Study On The Engineering Geological Properties And Stability Of The Fine-grained Phosphorite Tailing Dam At Jiache County, Huize City, Yunnan Province

The tailings dam is mainly used for stockpiling tailings or other industrial waste residue. The main risks of tailing dam are the harmful effects to life, ecological environment and water environment resulted from dam breach. The composition and particle structure and microscopic characteristics and pore structure of the fine-grained phosphorite has an important influence on the physical and mechanical properties of tailings material. Therefore, it has important theoretical value to study the microscopic characteristics and mechanical characteristics. Geological conditions and dam structure are the main reasons of tailings dam break. All kinds of research methods including the field investigation, the experimental simulation, theoretical analysis and the numerical calculation are used in this paper for the engineering project of the phosphate tailing dam in the Huize county of Yunnan province. The characteristic parameters including the neotectonic movement, the microscopic characteristics, the initial cracking model, the layered consolidation property, the physical and mechanical characteristics, the seepage field, the stress field, the displacement field, the stability and the dynamic response are discussed in this paper. The main works are as follows:(1) On the basis of the drilling data and the geological documents, the neotectonic movement characteristics of the near-zone around the tailings dam are analyzed that includes the structure characteristics of the formation of deep and the seismotectonic characteristics before cenozoic geology. The feature of neotectonic movement in the near-zone of the tailing is presented that this block-region is inclined uplift, and the fault movement is obvious. The results show that the landslides probably occur along the fault zone, and the landslide and flow slide of the dam may occur under the action of earthquake.(2) The microscopic characteristics of tailings are studied through the experiment by compare with the characteristics of sand and red clay. The microscopic characteristics include the composition, the pore structure, the particle composition and the microstructure of phosphate rock tailings. The experimental methods include the X-ray fluorescence spectrometer, the BET test, the desorption-adsorption test and the scanning electron microscope. The results show that the composition of tailings mainly includes the dolomite. The particles are fine because of the flotation process. It is very different with the other metal ore tailings, the red clay and the sand. The pore structure of phosphate tailings is mainly mesoporous. The absolute content of matrix pore is relatively low, but the relative content is relatively high that is compared with the the red clay. The matrix pore volume ratio controls the value of the matrix suction of unsaturated tailings. The matrix pore is the main cause of the permeability and compressibility of tailings. The tailing of phosphate rock is made up of the fine particle, and the structure is a single particle structure.(3) A new particles model is advised in this paper on the base of the analysis about the particle structure of the tailings material. The values of matrix suction are tested by the filter paper method of phosphate that includes the tailings, the sand, the silt and the red clay. The experimental results show that the maximum value of the matrix suction is relatively small, and is much smaller than the value of the clay, and is slightly less than the value of the general silt, and is is similar to the value the sand. The shear strength tests of tailings are done with different dry density and different saturation degree. The results show that the shear strength indexes of tailings with different dry density and different saturation degree are changed, but the difference was not significant. This is related to the matrix suction of unsaturated tailings. (4) A new crack development model based on the wet tailings fracture is advised that is satisfied with the Saint Venant principle. The stress state and the cracking of tailings materials were studied by using the variation of moisture stress. The quantitative expression is advised to compute the initial crack spacing and the width of the tailings.(5) The consolidation characteristics about the saturated and unsaturated tailings are tested in this paper. The compression coefficients under the same saturation of the different dry density are compared in this paper. The results show that the initial consolidation pressure can eliminate the impact that initial dry density influent to the compression characteristics of tailings material. The compression characteristic of tailings is not sensitive to the stress history. Unsaturated tailings material has obvious periodic compression features. In the initial stage of test, the gas pressure in the tailings material dissipates quickly, and almost shows no gas compression. The initial deformation equation is advised without using the Biot equation. The experimental data of the consolidation characteristics about the saturated tailings show that the compression and consolidation behaviors of the saturated tailings are silimar to the behaviors of the unsaturated tailings. The soil samples are tending to be stable in the small initial consolidation pressure. The stress history has little influence on thecharacteristics.(6) Based on the analysis of coupling mechanism of the seepage field and the stress field of a saturated-unsaturated soil, the new two-dimensional and three-dimensional finite element model of the tailings dam are used to analyze the seepage field, the stress field and the stability under all kinds of load case. The minimum depth of the saturation line and stability safety factor under different load cases can meet the requirements of the specification. The stress distribution is nearly horizontal, and increases with the increase of depth. The distribution of the extreme value of displacement has the symmetric characters and the trend of diminishing along the way from the center to the outward.(7) The dynamic finite element model of the tailings dam in the high intensity areas is used to analyze the dynamic responses under all kinds of load case with the nonlinear time history analysis method. The dynamic responses, the whole analysis process of the dynamic stability, and the liquefaction analysis are done. The safety factor values are obtained that they are greater than the requirement values in the standard. The areas around the initial dam and the sub-dam are more dangerous areas.