Yuanbaoshan open-pit coal mine was discovered in 1954. After exploration, the preliminary design of the open pit mine was completed by Shenyang Coal Mine Design Institute on 1987, and the construction was officially started on 1990, 10 and 15. There is no mining record in the history of Yuanbaoshan open-pit mining area. There are Yuanbaoshan 1, No.2, No.3 and No.4 wells in the southwest, Laogongyingzi and Xiaofeng Shuigou mine fields in the northeast, and two local small coal mines in the southwest of the open pit mine are mined along the outcrop of No.7 coal. The open pit mine has now built two dump sites in the west and south, and the first and second mining areas are also under construction. The southern boundary of the mine field is formed at the foot of Yuanbaoshan, and the eastern boundary is bounded by F 1 fault. The western boundary is formed along the floor of six coal seams. Adopt the method of ditching in the first and second mining areas at the same time, and advance from south to north, with the north side as the working face. The designed final mining area of open-pit mine is 12.32km2 ... The open-pit mining reserves are 542.89 million tons, A+B reserves are 532.65 million tons, and the design scale is 5 million tons/year of raw coal.
Since 1954, geological, coal, hydropower and other departments have done a lot of fruitful work in this regard and obtained rich information. From 1954 to 1955, the geological team of Pingzhuang Mining Bureau and the team of East Coal Geology Bureau 107 conducted a coalfield geological survey in this area. The Geological Survey Report of Yuanbaoshan Open-pit Coal Mine was submitted by the East Coal Geology Bureau/team KLOC-0/04 on 1973, and the Hydrogeology and Engineering Geology Survey Report of Yuanbaoshan Open-pit Coal Mine (Belt Strength) was submitted on 1982. Northeast Electric Power Design Institute of Ministry of Water and Electricity submitted the Hydrogeological Investigation Report of Water Supply for Yuanbaoshan Power Plant New Project on 1975, and Nanjing Water Conservancy Research Institute of Ministry of Water and Electricity submitted the Calculation Sheet of Yuanbaoshan Open-pit Coal Mine Affected by Jinyinghe Leakage on 1987; 1993, Xi Branch of the General Research Institute of Coal Science submitted the Preliminary Design Description of the Curtain Project of Yuanbaoshan Open-pit Coal Mine and the Geological Survey Report of the Curtain Closure Project of Yuanbaoshan Open-pit Coal Mine. The preliminary work has accumulated rich geological and hydrogeological data in this area, which has laid a good foundation for future work.
However, during the stripping construction of open-pit mine, the abundant groundwater in the Quaternary loose sedimentary aquifer brings great difficulties to the stripping of open-pit mine. At present, high-intensity drainage of groundwater is being carried out in a large area around the stripping area. Since 1990, nearly 120 drainage holes have been constructed successively, with a total daily displacement of about 400,000 ~ 500,000 m3. The groundwater level in the open-pit mine and its nearby Quaternary aquifer drops by about 8 ~ 20 m, and the maximum water level in the mining area drops by about 27 m. The groundwater flow field in the Quaternary aquifer in the area is basically stable. However, the current underground water flow field is far from meeting the stripping requirements of the excavated stripping areas (the first mining area and the second mining area), especially the water level drop value of the second mining area should be about 20 ~ 50 m. Therefore, in order to produce coal safely in the second mining area, it is necessary to reduce the quaternary underground water level in this area by about10 ~ 30 m. Therefore, the current drainage scheme, drainage project and drainage amount are far from meeting the needs of mine construction and production. How to choose the best drainage scheme to meet the drainage requirements with the minimum total drainage amount is an urgent problem to be solved.
7.3. 1 Groundwater System and Hydrogeological Model
7.3. 1. 1 Overview of the study area
(1) Geographic traffic
Yuanbaoshan Open-pit Coal Mine is located 35km east of Chifeng City, Inner Mongolia Autonomous Region, and belongs to Jianchang Ruth Township, yuanbaoshan district, Chifeng City. Its geographical coordinates are east longitude11917' 55 "~119' 55"; North latitude 4219'13 "~ 42 22' 21".
There is a special railway in the south of the mining area, which is connected with Yechi Line (Yebaishou-Chifeng) at Yuanbaoshan Station, and the mining area is connected with Chifeng and surrounding counties and cities with a third-class highway, so the traffic is very convenient.
(2) Topography and landforms
Yuanbaoshan Open-pit Coal Mine is located in Jinying Valley Plain, which runs through the middle of the mining area and divides the mining area into north and south parts. The southern part of the open-pit mine is located in the first terrace on the right bank of Jinyinghe River, with a step width of 500 ~ 4,000m, a ground slope of 1 ‰ ~ 1.5 ‰ and a ground elevation of 472 ~ 482m. The northern part of the open-pit mine is located in the first and second terraces on the left bank of Jinying River, with a ground slope of 1‰~ 2‰, a bench width of 500 ~ 5000m and a ground elevation of 482 ~ 490 m. There are modern aeolian sands on the terraces, and sand dunes are distributed in waves.
Generally speaking, the mining area is an alluvial and diluvial plain, surrounded by low mountains and hills. The elevation of the surrounding hills is generally 500 ~ 600 m, and the alluvial and diluvial deposits form an extremely water-rich aquifer in this area, and there are abundant extremely thick coal resources hidden under the Quaternary aquifer.
(3) Meteorology and Hydrology
This area belongs to semi-arid continental climate. It is characterized by long, dry and cold winters, concentrated rainfall in summer, less rain and snow in spring and autumn, and more wind.
According to the data of Chifeng Meteorological Observatory, the highest temperature in Yuanbaoshan area is 42.5℃ and the lowest temperature is -3 1.4℃. The freezing period is generally from mid-June 165438+ to the end of March of the following year (average temperature-10.8℃, minimum temperature -27℃, maximum temperature 7.6℃), and the maximum freezing depth is 2.01m. ..
The annual average relative humidity is 49%, the average evaporation is 1867. 1 mm, and the annual average precipitation is 372.34 mm (according to the data of 1950 ~ 1994). The annual precipitation is mostly concentrated from June to August in summer, accounting for 68.55% of the whole year.
Jinying River flows through the mining area from northwest to southeast and joins the Laoha River at Dongbajia, which is the largest tributary on the left bank of the Laoha River. Jinying originated in the northern mountainous area of Hebei Weichang (Qilaotou Mountain). Flow length 194.6 km, drainage area 10598 km2. Over the years, the maximum flood peak discharge is 2650 m3/s, the minimum discharge is 0.5 m3/s, and the multi-year average discharge12.8m3/s. The riverbed width varies greatly, ranging from 200m to 900m. The mainstream swing has a strong lateral erosion on both banks, which often causes bank collapse during floods. In recent years, due to the development of upstream reservoir water storage and agricultural irrigation, the open-pit mining area located downstream is often cut off in spring, autumn and winter. The flow distribution of rivers in dry season, normal season and high season is the same as that of precipitation. Groundwater in Yuanbaoshan open-pit mining area is replenished in the form of vertical infiltration.
Laoha River flows from southwest to northeast through the southern part of the valley plain, 3 kilometers away from the open-pit mining area. The river originates from Guangtou Mountain in Qilaotou Mountain range, Pingquan County, Hebei Province, and joins the Xilamulun River near Hailitu in Daxing Township of Zhaowuda League to form the West Liaohe River. The total length is 421.8km. The drainage area is 33,076km2. The maximum flood peak flow over the years is 9840 m3/s, the minimum flow is 0 m3/s, and the average flow for many years is13.6m3/s. The riverbed width in the study area is between 500 ~ 1000 m, and the riverbed and floodplain are mainly composed of sand, sandy soil and gravel. The flow distribution of rivers in dry season, normal season and high season is the same as that of precipitation. Groundwater in Yuanbaoshan open-pit mining area is replenished in the form of vertical infiltration.
7.3. 1.2 Geological and hydrogeological conditions
(1) Main strata in the mining area
A. Upper Jurassic Xingyuan Formation: mainly grayish white medium-fine sandstone mixed with purplish red sandstone and mudstone, with a thickness greater than100 m. The middle part is grayish green sandstone and glutenite mixed with black mudstone, with a thickness of 60-230 m, and the upper part is grayish green and grayish brown thick mudstone mixed with grayish white sandstone, with a thickness of about 200 m, distributed in F/kloc-.
B. Yuanbaoshan Formation of Upper Jurassic: mainly grayish white medium-fine sandstone, mixed with coarse sandstone, mudstone and coal seam, with a general thickness of about 340 m, of which minable coal seam 12, with an average cumulative thickness of 84.29 m, and the main minable coal seams are No.5 and No.6..
C Neogene Pliocene (N2): the bottom is purplish red sandstone, mudstone conglomerate and mudstone, which is unconformity in Yuanbaoshan coal-bearing stratum, with a thickness of 0 ~ 1 15m, and only distributed in the dome anticline in the south of the open pit mine. The upper part is basalt, laterite and gravel layer, which only appears in the north of the mining area or covers the purple gravel in the south of the open pit mine.
D. Quaternary (Q): It is mainly composed of modern alluvial deposits, diluvial deposits and glacial water deposits, and is composed of pebbles, mud pebbles, pebbles, andesite, basalt and other gravels. Thickness 14 ~ 85 m, generally 55 m, distributed in the whole Yuanbaoshan basin. Near the mining area, the thickness is generally 14 ~ 60 m, which gradually becomes thicker from west to east.
(2) Main geological structure characteristics of the mining area
Yuanbaoshan coalfield is a faulted coal-bearing basin controlled by Yanshan tectonic movement, which is distributed in NNE-NE strip shape. The coal-bearing basin is a broad and gentle compound syncline structure, which consists of three synclines and two anticlines. From southeast to northwest, it is: Fengshuigou short axis oblique, Wu Jia anticline, Nanhuang syncline, Longtoushan anticline and Laoyao short axis oblique. There are minable coal seams in syncline structure, the syncline axis is NNE, and the stratum dip angle is 3 ~ 5.
Generally speaking, the underlying coal measures of Quaternary in this area are relatively flat, and the thickness of the main coal seam is about 60 m. The occurrence and occurrence conditions of coal seams are very conducive to the open-pit mining of coal resources.
(3) Hydrogeological conditions in the mining area
Quaternary pore aquifer is composed of round gravel, gravel, pebble and mud gravel accumulated by alluvial, flood and ice water, with the particle size of 5 ~ 60 mm accounting for more than 50%, larger than 60 mm accounting for 20%, and there are boulders in some areas. Gravel is mainly composed of andesite and granite with good roundness and poor sphericity. The thickness increases from southwest to northeast, but the change trend is gentle, and the thickness changes greatly only at the step ridge between two terraces on the bedrock surface.
Because of the different genesis of Quaternary strata, the permeability of the upper and lower layers is also different. According to the previous exploration test data, the permeability coefficient of the upper stratum is relatively large, about 256 ~ 7 10 m/d, and the permeability coefficient of the lower stratum is relatively small, about 16 ~ 146 m/d, but they are closely related hydraulically. The water quality belongs to magnesium heavy calcium carbonate water, the pH value is mostly between 7 and 8, the solid content is 240-400 mg/L, the total calcium and magnesium content is 260-365 mg/L, and the water temperature is 8- 1 1℃.
The Jurassic pore and fissure weak aquifer is composed of sandstone, glutenite, siltstone and coal seam, and there are a few cracks in the coal seam. According to the pumping test data, the general thickness is 50 ~ 150m, the average thickness is 1 13.9 m, and the permeability coefficient is 0.001~ 0.38m/d.
It can be seen that the Quaternary unconsolidated sedimentary phreatic aquifer is the only major aquifer in this area. While other bedrock fissure water can be ignored.
Under natural conditions, the flow direction of groundwater in this area is consistent with that of surface water, that is, it flows from northwest to southwest of the basin. The hydraulic gradient is gentle (see Figure 7.23). In recent years, with the pumping of the water source of Jianchangying Power Plant and the stripping drainage of the open-pit mine, the Quaternary groundwater level has formed a descending funnel centered on the stripping area of the open-pit mine. Groundwater forms a new runoff condition and converges from the periphery of the funnel to the center.
The recharge of Quaternary groundwater mainly comes from seasonal precipitation in the basin, leakage of Jinying River and Laoha River flowing through the basin, and upstream lateral runoff from northwest and southwest. It is particularly important to note that at present, the groundwater level drop funnel has expanded outward across Jinying River and Laoha River, so the recharge of groundwater by rivers is mainly infiltration, not injection. From the above analysis, it can be seen that the main recharge, diameter and discharge relations of Quaternary groundwater in the study area are shown in Figure 7.24.
(4) Current situation of Quaternary groundwater discharge.
Since the dewatering project started on August 5th 1990, * * * has been put into operation 13 row, of which dewatering boreholes 12 1 have been partially scrapped and partially shut down. The average displacement is 400,000 ~ 500,000 m3/d (see Table 7.23). The center of the mining funnel is near Guanwu Cave, and the water level elevation changes at (438 m 1 m). See Figure 7.25 for the main drainage holes.
Figure 7.23 Natural Flow Field Diagram of Quaternary Groundwater in Mining Area
Fig. 7.24 Schematic diagram of Quaternary groundwater recharge and drainage in Yuanbaoshan mining area.
Jianchangying Power Plant, which is adjacent to the open-pit mine, uses group holes to centrally extract Quaternary groundwater as the power plant water supply source, and the daily pumping capacity is about 654.38+ 1 100 million m3. Due to the long-term pumping of two water sources, the regional groundwater flow field has been basically stable in recent years, and the hydrogeological conditions in the mining area have changed greatly due to drainage. The main manifestations are as follows: ① the groundwater runoff along the Jinying River is basically drained and intercepted; (2) The groundwater level is lower than that of Jinying River and Laoha River, making the second river a "suspended river"; (3) Groundwater is replenished by the infiltration of Jinying River and Laoha River.
(5) Mathematical model of Quaternary groundwater flow in Yuanbaoshan open-pit mining area.
According to the hydrogeological conditions of Yuanbaoshan open-pit coal mine mentioned above, the main aquifer selected by simulation calculation is the Quaternary phreatic aquifer located in the alluvial plain of Jinying River and Laohahe River, with an area of about 2 10 km2. The northwest boundary, southwest boundary and southeast boundary are generally far away from the basin, which can be treated as constant head boundary and the rest as water blocking boundary.
Figure 7.25 Distribution Map of Main Drainage Well Groups in Mining Area at Present
Table 7.23 1990 to 1995 Statistics of Drainage and Displacement (ten thousand m3/ month)
Quaternary aquifer is regarded as heterogeneous and anisotropic aquifer. Although the floor of Quaternary aquifer fluctuates to some extent, groundwater flow can still be regarded as two-dimensional unsteady flow of groundwater. Jinying River and Laoha River recharge the phreatic aquifer through infiltration.
According to the characteristics and boundary conditions of Quaternary groundwater in Yuanbaoshan mining area, a two-dimensional anisotropic movement mathematical model of Quaternary groundwater in Yuanbaoshan open-pit coal mining area is established.
Stochastic simulation and management of groundwater system
Where: h- groundwater level [L];
μ-unit output (dimension1);
Kxx, Kyy—— the main permeability coefficients of Quaternary aquifer in X and Y directions;
t-time；
(x, y)- Cartesian coordinates;
ω-groundwater seepage area;
γ 1- boundary conditions of the first kind;
γ 2 —— boundary condition of the second kind;
H0(x, y, t)- initial head distribution;
ε-infiltration recharge intensity per unit area [L3/(t L2)], mainly including atmospheric precipitation recharge and river infiltration recharge;
W- source and sink terms. The model mainly reflects the pumping capacity of open-air drainage and power plant intake;
Z—— aquifer floor elevation (L).
Model (7.3) is solved by Galerkin finite element method and divided by triangular elements. The plane is divided into 1788 triangular elements and 952 computing nodes. See Figure 7.26 and Figure 7.27. Because of the large calculation area, Figure 7.27 is an enlarged view of the open-pit stripping and well group drainage area.
Figure 7.26 Subdivision Diagram of Finite Element Calculation of Groundwater in Mining Area
7.3.2 Randomness of hydrogeological parameters and parameter identification
The hydrogeological conditions in the study area are similar to other groundwater systems, and the main parameters controlling groundwater flow are also very random and uncertain. As far as the structure of aquifer medium is concerned, due to the limitation of its genetic conditions, its sediment properties change greatly in vertical and horizontal directions, and sand, gravel and silty loess are often produced in the form of lenses. The randomness of the spatial distribution of this medium property determines the randomness of the main hydrogeological parameters (Kx, Ky, μ) of the aquifer. Secondly, the recharge conditions of aquifers (river infiltration, atmospheric precipitation and lateral runoff) are strongly influenced by the precipitation law in this area. Due to the randomness of atmospheric precipitation factors, the recharge conditions of groundwater in this area are also random. Finally, the main discharge conditions of groundwater (power plant water supply, open-pit mining drainage, agricultural water use, etc.). ) are interfered and influenced by many human factors such as equipment and water consumption, so the emission situation can also be regarded as a random factor. It can be seen that many factors affecting groundwater recharge, runoff and discharge in this area are random and uncertain. It is inaccurate to describe the dynamics of groundwater system in this area with any set of certain parameters. Therefore, it is more practical to introduce stochastic theory and statistical concepts to study the characteristics of aquifer system and plan and manage it.
Figure 7.27 Subdivision Diagram of Finite Element Calculation of Groundwater in Mining Area
According to the above analysis, five groups of hydrogeological parameters are obtained by using the hydrogeological data (pumping data, precipitation data and groundwater dynamic data) of the study area1991995. Trial estimation-correction method is adopted in the process of parameter adjustment. That is to say, the initial parameters are given according to the existing data, the mathematical model of groundwater is calculated, and the groundwater level value is solved. The obtained results are constantly compared with the measured results, and the parameters are corrected until the required fitting accuracy is achieved. On the basis of hypothesis testing, the mean, variance and probability distribution of each parameter are calculated statistically. Table 1. 1 gives the inversion value, mean value and variance of hydrogeological parameters Kx, Ky and μ in 15 parameter area. Through hypothesis testing, Kx, Ky and μ all obey the uniform distribution of [a, b].
In the process of parameter inversion, the following problems are especially dealt with:
(1) initial flow field. Based on the observed regional water level of 65438+ 10 in each simulation year, the water level values of each node are interpolated by kriging interpolation method as the initial flow field of simulation calculation in that year. The water level observation data from/kloc-0 to June every year is used as the adjustment parameter of fitting water level.
(2) precipitation. The precipitation in the simulation process is based on the actual observation data of Chifeng weather station. The monthly average precipitation data of 1950 ~ 1995 are used in the precipitation prediction. The infiltration coefficient of precipitation is 0.3.
(3) River infiltration recharge. According to the measured data, the maximum leakage of Jinying River and Laoha River are 4. 12× 104 m3/s and 1.6× 104 m3/s respectively. When calculating, the total seepage flow is divided by the area of two rivers in the calculation area to get the seepage flow per unit area, and then it is added to the corresponding area unit. Due to the lack of data, the model does not particularly consider agricultural water intake and irrigation infiltration.
(4) Because the phreatic flow is a nonlinear partial differential equation, Boussinesq equation is used to treat it as a linear problem when the management model is established by the response coefficient method.
As mentioned in the previous chapters, the biggest problem in the production and construction of Yuanbaoshan open-pit mine is the control and management of Quaternary groundwater. Predecessors have done a lot of research work on this issue and put forward many technical schemes, such as building impervious curtain wall, combining drainage with recharge, etc. Its core goal is to protect groundwater resources as much as possible and reduce the large-scale decline of groundwater level while ensuring safe production in mines. However, due to economic and social reasons, these programs have not been put into practice. At present, the drainage of large well groups is still the main technical measure to prevent and control water, and the existing drainage engineering and design capacity can not meet the production requirements. Therefore, based on the existing engineering situation, it is particularly necessary and urgent to put forward a new design scheme and hole arrangement principle of mine drainage well group, which can not only meet the requirements of mine construction and production, but also ensure the minimum total displacement of mine. Based on this purpose, a stochastic groundwater control and management model is established, and it is solved by taking the water level around the mining and stripping area as the constraint condition, taking the stable minimum total drainage as the objective function and the drainage as the decision variable. The optimal design principles of total displacement and drainage hole position under different confidence levels of constraints are put forward.
7.3.3 Establish groundwater management model with opportunity constraints.
According to the constraints, objective functions and decision variables of Quaternary groundwater drainage management in Yuanbaoshan open-pit mining area, an opportunity-constrained stochastic management model for optimal design of Quaternary groundwater drainage in mining area is established:
Stochastic simulation and management of groundwater system
The symbol in the formula has the same meaning as before, where n= 1, that is, the dehydration stage is considered. M=96, that is, two rows of nodes (96 in total) along the periphery of the stripping area are selected as the candidate positions (decision variables) of the drainage well, as shown in Figure 7.28. J=35, that is, 35 nodes (in total) along the edge of the stripping zone are water level constraint control points.
Substituting relevant parameters into the model (7.4) and making appropriate transformation, the following management model can be obtained:
Stochastic simulation and management of groundwater system
Where: s (j) = h0 (j)-ZL (j)-5;
i= 1，2，…，96；
j= 1，2，…，35 .
Model solution
The inversion results and random distribution forms of random hydrogeological parameters of Quaternary aquifer in this area are substituted into the stochastic finite element model, and the mean value E[β(i, j)] and variance r2(i, j) of the response coefficient of the stochastic management model are obtained by Monte Carlo stochastic finite element method. Taylor expansion stochastic groundwater management model is used to solve the total groundwater discharge and its drainage location distribution under different stochastic constraint confidence levels. See Table 7.24 for the calculation results. The relationship between total displacement and constraint confidence level is shown in Figure 7.29.
Figure 7.28 Location Distribution of Alternative Drainage Wells
Table 7.24 Calculation results considering the current excavation area.
Figure 7.29 Relationship between Total Drainage Capacity and Confidence Level
7.3.5 Discussion on Calculation Results
(1) As can be seen from the calculation results, due to the randomness of hydrogeological parameters, in order to meet the drainage requirements, the total amount of drainage is closely related to the confidence of meeting the constraints. With the decrease of confidence level of constraint conditions, the total drainage capacity decreases obviously. However, with the improvement of the confidence level of constraint conditions, the total drainage capacity increases rapidly. It shows that when the hydrogeological parameters have a small probability event that is not conducive to drainage, if the drainage requirements can still be met, the total amount of drainage will inevitably increase, which is consistent with theoretical analysis and actual situation.
(2) At present, the drainage capacity of 400,000 ~ 450,000 m3/d is obviously small, and even the constraint confidence of 80% cannot be reached. Therefore, the increase of total displacement is inevitable.
(3) As far as the distribution of drainage wells is concerned, it is mainly concentrated in the northeast and southeast of the present stripping area (the thickness of Quaternary is relatively large). This is also consistent with the analysis results of hydrogeological conditions. Because the general trend of aquifer basement in the basin is high in the west and low in the east. In order to ensure the drainage of the groundwater level in the whole drainage area, as long as the eastern region can meet the drainage requirements, the water level in the western region can naturally be reduced to the drainage requirements (because the aquifer has good permeability, the groundwater level will be very flat). Therefore, if the drainage holes are arranged in the western region, it will be wasteful and unnecessary.
(4) In order to test the superiority and correctness of the management results, the constraints of the current drainage water level decline are substituted into the management model to solve. If the variance of the parameters considered is zero (i.e. deterministic model), the calculation results are shown in Table 7.25. If the variance of parameters and the confidence level of constraints are considered, the calculation results are shown in Table 7.26. The results in the table show that, for some parameters, the total drainage can be reduced by about 65438+100000 m3/d compared with the current actual drainage if the drainage is conducted according to the optimized drainage well group. If the stochastic parameter model is considered, the current displacement is exactly equivalent to the calculated water volume with the confidence of constraint conditions between (95% ~ 100%). Therefore, the calculation result of the management model accords with the actual situation.
Table 7.25 Calculation Results of Optimal Water Allocation by Deterministic Model of Drainage Conditions in Yuanbaoshan Open-pit Mine
Table 7.26 Optimal Calculation Results of Current Drainage Conditions
sequential
(5) In order to further test the correctness of the management results, the management results are substituted into the groundwater simulation and prediction model to simulate and predict the main flow field of groundwater drainage. The predicted flow field shape better reflects the satisfaction degree of drainage requirements under different confidence constraints.