Open Pit Modeling


Courtesy of SoilVision Systems Ltd.

The Problem

Open pits are a common method of mining in the world. The design of such pits requires the application of technologies from a number of fields. One of the common scenarios is that the bottom of the pit may possibly extend beneath the water table of the surrounding geo-strata. If this is the case then a suitable set of pumping wells must be designed in order to ensure that the bottom of the pit does not flood. The spacing and location of these wells can be complicated by the conductivity of the surrounding material layers. Thus it is unlikely that such calculations can be performed by hand.

It is also known that many open pits are not geometrically symmetric. Therefore, it is difficult to create a 2D or axisymmetric numerical model that adequately represents the real world situation. Many numerical models only support 2D or axisymmetric types of analysis and are therefore inadequate for complex non-symmetric pits.

Many open pits also have complex soil or rock layers intersecting the pit. This introduces the added complexity of pinching out 3D layers. This pinching out process has traditionally been difficult to do in any three-dimensional model. 3D finite difference numerical models typically require the user to extend all layers throughout the entire numerical model. This creates additional difficulties with layers which intersect the pit walls and often causes numerical modeling projects to extend from weeks to months in duration.

Each pumping well installed around an open pit has a limited zone of influence. If the pumping wells are not spaced close enough then there is the danger that seepage between the wells could cause flooding in the pit. If the wells are spaced too close together then the mine site has incurred the additional and unnecessary cost associated with the installation of pumping wells which are not needed. There is need to optimize the spacing, depths, and pumping rates of such wells such that the costs are minimized.

The Solution

The SVFlux™ finite element seepage modeling software implements state-of-the-art meshing algorithms which can handle complex pinch-outs of layers intersecting pit walls. Therefore intersections with geo-strata can be represented as exact intersections and layers do not need to be extended throughout the model. Stated another way, layers can pinch out to a true zero thickness.

In northern climates, it is also an option to investigate the use of thermosyphons to freeze saturated areas around the pit. The frozen areas than represent zones of reduced flow. It is possible using the SVHeat™ software to model the potential behavior of thermosyphons around an open pit. This scenario will also be investigated and a typical model presented which solves this type of simulation.

Well objects have also been implemented such that complex pumping scenarios can be simulated in the software. Pumping rates or target elevations can be entered into each well object. Well objects can also be entered at any depth and the pumping well screen can intersect any particular geo-strata.

There are a number of technical challenges which present themselves during the creation of a pit dewatering or a freezing numerical model. The challenges for creating such a numerical model may be summarized as follows:

  • Well placement / mesh design: during the design process, it may be desirable to try a number of different well placement strategies. This requires the creation of a number of different numerical models with wells placed in different locations. Each time such a numerical model is created, it requires an optimized mesh such that mesh density is placed around well locations. The automatic mesh generation feature in SVFlux™ is ideal for the fast generation of multiple modeling scenarios.
  • Unsaturated flow: the creation of the watering scenarios implies the correct handling of unsaturated soils above the water table. SVFlux™ has been optimized to handle the nonlinearities associated with unsaturated flow. The automatic mesh refinement can automatically refine the mesh around zones of unsaturated flow to minimize numerical instability and optimize model accuracy.
  • Complex pinch-outs with geo-strata: another potential problem area is where layers of geo-strata can pinch out as they meet the pit walls. SVFlux™ makes use of tetrahedral elements in the mesh and therefore can represent true pinch outs of geo-strata layers against the pit.
  • Mesh refinement: pumping of wells around a pit implies that high gradients may be present next to the pumping wells. Such high gradients have traditionally been the source of numerical convergence issues in fixed mesh simulations. The dynamic mesh refinement abilities of SVFlux™ result in automatic mesh refinement around well objects. This ability enhances model stability and accuracy.
  • Optimized modeling of wells: the latest version of SVFlux™ implements a new method of representing pumping wells. This new method offers a more stable way of representing flow in and out of a large number of wells.

Engineers at SoilVision Systems Ltd. have been working hard on adding features to simplify the creation of such open pit models. The latest versions of the SVHeat™ and SVFlux™ software allow creation and solution of such open pit models. Therefore models which were previously technically challenging are now made routine.

The first example in the figures below shows the application of SVFlux™ to an example pit. This pit is irregular in shape and must be modeled in 3D. In this example, a number of wells were installed around the edges of the pit. Pumping rates were applied in the numerical model such that constant heads were maintained in each of the wells. This allowed solution of the numerical model and plotting of the resulting water table. It can be seen that the resulting water table is lowered to the point that it does not intersect the bottom of the pit. The comprehensive graphics in the software allow for a number of visualization scenarios. The pumping rates required to maintain these heads can be output by the software.

The zone of influence of each pumping well can be seen in this example by plotting the water table as an isosurface. The true 3D nature of the geo-strata is represented along with the resulting impact on the water table. This model could be further extended with a few clicks of the mouse to simulate the influence of additional pumping wells.

What sets SVFlux™ apart for modeling seepage in open pits?

  • Fully 3D
  • Complex 3D geo-strata can be modeled
  • Intersections between pit walls and surrounding geo-strata can be modeled with true pinch-out zones to zero thickness
  • Well objects allow for fast application of internal boundary conditions
  • Unsaturated flow or climatic influences can be modeled
  • Anisotropic flow at any 3D angle
  • Models can be transferred to SVSlope® or SVSolid™ for further analysis

A number of additional numerical models which illustrate the concept of solving content models were created to demonstrate the abilities of SVFlux™ and SVHeat™. These numerical models are shown below. A new project called 'Pits' has been created in the distribution models. All the numerical models presented in this article may be opened and run with the student version of the SVOFFICE 2009 software.

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