environment problems in the process of mineral resources exploitation. In order to protect groundwater resources in the mining area and ensure mine safety, this dissertation takes Dajiazhuang iron mine and Dahaize coal mine as the research object, according to basic theories such as coal geology, geology of ore deposits, hydrogeology etc., on the basis of summarizing previous work, through research methods like field investigation, data analysis, hydrogeology test and numerical simulation etc., evaluates the influence of underground mining on the groundwater environment, predicts the dynamic water inflow, respectively, and provide a scientific basis for guiding the coordinated development of groundwater resources protection and safety and high effective mining. The main research results are as follows:(1) Considering the physical geography, geological background and hydrogeology conditions respectively, the aquifers of Dajiazhuang iron mine are divided into Quaternary pore water aquifer, weathering bedrock fissure water aquifer and bedrock structure fissure water aquifer based on the groundwater occurrence conditions. According to the lithology, permeability and water yield property, the Quaternary pore water aquifer can be further divided into upper high water-bearing aquifer and lower medium water-bearing aquifer. The main water-resisting layers in the mining area, which block the hydraulic connection between the pore water and fissure water, are ?the main water-resisting layer? in the middle of Quaternary and ?the bottom water-resisting layer? at the bottom of Quaternary. The direction of groundwater flow is mainly from north to south, with a gentle hydraulic gradient in the mining area. Fractures are developed, multiple faults influence and contact with each other, allows the mining area to form a ?flower-shape? structural-fissure water zone that represents wide in the north, narrow in the south, large at the top and small at the bottom, and becomes water-filling factors and water-conductive channel. Dahaize coal mine has a multiple-layered groundwater aquifer system which can be divided into Quaternary loose bed pore water aquifer, Cretaceous Luohe group sandstone fissure water aquifer, Jurassic Anding group sandstone fissure water aquifer, Jurassic Zhiluo group sandstone fissure water aquifer and Yanan group sandstone fissure water aquifer. The direction of groundwater flow is mainly from north east to south west and the geologic structure is simple in the mining area. After determining the Luohe group and Zhiluo group aquifers as the main research target, this dissertation analyzes the water-filling conditions of Dahaize coal mine, and determines the water-filling source, channel and intensity during the mining of 2# coal seam.(2) By analyzing the overburden rock failure mechanism during mining active process, it is concluded that the influence of underground mining on the groundwater environment due to the damage of underground water storage spatial structure and the changes of its physical properties, as well as a large amount of drainage from the mines. The damage of underground water storage spatial structure characterized by the damage of aquifer or aquiclude, further the changes of properties such as permeability, storage capacity, infiltrating ability and the aquifer thickness etc. Analysis to determine the main controlling factors of underground mining affecting the groundwater environment, including the combination mode of aquifer or aquiclude, geological structure, mining method and roof management method. According to the occurrence spatial relations of aquifers, aquiclude and ore bodies, the groundwater system is divided into two types of combination modes of aquifers or aquiclude in typical mining area: double-layered strong leakage pattern and multiple-layered leakage pattern. During mining active process, the damage of aquifers or aquiclude and large amount of drainage from the mines, increases the hydraulic connection of each groundwater subsystem, and gradually change the groundwater environment in the mining area.(3)Based on the hydrological data of Dajiazhuang iron mine, this dissertation analyzes the natural flow characteristics of Quaternary pore water and bedrock fissure water, the analysis results show that the direction of groundwater flow is consistent with formation tilt direction, and the water level of the bedrock fissure water is 0.2 ~ 0.3 m higher than the Quaternary pore water. After that, establish the concept model and mathematical model of groundwater system under the condition of mining, conduct model identification and verification by curve fitting of observation well data, to obtain spatial distribution of corresponding hydrogeological parameters and sizes of source sink term, and carry out the analysis of groundwater balance.According to the mining plan to predict the flow field of each aquifer in the next 30 years, the results show that the flow field of each aquifer has been disturbed on different levels and formed obvious descent funnels around the stope. Groundwater drainage from around to concentrate in the pit, the flow field represents obvious three dimensional flow state, meanwhile multiple classes and chain-type leakage characteristics are performed between aquifers. Mining has the greatest influence on bedrock aquifer, horizontally, the drawdown funnel distribution represents a north-south banded shape along the main fault zone and its influence zone, spreads farther up along the north-south direction and basically remains unchanged in the west-east direction, this is consistent with the development law of bedrock fracture; vertically, the funnel area expands increasingly following the improvement of mining, that is consistent with the vertical development law of bedrock fracture, while the shape changing rule of drawdown funnel shows that the bedrock tectonic fracture zone become mine water-conductive channel. Although there are multiple water-resisting layers, large amount of drainage from the bedrock pit will affect the overlying Quaternary aquifer through the leakage step by step. The medium water-bearing aquifer at the bottom of Quaternary is affected first, forming an obvious drawdown funnel in the upper panel, with a drawdown about 45 m in the center. Water level of the upper Quaternary aquifer, due to the high permeability and fast groundwater flow velocity, represents an overall downward trend and influences the overlying high water-bearing aquifer.(4) Applying FLAC3 D to simulate the roof stability of 20602 working face during mining, calculate the height of crack zone is about 125 m. According to the computation result, then modify the empirical formula in the ?Mining under Three Procedures? to get a new modifier formula for calculating the roof crack zone development height of 2 coal seam, getting the calculation result of 97.15 ~ 153.24 m, 121.05 m on average. The calculation results show that water-conductive fracture zone completely connects the Zhiluo group sandstone aquifer, but does not influences the other overlying aquifers. Consequently, determines the Zhiluo group sandstone aquifer as the direct water filling aquifer and the other overlying aquifers as the indirect water filling aquifer of 2 coal seam during mining.According to the groundwater system characters of Dahaize mining area, establish the hydrogeology conceptual model, complete the groundwater flow numerical simulation by identification and verification, and predict the groundwater flow fields of the Zhiluo group aquifer and Cretaceous aquifer in future 1, 3, 5 and 10 years, respectively. Compared with the initial flow field, the analysis results are as follows: as the direct water filling aquifer of 2 coal seam, the Zhiluo group aquifer has dramatic changes in groundwater flow field, forms a drawdown funnel that always keep changing with the working face as the center. Because tectonic condition is simple in the study area, the drawdown funnel is mainly controlled by the working face, and with the increase in the number of working faces, the area of the drawdown funnel is also increasing, thus groundwater discharges into the goaf from the surrounding aquifers, the supply, runoff and discharge conditions change, and gradually form the local flow field controlled by the working faces. The Cretaceous aquifer, located a longer distance above the coal seam, has also been a certain degree of influenced. The local flow field above the goaf gradually changes and finally forms a groundwater drawdown funnel, at the end of the tenth year, the water level falls by nearly 100 m in the center of the drawdown funnel, while the surrounding groundwater flows into the drawdown funnel area, infiltrating vertically to recharge the beneath Zhiluo group aquifer. The Cretaceous groundwater flow field changes locally, but the overall form does not represent obvious changes. The prediction results show that mining influences the Cretaceous aquifer less than the Zhiluo group aquifer.(5) Mining is a dynamic process, and its disturbance to groundwater system is also a dynamic process. In this process, the position and extent of mining, as well as the supply, runoff and discharge conditions of groundwater flow field change constantly, that represents the water inflow is a dynamic value. This dissertation proposes the water inflow dynamic prediction basic method based on the numerical simulation, according to this and combining with the mining plan, predicts the dynamic water inflow of four working faces in the first mining area of Dahaize coal mine and three mining levels of Dajiazhuang iron mine. According to the forecast results, form the first to the third stage, the water inflow of Dahaize coal mine increases with the increase of mining area, with the normal water inflow increases from 580 m3/h to 2221 m3/h, and falls back to about 1500 m3/h in the fourth stage, which indicates the water inflow increases with the increase of goaf area within a certain range, when reaches a certain degree, the mine water inflow tends to be stable or even decreases gradually, and the characteristics of groundwater system changes to control the water inflow dynamic process. Dajiazhuang iron mine is mining from bottom to top with three mining levels including-450 m,-350 m and-250 m, the prediction results indicate that the normal water inflow increases from 47500 m3/d to 57800 m3/d, with the mining level gradually increases close to the Quaternary aquifer.(6) Comparing the prediction results of underground mining affecting the groundwater environment between Dahaize coal mine and Dajiazhuang iron mine, the main research conclusions are as follows: on the one hand, there are significant differences in the occurrence conditions, mining methods, combination types of aquifer or aquiclude, geological structures etc. between coal mines and iron mines, but the characteristics and manner influencing the groundwater environment are the same, i.e. long-term underground mining will affect each overlying aquifer, resulting in changes of entire mining area groundwater system and the groundwater flow field of each aquifer. On the other hand, it represents obvious different influence extent on the groundwater environment, due to different conditions of the two mines.