Study on the influence of the size of the near-pillar column method on the stability of goaf in gypsum mine

0 Preface

Due to the characteristics of expansion, low strength and rheology of weak rock in gypsum mine, when using the room-column method to return to the mining body, in order to strengthen and ensure the stability of the stope, it is often necessary to leave different types of pillars, while the pillars remain. The size of the set has a significant impact on the safe production of the mine [1], the pillar is too large, the ore recovery rate is reduced; the pillar is too small to make the pillar strength smaller, if the support strength is less than the upper cover Rock pressure may cause instability and collapse of the stope, which may cause a series of surface environmental geotechnical problems, such as ground subsidence, road water accumulation, house collapse, etc. [2]. Since 2013, a gypsum mine has experienced instability and ground collapse accidents in several mining areas nearby, which has caused great concern from relevant departments such as safety. In order to avoid similar problems occurring in the mine, it is necessary to be wide and high in different pillars. The stability of the goaf is analyzed under the conditions.
1 Project Overview
The gypsum ore body is a concealed ore body, and the near-surface surface of the ore-bearing layer is covered by the Quaternary system, and is produced in the gray-bearing mudstone of the third lithologic section of the Upper Cretaceous Luzhou Formation. The surrounding rock of the top and bottom plates is the gray paste mudstone or thick layered magenta silty mudstone of the section. Among them, the 11# ore deposit has an elevation of +20 to -70 m, and the NE strikes with an inclination of 12°. In the ore section, the strike layer is 1200m long, with a tendency to extend 220m and an average thickness of 2.52m. The pillar is a gray mudstone containing gypsum layer. The gypsum and mudstone alternate in thin layers. The rock bedding is developed and easily swells when exposed to water. It is easy to split along the rock layer after dehydration. In this paper, the mining room parameters of the gypsum mine 11# ore body are taken as the research background, and the stability of the goaf under different pillar height and height conditions is analyzed to determine the reasonable safety pillar size.
2 theoretical analysis of pillar size versus pillar stability
Since the stability safety factor Fs of the pillar can be obtained from the ratio of the pillar strength Sp to the pillar average stress σp [3], namely: Fs=Sp/σp, wherein the pillar strength Sp is calculated by the Lunder formula [4], and the calculation is performed. The formula is:

Where, Ka is the coefficient of friction of the pillar; Cp is the average strength coefficient of the pillar; Wp and h are the width and height of the pillar, respectively. Sl refers to the integrity of the rock sample. According to the area bearing theory [5], the roof and overlying rock pillars are subjected to the gravity of the pillar. The average stress σp of the pillar can be considered as the gravity of the roof and overlying rock pillar and the area. The ratio of the bottom area of ​​the inner column. Common pillars are strip, square, rectangular, etc. The gypsum mine is a strip-type and square-mixed pillar. The simplified model is shown in Figure 1.

Then the average stress of the pillar is calculated as:

Where: σp is the average stress of the pillar; γ is the bulk density of the rock strata on the pillar; H is the depth of the stop; lo is the length of the mine; wo is the width of the mine; lp is the length of the pillar; wp is the mine The width of the column; wp1 is the width of the mine wall.
Substituting various parameters into the mine: γ=23KN/m, lo=20m, wo=7m, wp1=3m. When the average buried depth H of the ore body is 60m, the safety factor Fs of the pillar and the height-to-height ratio of the pillar wp/h The relationship between them is shown in Table 1.

It can be seen from Fig. 2 that the safety factor of the pillar increases with the increase of the aspect ratio of the pillar. According to Bieniawski [6] (1992), the safety factor of the pillar is taken, and the long-term load of the pillar is considered. Stability may cause damage to non-essential buildings such as surface motor vehicles. The safety factor of the pillar is 1.5, so fit according to the fitting formula: y=0.259x2+0.407x+0.813 The correlation coefficient is 0.999. When the column height to height ratio is 1.02, the safety factor of the pillar is critical, that is, the reserved pillar width is above 2.55m to ensure the mined area of ​​the mine. Stable and meet the safety requirements of non-essential buildings on the surface.

3 numerical method simulation
3.1 Calculation Model Construction
According to the geological data of gypsum mine, a calculation model of 200m×120m×120m (X×Y×Z) is constructed. The X direction is 200m from -100m to 100m, the Y direction is from 120m to 120m, and the Z direction is from 120m to 120m. , wherein the Y axis is along the direction of the ore body. The established computational model has a total of 384,000 regular hexahedral elements and 440,271 initial nodes. The body size of the ore body: 1 m × 0.5 m × 0.5 m (X × Y × Z). The calculation model diagram is shown in Figure 3. During the simulation, the left and right boundaries, the front and back boundaries, and the bottom boundary of the model adopt zero-displacement boundary conditions. In this paper, the Mohr-Coulomb model is adopted, and the rock mass failure criterion is adopted by the Mohr-Coulomb yield criterion. The mechanical parameters of the ore used are collected from the mine. See Table 2 for details.

In order to save the calculation time, different levels of mines are excavated in order according to 1→2→3→4→5→6. The size of the mine is 20m×7m (length×width), and the thickness of the gypsum layer is 0.5m as the roof support layer. Under the condition of 2.5m height, the pillar width is set to 1, 2, 3, 4m. That is, the aspect ratio is 0.4, 0.8, 1.2, 1.6, and the stress and strain distribution of the rock mass around the goaf of the mine 3 is analyzed for the depth of the mining site Z=60m.

3.2 Analysis of simulation results
3.2.1 Displacement analysis
It can be seen from Fig. 5 to Fig. 8 that the amount of settlement of the roof decreases as the aspect ratio of the pillar increases. When the pillar height-to-height ratio is 0.4, the displacement is -24.91 mm; when the pillar height-to-height ratio is less than 1.2, as the aspect ratio increases, the roof panel settlement increases by about 1.0 mm each time. However, when the pillar height to width ratio is 1.2, the increase is only 0.4mm. At the same time, when the aspect ratio is 0.4, the bottom drum volume is 20.08 mm, and then when the pillar height-to-height ratio is less than 1.2, as the aspect ratio increases, each time The increase is about 3.0mm, and when the pillar height-to-height ratio is 1.2, the increase is 0.15mm, which means that the ceiling and the bottom plate have a sudden change in the settlement when the column height-to-height ratio is 0.8. The pillars with a height ratio of 0.8 are not enough to withstand the plastic yielding of the upper part of the stope, resulting in the stope on both sides of the pillar changing from the previous simple supported beam to the cantilever beam. The goaf is prone to occur. Large-scale instability.

3.2.2 Stress Analysis
It can be seen from Fig. 9 to Fig. 12 that there is obvious pressure stress concentration at the end of the pillar. When the pillar height to height ratio is 0.8, the maximum compressive stress is the largest, the value is -5.7670 MPa, and then with the pillar When the aspect ratio is increased, the compressive stress concentration is alleviated, and when the column width to height ratio is 0.4, the maximum compressive stress is at least 4.634 MPa. Correspondingly, when the pillar height-to-height ratio is 0.4, the maximum tensile stress of the roof is 0.178 MPa, and when the pillar width-to-height ratio is 0.8, the minimum tensile stress is 0.147 MPa. This shows that when the pillar height-to-height ratio is 0.8, the roof of the goaf may locally collapse and collapse, resulting in a significant reduction in the maximum tensile stress.

3.2.3 Plastic zone analysis
It can be seen from Fig. 13 to Fig. 16 that as the size of the reserved pillar width ratio increases, the plastic region gradually becomes smaller. When the pillar height to height ratio is 1.2 and 1.6, it is only in the pillar portion. Shear failure occurs, and when the height-to-height ratio of the pillar is 0.8, shear failure occurs in the entire pillar, and shear failure occurs in the local roof. This will cause instability of the pillar and promote the roof of the goaf. slate body integrity severely damaged, more fissures developed when cracks run through to the surface, the surface subsidence induced phenomenon.

Therefore, combined with the previous analysis of the displacement, stress and plastic zone of the stope, it can be preliminarily judged that when the column height-to-height ratio is 0.8, the pillar is subjected to compressive stress damage, which causes the local stope to be unstable and destroyed. When the height ratio is 1.2, only plastic deformation occurs locally in the pillar. In order to better maintain the stability of the stope, the size of the retained pillar should meet the ore column height-to-height ratio of 1.2 or more.
4 Conclusion
For the mine section with a depth of 60m buried in a gypsum mine, the steady state of the goaf under different pillar sizes was numerically simulated. By comparing the simulation results, the following conclusions were obtained.

(1) According to the area bearing theory, the gypsum ore pillar arrangement is simplified into a strip-type and square-column mixed arrangement, and according to the Lunder pillar strength formula, the safety factor between the pillar and the pillar width ratio is obtained. The relationship is that when the gypsum mine column width is above 2.55m, it can meet the safety requirements of non-essential buildings on the surface.
(2) Using FLAC3D software to construct a gypsum mine excavation calculation model, analyze the displacement, stress and plastic zone change of the stope at H=60m. It is considered that when the pillar height-to-height ratio is 0.8, the pillar is plasticized. Yielding, and causing serious damage to the integrity of the roof rock mass in the goaf, may induce surface subsidence, and the column height-to-height ratio is 1.2, that is, when the pillar is 3m wide, only plastic damage occurs locally in the pillar. The stability of the stope can still be maintained, which is consistent with the theoretical analysis.
(3) Combining the theoretical and simulation analysis results, it is considered that the gypsum mine pillar should be reserved for more than 3m to ensure the stability of the stope.
references:
[1] Song Hua. Study on distribution law and control of ground pressure of security pillars under buildings [D]. Wuhan: Wuhan University of Technology, 2013.
[2] Li Yaru. Discussion on the construction of roadbed in highway goaf [J]. China's new technology and new products, 2012 (17): 61-61.
[3] Zheng Xuemin. Safety evaluation of mining under the extra large goaf [J]. Mining Research and Development, 2002, 22(6): 16-18.
[4] Huang Yinghua. Study on instability mechanism and stability of goaf in gypsum mine by room and pillar method [D]. Changsha: Changsha Mining Research Institute Co., Ltd., 2008.
[5] Liao Wenjing. Analysis of the influence of water accumulated in the goaf of gypsum mine on the stability of the pillar [J]. Mining Technology, 2009, 9(3): 52-53, 58.
[6] Liao Wenjing, Xu Bigen, Tang Shaohui, et al. Orthogonal test analysis of main influencing factors of gypsum mine goaf stability [A]. Proceedings of 2010 China Mine Safety Management and Technical Equipment Conference [C]. 2010: 15-18, 35.