Figure 1: Re-contouring – excavation detail
There are many factors to consider when planning the costs of final reclamation and closure. Re-contouring the slopes of heap leach pads, waste rock storage areas, or tailings facilities is just one of those. CostMine recently completed a study to compare the costs of a straight slope reduction and a geomorphic reclamation.
Straight slope recontouring uses a bulldozer to excavate horizontal cuts into the stockpiled material and then cast material down the face of the slope starting at the top of the stockpile. The process is repeated until the desired final slope is achieved.
Figure 1 shows how a bulldozer would make horizontal cuts starting at the point of the final desired crest. At a point approximately halfway down the slope, all of the material being moved is rehandled material.
The end result of a straight slope recontour is that the stockpile has a straight line topography. This could look very unnatural in many environments, but is often the method used when a stockpile has reached the edge of a permitted area. With this method, the cost of planned long-term or even perpetual monitoring and maintenance is high due to the erosion potential.
The geomorphic reclamation method creates a more natural-looking final landscape that blends into the local topography and minimizes erosion by mimicking the natural adjacent terrain. The costs for perpetual monitoring and maintenance are reduced or eliminated by using this method, which could balance the initial difference in the costs of recontouring this way.
Of course, the distance that material must be moved is greater for a geomorphic result because the material is not cast directly downslope, making the costs of geomorphic reclamation greater. Material from deeper cuts will be moved to the adjacent extension.
Figure 2: Geomorphic re-contouring plan – crest
Straight slope recontouring minimizes the amount of material moved and maximizes the efficiency of the bulldozer, while the geomorphic contouring will demand more from the dozer for the top half of the slope. Once the slope recontouring reaches the halfway point on either method, the dozer is no longer excavating previously placed material.
Below are samples of the cost comparisons CostMine did for straight slope and geomorphic slope reduction for a 75-metre-high stockpile with various underlying slopes. The cost difference gradually increases as the underlying slope increases. In this example, there is a 2.6-2.8% difference. The difference in costs increases to an 11.9% difference for a stockpile with a 19° final slope, 16° underlying slope, and a 100-metre-high stockpile height (30-metre face width).
Knowing the estimated costs for final reclamation can help make important decisions when placing material in stockpiles. The footprint of the placed material and the height of the stockpile have significant impacts on the final closure costs.
Krista Noyes is a cost analyst/geologist with CostMine, publisher of Mining Cost Service and part of the Glacier Resource Innovation Group, based in Spokane, Wash. She can be reached at email@example.com. Scott Stebbins of Aventurine Engineering provided the figures and cost model engineering used in this article.
CostMine publishes the Reclamation Cost Guide as well as the Reclamation Cost Estimator software tool. The complete guide – including the detailed tables for various scenarios regarding slope and equipment are available for purchase from CostMine (www.costmine.com).