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Oil Sands Tailings Consolidation

Problem:

The dewatering of oil sand tailings has become more imperative in the past few years with the creation of Directive 074. Tailings producers are now being required to have plans in place such that it is possible to demonstrate the achieving of five kPa of shear strength within oil-sand tailings deposits in reasonable times. Most existing oil--sand tailings are stored in large ponds which have been shown to be consolidating very slowly. The difficulty in the coming years is to develop strategies for the successful dewatering and closure of tailings storage facilities. The primary technical challenge to overcome is that the numerical modeling of the watering strategies is highly complex as it involves a combination of both large strain consolidation technologies and climate interaction.

Solution:

SoilVision Systems has been investing significant amounts of research in the development of large strain software solutions and applying them for use in the oil-sand tailings area. The SVFlux™ and SVSolid™ products have been modified such that they can handle large strain consolidation behavior. Consulting solutions have been provided for oil companies in the following specific areas:

Figure 1: Pseudo 3-D example numerical model solving tailings deposition

Tailings ponds:

SoilVision Systems Ltd. has been involved in the large-scale multidimensional numerical modeling of the settlement behavior of tailings ponds. The benefit of these models is that the long-term ability of ponds to de-water through self-weight consolidation can be predicted.

Rim-ditching:

A strategy which can be employed is to place either Mature Fine Tailings (MFT) or Thickened Tailings (TT) in smaller holding areas and allow self-weight consolidation and the effects of evaporation to dry the tailings. Tailings in rim ditch facilities are also exposed to freezing/thawing effects of the colder climate. SoilVision Systems Ltd. has been involved in numerical modeling of rim ditch facilities including the related aspects of coupling evaporation effects and accounting for the influence of freeze-thaw effects.

Thin-lift deposition:

One of the strategies which might hold promise is the successive deposition of Oil-Sand tailings in thin lifts. Placement in thin lifts can maximize the influence of evaporation on the dewatering process. Subsequent placement of thin lifts on top of each other becomes challenging to represent in a numerical model. It is also challenging to couple the influences of both large-strain consolidation and Actual Evaporation (AE) from the surface of the tailings material. SoilVision Systems has been involved in being modeling of multi-stage thin-lift deposition while considering both the effects of large strain consolidation and climatic evaporation. SoilVision Systems Ltd. provides consulting services related to geotechnical numerical modeling of oil-sand behavior. If you are interested in discussing potential solutions please contact us here.

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Tailings Consolidation

Problem:

Mine tailings from various types of mining operations are often deposited on the area surrounding the mine site in the form of a slurry. The solids settle out over time and a tailings pile may have a number of lifts corresponding to deposition over different time intervals. The rate of drain-down of the wet tailings pile is of interest for understanding the mine site hydrology and stability of the tailings pile.

Solution:

Numerical modeling of mine tailings can provide value in a number of specific areas: 1) Understanding the rate of discharge of contaminated water out of the tailings, and, 2) obtaining the pore-water pressure conditions at different deposition stages to aid in stability analysis, and, 3) estimation of long-term groundwater conditions. Problems are commonly encountered in numerical models of mine tailings due to the following reasons:

  • Mine tailings typically have irregular 3D geometry.
  • Mine tailings are often deposited in different lifts at different stages of the mine life requiring proper specification of initial conditions and staged models.
  • Mine tailings deposited as a slurry become unsaturated over time and therefore require a numerical model capable of handling the particular nonlinearities associated with unsaturated seepage modeling.

Figure 1: Pseudo 3-D example numerical model solving tailings deposition

SVFlux™ has been successfully applied to mine tailings drain-down applications and proven itself capable of matching results obtained in the field. Specifically, the following features make SVFlux the premier tool for evaluating mine tailings drain-down operation.

SoilVision Systems Ltd. also provides solutions for the large-strain numerical modeling of the consolidation process. Current state of practice typically involves running a 1-D numerical model and assuming that it reasonably represents the tailings long term behaviour in 2-D and 3D. Research has shown that this is not necessarily the case. SVS developmental software provides the solution of fully coupled large-strain multi-dimensional solutions. Such an approach is consistent with historical stress-deformation formulations and is theoretically defensible. Click here to contact us about such a solution.


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Long-term Performance of Earth Covers

Problem:

The long-term performance of earth covers is a crucial component to most remediation scenarios. In particular the long-term percolation through earth covers is critical to the successful closure. The majority of covers attempt to limit the ability of air and water to enter the covered material. The performance of such covers is highly dependent on the intricate balance between precipitation and evaporation and cannot be estimated with empirical or closed-form analytical solutions.

Solution:

SVFlux™ offers climatic coupling using the Penman-Wilson formulation in order to provide an accurate account of the impact of climate on the subsurface flow regime. Solutions are available in 1D, 2D, and 3D. 1D solutions offer a quick and easy solution which may be run out for a period of 30, 50, or 100 years and result in a reasonable estimation of long-term cover performance.

A particular advantage to having SoilVision Systems Ltd. personnel run your numerical analysis is related to the ability to provide advanced probabilistic analysis. If an analysis is run and the answer is that 5% of precipitation percolates through a cover, the obvious question is “How certain are you of this number?”. SoilVision Systems Ltd. offers methodologies for quantifying the uncertainty in your soil cover analysis.


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Tailings Pond Water Balances

Problem:

Rehabilitation of tailings impoundments for final closure is one of the most challenging aspects facing engineers, as not only does the potential for producing leachate pose a challenge to the rehabilitation designer, but also other aspects such as stability and settlement must be considered. The water balance of tailings impoundment is unique in the sense that it usually hosts a pond that in turn causes a phreatic surface in the impoundment. The position of the phreatic surface defines the saturated and unsaturated zones in the impoundment which of course vary spatially and temporally. Predictive modeling for this hydrologic system becomes difficult, as numerical models capable of analyzing the combined saturated/unsaturated zones are not adequately refined to accurately solve the flux boundary problem for infiltration at the surface of the tailings.

Solution:

SVFlux™ and SVSlope® offers significant advantages in the analysis of tailings facilities. In particular, the following advantages may be realized:

  • Saturated / unsaturated analysis :SVFlux provides the ability to model both the saturated and unsaturated zones in a tailings facility. Automatic mesh refinement also increases the accuracy of the unsaturated calculations.
  • Climatic coupling : a significant portion of the water balance for a tailings facility is controlled by the climate in the region. SVFlux implements the comprehensive Wilson -- Penman method to calculate actual evaporation and increase the rigor of the model.
  • 3D analysis :many Tailings ponds are irregular in shape and bass are not well-suited for a 2-D analysis. SVFlux offers the advantage of full three-dimensional analysis of tailings facilities.
  • Slope stability analysis :any model created in SVFlux whether it is 2-D or 3-D can be transferred straight across to the SVSlope package and analyzed within the context of stability. The pore-water pressures can be transferred from the SVFlux analysis straight into the SVSlope analysis.

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Waste Rock Seepage Analysis

Problem:

Waste Rock piles are a common earth structure resulting from extensive mining operations. Such structures can often lead to acid rock drainage (ARD) if the flows into waste rock are not minimized. There are also potential slope stability issues.

Solution:

SVFlux™ and SVSlope® can be used in order to model the flow through a waste rock pile or the stability. Complex geo-strata can be modeled with either package. This complex strata is often necessary due to the coarse-fine layering of rock material. The influences of any particular long-term cover design can be evaluated with SVFLUX.


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