Back Analysis of Mount Polley Tailing Dam Failure

The motivation of this research was to verify the findings of Independent Review Panel for the failed perimeter embankment at Mount Polley Tailings Storage Facility. The finite element analysis conducted in industry standard geotechnical softwares indeed confirmed that the foundation of the perimeter embankment would have failed along the weak GLU layer because of excessive shear deformation in the layer. Additionally, the finite element models also highlighted the importance of choosing correct relative stiffness values for various foundation layers for a realistic development of failure mechanisms for stability analyses done using the Strength Reduction technique. This observation is novel and is of considerable practical significance because designers pay little attention to obtaining stiffness parameters for foundation soils and focus more on obtaining realistic shear-strength parameters. A numerical parametric study conducted to investigate potential ways of preventing failure of the perimeter embankment revealed downstream slope flattening to be the most effective solution

The remediation of surface water contamination: Wonderfonteinspruit

When mining activities in some parts of the Witwatersrand were discontinued in 2000, the defunct workings started to flood. In September 2002 the mine water started to decant from the West Rand Mine Basin (WRB) next to the Tweelopie East Stream. Treated water is currently used in the mine's metallurgical plants and 15Ml per day of treated water is disposed firstly into the Cooke Attenuation Dam and then discharged into the Wonderfonteinspruit. The aim of this study was to find and provide remediation measures as a result of acid mine drainage and other impacting factors on the water quality and volume in the Wonderfonteinspruit. Conductivity and total dissolved solids (TDS) were highest at the point where the tailings dam leached into the Wonderfonteinspruit. Sulphate was very high as was expected due to acid mine drainage. The best way to treat the high sulphate levels is with sulphate-reducing bacteria. To avoid the fatal flaw of many other constructed wetlands, a continuous carbon source is provided to the bacteria in the form of activated sewage from the Flip Human sewage treatment plant. Iron and other heavy metals are being precipitated through oxidation reactions to form oxides and hydroxides from the aerobic cell in the wetland. The wetlands are also known for their ability to reduce nitrate and microbial values with great success. In the remediation, four elements that currently do not comply with the SABS criteria for class 0 water, were chosen for improvement: conductivity, dissolved solids, sulphate and iron. Conductivity falls within class 1 and has a maximum of 178 mS/m @25ºC that should be reduced to under 70 mS/m. Total dissolved solids have a value of 1585 mg/l, which is much higher than the prescribed 450 ml/l, making it class 2 water. The last two problematic elements are both considered as class 2 water: sulphate peaks at 592 mg/l where the preferred value is 200 mg/l, and iron should be 0.01 mg/l, not the staggering 0.3mg/l. iv Alternative mitigation methods were identified and analysed for the impacts of the five major contaminators and ultimately the solution comes down to constructed wetlands. This is not a straightforward solution, however, and a specific design to accommodate all the different pollutants and water quality ranges was proposed. The other mitigation methods include a cut-off trench and pump-back system for the tailings dam, as well as the implementation of a monitoring programme. The sewage works should be optimised and better managed. Both the settlement and agricultural sector need to be educated on their representative impacts on the environment and government assistance should be available.ENVIRONMENTAL SCIENCESMSC (ENVIRON MANAGEMENT

Nonwoven Geotextile Filtration Performance with Coal Refuse

The project involved bench scale testing investigating geotextile filter clogging by fine particle intrusion and fabric blinding from coarse and fine coal refuse with nonwoven geotextile filter fabric. The research explored the fine particle distribution through the refuse material and interfacial contact with the geotextile. The coal refuse was obtained from an active mining and coal waste impoundment operation located in Boone County, WV. The research involved testing of non-woven geotextiles with coarse and fine coal refuse under standard and reduced compaction energies in order to simulate field performance of the fabric to the effects of loose compaction conditions, such as direct end dumping, compared with a maximum compaction effort, standard Proctor, occurring in field conditions.;Conclusions of the research identified several key findings. During the compaction of the rigid wall permeameter samples the refuse experiences crushing and slaking effects which for a coarse grained refuse will produce an increase in the percentage of fines. This effect occurs at compaction energies ranging from optimum to reduced (13,288 to 1,417 ft-lb/ft3). The percentage of fines produced appeared to be a consistent percentage increase of approximately 32%. The characteristics of the grain size shift can result in filtration concerns when evaluating the D15 criteria used in the clogging evaluations.;Graphs of the hydraulic conductivity versus time indicated that there was no stable filter developed within the coarse coal refuse specimens. The GSE Lining Corporation\u27s NW 6 nonwoven geotextile was selected as the worst case filter for comparison of the soil-geotextile system because it has the largest Apparent Opening Size (AOS) of 0.212 mm. Results indicated that for both the initial (non-compacted) and the compacted refuse at all compaction density ranges that the Retention and Permeability Criteria were met (passed) both the MSHA and Giroud design criteria. The corresponding results of the Clogging criteria indicated that the refuse failed under non-critical conditions.;Blends of fine and coarse coal refuse were developed in the laboratory which provided for a uniformly graded grain size distribution. This distribution was at 80% coarse to 20% fines (80/20) and at 60% coarse to 40% fines (60/40) which passed the No. 100 sieve. Results of the post compaction grain size distributions for the blended refuse samples identified that the crushed particles formed aggregates and produced a shift in the shape of the curve which resembled more of a coarse refuse material. The aggregation of the fine particles was consistent across all tested samples and exhibited similar ranges of increases to the D85, D60, D15 and D10 particle sizes.;A series of rigid wall permeability tests were conducted on Coarse Coal Refuse and Blended 80/20 and 60/40 refuse samples using a strong pH2, sulfuric acid. Results of the Coarse Coal Refuse and the Blended 80/20 and 60/40 mix, the hydraulic conductivity versus time showed a one-order of magnitude decrease when compared to the water permeated samples at similar testing parameters. For the acid permeated Coarse Coal and 80/20 Blended samples the post grain size distribution indicated that the acid does not alter the Retention, Permeability, or Clogging criteria. For the 60/40 Blended samples the post grain size analysis indicated that specimen passed the Clogging criteria.;The following recommendations are presented for the use of nonwoven geotextiles in non-critical conditions. Post grain size distribution tests should be performed on specimens at the optimum compaction level to observe changes in D 85 for meeting the Retention Criteria requirements. This will take the particle crushing and slaking effects into consideration. It should be noted that the continued or repetitive compaction of the CCR material may reduce the particle diameter to less than the geotextile\u27s Apparent Opening Size (AOS) which then renders potential failure in achieving the Retention Criteria. (Abstract shortened by UMI.)

Use of cement to reduce erosion of the slopes of mine tailings dams

Erosion on the slopes of mine tailings represents one of the most important environmental problems arising from the disposal of mine tailings. Tailings dam erosion is the main source of pollution that contaminates agricultural land and streams around mining areas. There is an urgent need to reduce erosion of the slopes of mine tailings in-order to limit further devastation of natural ecology. The present study investigates the possible use of cement to reduce erosion of the slopes of mine tailings dams. The pinhole erosion test was used to measure erodibility of tailings stabilized with cement. The strength properties of cement-stabilized tailings have been evaluated by the unconfined compression test. Conbex and ordinary Portland cement have been used for tailings stabilization, with contents of 0, 3, 5, 7, and 10% by mass. It has been established that cement can be used to reduce erosion of the slopes of mine tailings dams. At least 3% by mass cement is required to produce zero erosion loss. The resistance of tailings to erosion can be enhanced by increasing compaction density, curing period and cement content. Ordinary Portland cement produced higher strengths and erosion resistance than Conbex, although Conbex may be useful for stabilizing fine tailings. Cement-stabilized tailings could be used to make bricks and rubble for use in reducing erosion of the slopes of mine tailings dams. As little as 10% of cement is necessary to produce unconfined compressive strengths of 1600- 2600 kPa