DumpSolver Description

The software component of DumpSolver has been specifically designed and written to optimise waste dumps.  It is not an "inverted" pit optimiser and therefore the set-up, processing and output for a project are relatively simple to implement and comprehend.

DumpSolver uses blocks, or cells, to model waste dumps.  Unlike block models used to define mineralisation, which may use small blocks or blocks capable of being sub-celled, waste dump modelling can adopt a coarser approach without any loss in accuracy.  Typically the waste dump model blocks will be 25-50 metres in the lateral dimension with the vertical dimension representing the dump lift height.

The basic concept behind DumpSolver uses the incremental horizontal and vertical haulage costs for a specific truck fleet to calculate the dump cost from a pit exit point to every possible block within the model.  Additionally, the program evaluates the benefit of using a block for dumping or leaving it undisturbed.  The program sorts the block values and chooses the cheapest combination of blocks to satisfy a particular dumping schedule.   It is the only software that can derive the current most economic optimal waste landform. 

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DumpSolver takes account of:

• Current topography and rill angles
• Surface constraints such as existing infrastructure.  Primary constraints (unmoveable) and secondary constraints (impact on cost of dump)
• Land costs (acquisition, clearing, rehabilitation etc.) and dozing costs
• Different materials with individual swell factors or material that requires encapsulation (e.g. ARD potential)
• Haulage costs or efficiency (effective flat haul)
• Alternative dumping locations to control dump progression (either single entry point or 3-D ramp line)
• Required sequencing of dump in multiple stages. Uses "bench by bench" or "best NPV" basis.

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DumpSolver determines whether a destination cell for a dump block is located within the rill zone or dumping zone.  An extreme example would be when a mine is tipping waste off the side of a mountain when the whole dump will be in the rill zone.  In complex tipping mode, the location of a destination cell will take account of a user-defined rill angle.

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Dumping constraints can be one or more or a combination of horizontal planes, polygons,  and surfaces (or DTMs).  The individual constraint components are usually prepared using SURPAC mining software which can import and convert data from most of the popular mine planning package.

Primary constraints are used to simulate immoveable boundaries such as the final pit perimeter (unless backfilling is envisaged), the treatment plant etc.

A secondary constraint may be moved if economic and/or environmental justification can be made.  An example might be, "no dumping above" a specific horizontal plane. In which case, DumpSolver can evaluate the impact of a height restriction on the waste dump.  Dumping may also be restricted "inside" or "outside" a polygon that may represent permanent infrastructure, so-called immovable objects or a lease boundary etc.  As a practical example, the orebody may be potentially open along strike and a polygon can be used to evaluate the impact of deferring sterilisation drilling.

Surface-based constraints can be used to model and evaluate pre-determined landform profiles (like a jelly mould).  This can be useful to define a benchmark for an existing dump design.

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Land costs are represented as area costs and usually expressed as $/Ha.  With this, DumpSolver has the ability to evaluate if it is more economic to leave a portion of land untouched or to be used for waste dumping.  DumpSolver will require a default area cost but in more complex cases, specific area costs can be modelled using polygons.  In a recent case, the mine had different site preparation costs for different portions of the lease. 

If area costs are high, there will be a penalty if a dump has a large areal extent.  The Dump Reforming module controls the method of "clawback" used to ensure an optimal balance is maintained between land costs and dumping costs.  The program will relocate donor block cones to recipient locations either inside or outside the dump zone.  In circumstances where area costs outside the dump zone are significantly lower, waste blocks will only be relocated to the new area provided the additional haul costs are compensated by a commensurate reduction in area costs.  The higher the area cost, the more impact this function will have on the shape and ultimate construction cost of a waste dump.

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DumpSolver takes account of multiple material types since waste from different weathering profiles may exhibit different swell characteristics.  During sequencing, the program is also able to track the whereabouts of specific rock types within the model.  This is particularly useful for example when certain rock types are inert and others are susceptible to acid rock drainage (ARD) .  In such circumstances, the program will report whether the ARD rocks are partially or fully encapsulated.  While the program cannot solve the problem directly, it will show which blocks will not be encapsulated and allow an engineer to take specific remedial action (using constraints) to model and evaluate a solution within the optimal dump profile.

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Horizontal and vertical incremental haul costs are input as c/100m and c/10m respectively.  They are generally calculated using a truck and loader simulation program, such as TALPAC or other analysis.  A haul system is set up using the appropriate material characteristics, haul profile, shift roster, loader and truck types along with equipment operating costs.  The vertical incremental haul cost is estimated for a gradient representative of the waste dump haul ramp.

The program can also take account of costs associated with each lift of the dump, referred to as "dozer costs".  One study determined the optimal dump lift height.  As lift height is increased, the vertical haul cost for every tonne of waste dumped will increase but the the dozer costs will decrease.  Conversely, if lift height is decreased, the vertical haul cost will decrease but dozer costs increase.  There is a point where total costs for the dump will be minimised, this is the optimal dumping height.

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DumpSolver uses a variety of dumping point methods.  The simplest approach adopts the Entry Point method, which will often be the pit exit.  This is represented as a single north, east, elevation coordinate.  Ramp Strings provide a more complex technique.  Strings may be either 3-D or 2-D.  The 2-D method, DumpSolver calculates the levation which will depend on the shape of the dump and the user defined gradient.  Both 2-D and 3-D methods control the direction of the progression of a dump and are usually applied once the Entry Point method has been applied and the engineer must take account of practical issues in the location of the ramp.

Other more complex techniques use the Hoop or Finger Ramp methods.  The Hoop Ramp is applied in situations where the shortest practical route between the start point and a dumping point is not a straight line.  A good example of this would be a curved pit perimeter as trucks are not able to fly!  The Finger Ramp is appropriate for detailed planning work where a network of ramp strings is defined for each lift of the dump from a common start point.  DumpSolver will choose the cheapest path to fill a dumping block.

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Currently 15 individual stages can be specified to define the dump construction.  Typically, each stage can represent either a time increment, a material type or benches within the pit.

DumpSolver allows two sequencing options.  The Best NPV method defines a construction sequence that minimises haul costs - in other words cheapest blocks are dumped first.  This may not be feasible as multiple dumping faces may be established.  The other construction method is By Bench.  This is always feasible but discounted cash flows will not provide the highest NPV.  The two sequencing methods are conceptually similar to the Best and Worse Case schedules developed by Whittle Four-X in that they define upper and lower benchmarks for discounted cash flow analysis.

The Stage Analysis module will also prepare a detailed report in Excel format that will provide the dump centre of gravity for each stage and the tipping centre of gravity and extents.  This centre of gravity information is invaluable for detailed productivity and truck and shovel matching studies.  For owner operated fleets, this will allow a more accurate assessment of capital cost expenditure over time.  For earthmoving contractors, it is invaluable information as it will ultimately enable a more accurate calculation of load and haul rates and avoid risk factors that are commonly incorporated into such rates.  After all, how many mining contracts specify where waste from a particular bench is to be dumped?

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