The Dawenkou mining area is rich in gypsum reserves, with proven reserves of 33.7 billion tons, accounting for 48% of the country's total reserves [1]. The gypsum ore deposit occurs in the Tertiary, and the buried depth is mostly in the range of 100-300 m. The thickness of the Quaternary covering layer above the Tertiary is more than 20 m, which is the main aquifer in the mining area. It has been repeatedly seen in the history due to the collapse of the roof. Four series of aquifers and flooded wells.
The Dawenkou mining area has fertile land, abundant water resources, and drought and flood protection. It is a famous high-yield grain area in Shandong. In addition, there are a large number of villages on the ground. Therefore, underground mining does not allow large subsidence on the surface.
Based on the above reasons, the gypsum mining in the Dawenkou mining area must avoid the roof falling and keep the pillar stable. Therefore, the continuous mining room and pillar method is generally used. The mining span is 4m, the pillar width is 4~5m, and the mining height is 3. ~6m. Although the mining of the continuous pillar column method ensures the stability and reliability of the goaf in a long period of time, due to various factors such as weathering and erosion and creep and splitting, even after the closure is closed for several years, it may eventually It caused the collapse of the pillars of only 5m wide, which led to the collapse of a larger area of ​​the pillars, resulting in large-scale subsidence of the surface and destruction of ground villages and high-yield cultivated land. Therefore, it is necessary to control the size of the mining block and to set up an isolated pillar around the ore block so as not to spread the large area of ​​the collapse and remain permanently stable. To this end, in view of the hidden dangers of the continuous mining of the Dawenkou gypsum mine, the need to study the isolation range of the goaf and the size of the isolated pillar after mining.
1 basic overview of the mine
The Dawenkou mining area in Shandong has a very rich gypsum reserve. The commonly mined gypsum deposits are 2III, 3II and 5 paste layers respectively. The average thickness of the ore layer is 5.4, 7.3, 5.2m respectively; the surface of the mining area is the Quaternary coverage area, the seam occurs in the upper portion of the gypsum group guanzhuang Sec dawenkou group, was syncline, dip both wings 4 ° ~ 7 °; paste layer of roof of mudstone, shale, sandstone a small clip The geological stability of the roof engineering is poor, and the lithology of the floor is mudstone and marl shale, which has poor stability.
In order to prevent the collapse of the goaf and the subsidence of the surface, the ore layer has been developed by shallow hole continuous pillar column method [2], and its parameters and arrangement are shown in Figure 1 [2].
2 isolation range determination
2.1 Determination principle
If a large area collapses in a pillar, it will inevitably cause overburden movement and surface subsidence. As long as the collapse is controlled within a certain range, it will ensure that the ground building does not cause serious damage, and the surface subsidence will not cause serious stagnant water. According to "buildings, water, railway and roadway main pillar for coal mining regulations and pressure" (the "order") [3], the value of the local table horizontal distortion, deformation and inclination curvature K i does not exceed Table 1 When the value of the Class II damage level is used, the building can be safely used with simple maintenance, and the surface will not be damaged. Therefore, the reasonable isolation range is determined based on the principle that the maximum deformation value of the surface when the building does not exceed Class II damage.
2.2 Surface deformation value is expected
According to the "Procedure", the horizontal deformation value ε, the curvature deformation value K and the oblique deformation value i in Table 1 are calculated by the following formula [4]:
Where b is the horizontal deformation coefficient, taking 0.3; q is the sinking coefficient; W is the surface subsidence value, mm; r is the main influence radius, m; M is the mining height, m; H is the mining depth, The average value of the multi-mineral layer is 260m; β is the main influence angle, taking 650.
If the goaf is not isolated, when the pillar collapses and collapses, the surface subsidence will reach full mining state, according to the factors such as the occurrence conditions of the ore deposit and the depth of burial, combined with the actual observation of the surface subsidence of the surrounding coal mine (Dawenkou gypsum) There is no collapse in the goaf of each mine in the mining area, no measured value), and the sinking coefficient q is 0.85.
At present, the mining layer in the Dawenkou mining area is 2III, 3II and 5 pastes, with an average mining height of 3.0, 4.5 and 2.7m, respectively, totaling 10.2m. Due to the continuous pillar, the pillar width is 5m and the mine width is 4m. If the pillar is unstable, the roof sinking space is only a part of the height. After the collapse, the crushed stone will accumulate on both sides of the long-shaped pillar, which makes the pillars unstable and scattered unevenly, and the support of the pillars and the drilled pillars are not completely collapsed, resulting in further reduction of the roof sinking space. According to the calculation and simulation, the sinking space after the instability of the pillar is 0.3 times of the mining height, that is, the maximum sinking space of the goaf is 3.06m.
According to formula (1), after the goaf is not isolated, the maximum sinking value of the surface is: ε=9.81mm/m; K=0.273×10-3m; i=21.256mm/m ;W=2581mm.
It can be seen from Table 1 that if the goaf is not isolated, the ground building will be seriously damaged, the damage level will reach Grade IV, there is a serious safety hazard, and the final value of surface subsidence can reach 2.58m, which is serious damage to the surface.
2.3 isolation range determination
To control the surface and buildings from damage, the use of isolated pillars to control the collapse of the goaf to a small extent, the surface subsidence is insufficient, the sinking value is reduced, and the degree of damage is reduced. Incompletely subsided subsidence, calculation of the maximum sinking value of the surface [2]:
Where C is the type of overburden type, hard overburden C=1, medium hard overburden C=2, soft overburden C=3; D is the maximum instability width of the pillar, m; m is the instability of the pillar Effective sinking space, mm.
Under normal circumstances, the slope deformation value i is most affected by the surface subsidence. As long as i≤6.0mm/m, the maximum damage level of the ground building will not exceed Grade II, and C=2[2], then according to the formula (1) And equation (2), when the i = 6.0mm / m, the maximum size of the goaf is D = 109m. That is to say, for each isolation of 109m, it is necessary to maintain a larger size of the pillar, which can remain stable during the collapse of the small-sized pillar in the goaf, and prevent the pillar from destabilizing and spreading.
3 pillar size determination
According to the elastoplastic theory and the design experience of the pillar, the isolated pillar width B can be calculated as follows [2]:
Where x0 is the plastic zone range on one side of the pillar, m; L is the elastic zone range in the middle of the pillar, m; n is the safety factor, take 2.
Calculation of the plastic zone range on one side of the pillar [2]:
Where k is the maximum concentrated stress coefficient, taking 3; γ is the density of the pillar, MN/m3, taking 0.024 MN/m3; R*c is the residual strength of the pillar, MPa, according to the test, R*c=7. 36MPa; 1 is the layer friction angle, take 10°;  is the internal friction angle, take 40°. The elastic zone in the middle of the pillar can be calculated as follows [2]:
Where η is the lateral pressure coefficient, which is 0.3; c is the internal friction of the column, MPa, and c=2.08 is obtained after testing.
Substituting the corresponding parameters, the calculated B=28m, and determining the width of the isolated pillar B is 30m.
According to the above calculations, it is determined that the maximum length of the mining area (mine block) mined in the Dawenkou gypsum mining area is 140m, the mining length should not exceed 109m, and the isolated pillar (inter-column) should be no less than 30m.
4 expected results
According to the above isolation range, using the formula (1) to calculate, if the continuous pillars left in the goaf are finally unstable, the maximum subsidence value of the surface is: ε=2.73mm/m; K=0.075×10-3 /m; i = 5.98 mm / m; W = 727.4 mm.
According to Table 1, after the isolation pillar is set, the surface damage range is controlled, the damage level of the building is not more than Grade II, the safety hazard is small, and the simple maintenance of the house is still habitable.
5 Conclusion
(1) Due to the weathering and cracking of gypsum, after a few years of mining, the underground gypsum pillar may collapse and become unstable. In order to avoid large damage to the ground building and the surface when the pillar collapses, it is necessary to set up an isolation pillar.
(2) The isolation range is that the maximum length of the goaf does not exceed 109m, and the width of the isolated pillar should be greater than 30m.
(3) After the pillars in the goaf are unstable, the isolated pillars prevent the collapse of the collapse range, and the maximum subsidence value of the controlled surface is not expected to exceed 727 mm. The maximum damage level of the ground buildings does not exceed Grade II, and can still be used after simple maintenance.
references
[1] Tai'an City Planning Bureau. Investigation and planning of mineral resources in Tai'an City, Shandong Province [R]. Taian: Tai'an City Planning Bureau, 2010.
[2] Xiang Baoyin, Li Hong, Li Shutao, et al. Demonstration report on key technologies for underground mining in Wenyang Gypsum Mine [R]. Taian: Shandong University of Technology, 2014: 26-40.
[3] National Coal Industry Bureau. Rules for coal pillars and coal mining in buildings, water bodies, railways and main shafts [S]. Beijing: Coal Industry Press, 2006.
[4] Xu Yongzhen. Coal mining science [M]. Xuzhou: China University of Mining and Technology Press, 2004.
Li Hong; Department of Resources and Civil Engineering , Shandong University of Science and Technology (Tai'an);
Article source: "Modern Mines"; 2016.5;
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