| In the process of mine production and discharge of tailings,tailings pond is gradually formed by piling tailings dam and intercepting wet discharge mud.Tailings dam has the characteristics of high potential energy and high infiltration line,which is a hazard source of artificial debris flow.Once the tailings dam breaks,it will flood and impact the downstream living and production facilities,and cause severe damage to the surrounding environment.The disaster of tailings dam is generally divided into natural factors and human factors,and the two often act at the same time to jointly induce geological disasters such as tailings dam break.Among them,the influence of tailings dam heightening and expansion,rainfall,earthquake and other factors on the stability of tailings dam cannot be ignored.Therefore,with the heightening and expansion project of Aketao tailings dam in Xinjiang as the background,the calculation parameters required for finite element simulation are determined based on the constant head permeability,the soil-water characteristic curve,the consolidation fast shear and moving test.Two-dimensional and three-dimensional finite element model of tailings dam was established based on field survey to study the stability of Aketao tailings dam before and after heightening of rainfall,earthquake and coupling,and to reveal the influence mechanism of rainfall and earthquake on the stability of tailings dam.At the same time,the influence of relative density of dam and layout of the seepage layer on the seepage field,safety factor,liquefied area distribution and permanent deformation of the tailings dam is analyzed,and a reasonable dam building scheme is proposed according to the calculation results.This study can provide a theoretical basis for the safely heightening of Aketao tailings dam.The specific research results are shown as follows:(1)Before the heightening and expansion,the leakage point of the downstream drainage surface of the dam and the minimum buried depth of infiltration line under the flood condition is 5.594 m which is greater than the standard allowable value of 4m.The minimum safety factor calculated by the quasi-static method is 1.407 and greater than the standard allowable value of 1.150,indicating that the seepage stability and skid resistance of the tailings dam are good.Both rainfall and earthquake will weaken the stability of tailings dam,and the seismic action will weaken the stability of dam more.(2)The stability of the tailings dam is weakened due to heightening and expansion,which is reflected in the decrease of safety factor of tailings dam under all working conditions after heightening and expansion.However,after heightening and expansion,the minimum buried depth of infiltration line of each working condition is 6.089 m greater than the standard allowable value of 6m,and the minimum safety factor of the special working condition calculated by the quasi-static method is 1.183,which still meets the requirements of the standard.The tailings dam is still stable,indicating that the heightening and expansion of Aketao tailings dam is feasible.(3)The dynamic time history analysis and calculation results of tailings dam after heightening and expansion show that the main range of the dynamic time history safety factor is between 1.150 and 2.000,and the safety factor at the end of the earthquake is greater than the allowable value of 1.150 in the specification.The liquefied area after the earthquake mainly occurs in the shallow soil layer of the sedimentary beach,and there are also local liquefied areas on the dam surface.Considering the overall structure of the tailings dam,the depth and range of liquefied area are not enough to cause dam failure.The permanent deformation caused by earthquakes mainly occurs in the tailings fine sand and tailings silty sand layers,and both horizontal and vertical permanent deformation exhibit a distribution pattern of smaller dam foot and larger upper part of the dam body.The maximum horizontal and vertical deformation values are 0.337 m and 0.317 m respectively,and the maximum seismic subsidence rate of the dam crest is 0.37%.According to domestic and international research,the deformation and instability control standards are selected as follows: the maximum horizontal and vertical deformation should not exceed 1.5-2.0m,and the seismic subsidence rate of the dam crest should be less than 1%.After comparison,it is found that the Aketao tailings dam will not experience deformation instability.(4)The calculation results law of three-dimensional finite element of tailings dam are basically consistent with two-dimensional,but the stability of three-dimensional tailings dam is less than that of two-dimensional.The reason is that the three-dimensional model reflects the valley landform of Aketao tailings dam,which leads to higher three-dimensional infiltration line in the downstream valley than the two-dimensional infiltration line,and the seismic waves are constantly reflected and refraction in the three-dimensional valley terrain,leading to the accumulation of seismic energy in the tailings dam.Considering that the three-dimensional model is closer to the engineering practice,the three-dimensional finite element method is recommended for the stability calculation of the valley tailings dam.(5)When building the dam,the drainage layer can reduce the height of the tailings dam and reduce the water content of the unsaturated soil in the dam,so as to enhance the seepage,anti-slip stability and seismic performance of the dam.When the relative compactness of the accumulation dam is 0.7,with the increase of the number of seepage layer layout,the height of the infiltration line of the tailings dam gradually decrease,and the safety factor shows a trend of increasing rapidly first and then increasing slowly,indicating that there is a marginal decreasing effect of the number of seepage layer layout on the improvement of the stability of the dam body.(6)Increasing the relative density of accumulation dam can effectively reduce the rainfall infiltration,reduce the moisture content of the unsaturated soil in the dam,and avoid the large loss of its strength due to the rainwater softening,so as to enhance the anti-slip stability of the tailings dam.In addition,when the number of drainage layer is arranged in 3layers,the seismic performance of the accumulation dam increases with the increase of the relative density,which shows that the scope of the liquefied area of the accumulation dam is gradually reduced,the horizontal and vertical deformation of the dam is gradually reduced,and the maximum deformation of the dam is transferred from the inside of the dam facing the dam body.When the relative compactness of the accumulation dam reaches 0.7,the dam surface liquefied area is completely disappeared,and the risk of overall instability of the tailings dam is significantly reduced.(7)Based on the on-site survey results,it is recommended to use methods such as excavation and backfilling or pouring cement mortar to treat erosion ditches and uneven cracks on the dam surface,and to lay anti-seepage materials or take measures such as vegetation slope protection on the dam surface to improve the anti-seepage performance of the dam surface.Based on the stability calculation results of tailings dams under different dam construction qualities,from the perspective of safety and economy,it is recommended to increase the height and expand the capacity of tailings dam as follows: ensure that the relative density of the dam is not less than 0.7,arrange three layers of drainage layers vertically and equidistant along the dam height,and reserve sufficient safety superelevation when building each level of sub dam to avoid flood overtopping caused by dam crest subsidence.In addition to the above plans and measures,the safety reserve of the dam can also be improved by slowing down the slope of the embankment and reducing the slope ratio of each level of embankment.Finally,it is recommended to conduct long-term monitoring of indicators such as dam displacement,infiltration line height,dry beach length,rainfall,and seepage flow to ensure safely heightening and normal operation of tailings dam. |
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