Engineering Properties Of Fine Grained Tailings And Dynamic Analysis Of Tailings Dam

The tailings reservoir is a major hazard in mine. Particle sizes of tailings are getting smaller and smaller in recent years, as a result of improvements in mineral processing technologies. And China suffers from frequent earthquakes. Now seismic stability of tailings dam is a major concern of the society. This article takes tailings pond of Changba lead-zinc mine as background. A series of tests are conducted in laboratory and field to investigate fine grained tailings’ deposition law, consolidation and deformation properties and dynamic characteristics. Theoretical analysises are made to interpret the mechanism of collapsed dam and numerical computation is carried out to study the seismic response of tailings dam. This thesis introduces dry-stacking technology of tailings and discusses the earthquake resistant behaviours of dry-stacking method. The followings show the main contents and the conclusions of the research:(1) Laboratory experiments and field tests are conducted to obtain engineering properties of fine grained tailings. Unclassified tailings of Changba lead-zinc mine are fine silts which have good grading distribution and the plasticity index is 7.4. For fine grained tailings, the sediments’ sorting becomes poor. A greater part of dam body is composed of silty clay tailings and the dam shell materials are silty tailings. An undulating model is proposed to describe the distribution characteristics of tailings’ average particle size. For fine grained tailings, geological section of the dam can not be built according to grain compositions, so a better approach based on average degree of consolidation is presented.(2) Cyclic strength tests and resonant column tests of unclassified tailings are conducted. It reveals that when confining pressure increases from 400 kPa to 800 kPa, cyclic shear stress ratio of different cycle number falls by about 0.01. Earthquake magnitude has little effect on the total stress shear strength indexes with internal friction angle between 21.0°and 22.2°. For tailings with optimum water content, there is significant linear correlation between themaximum shear modulus and confining pressure. The dynamic shear modulus and damping ratio versus the shear strain of unclassified tailings fit into three-parameter Davidenkov model.(3) Liquefaction resistance tests of tailings show that development of pore water pressure can be expressed with a double-S model under the condition of low shear stress. Growth process of pore water pressure can be divided into four stages: rapid growing period, steady increasing period, structure damaged period and fully liquefied period. The double-S model is also suitable for sand. Post-liquefaction stress-strain relationship of fine grained tailings can be divided into three stages: approaching zero effective stress period, effective stress increasing period, flowable deformation steady growing period. A three-parameter constitutive model is proposed to describe the stress-strain curve. Confining pressure and relative density influence obviously the flow deformation features of liquefied tailings, while loading rate has no noticeable effects. The value of the final pore-water pressure radio is from 0.7 to 0.9 in the cyclic-loading experiment under different confining pressures.(4) The seismic response of tailings dam is analysed by using Geo-studio software. For an earthquake intensity of 8, the calculating results are shown below. The embankment crest has a horizontal displacement of 7cm and a vertical displacement of 3cm. Horizontal acceleration amplification factor is 1.9, and 2.16 for vertical one. The starter dam crest has a horizontal displacement of-1.3cm and a vertical displacement of-0.1cm. Horizontal acceleration amplification factor is 2.2 and 1.98 for vertical one. Local failure occurs at the lower parts of embankment next to starter dam. It is necessary to take measures to strengthen the construction. Static safety factor of embankment is 1.926 and that of starter dam is 1.894. During the earthquake, the minimal safety factor of tailings dam is always greater than 1.4, which means the dam has favorable whole anti-sliding stability.(5) Engineeriing measures can be taken to improve the anti-earthquake performance of the tailings dam by the following methodologies including changing dam construction method, lowering the phreatic line in the dam, soil reinforcement and soil compaction. A new technology for tailings storage called dry-stacking is introduced. The key of the dry-stacking technology is dewatering of tailings. Relatively speaking, pressure filter has a smaller tailings processing capacity and a higher running cost. That makes it suitable for small-scale mines and paste tailings stacking technology more than for large-scale mines. Geo-studio software simulation results indicate that the dry-stacking dam construction method is significantly better than traditional ones whether in static stability or in seismic stability.