October 5, 2016

Solution Collection Pipes on Slopes

Marvin Silva, Tierra Group International, Ltd., USA;  Jose Cordova, Agnico Eagle Mexico, Mexico;  Francisco Barrios, Tierra Group International, Ltd., USA

ABSTRACT: The solution collection system commonly used in heap leach facilities consists of a basal drainage layer of crushed rock (overliner) and embedded perforated pipes installed above the geomembrane and below the ore heap. The solution percolates vertically downward through the entire ore column, flows along the overliner, and enters the pipes with zero hydraulic head. The pipes are designed and spaced to convey the operational leach and design storm flows while maintaining low hydraulic heads on the geomembrane. Some heap leach facilities are constructed in mountainous terrain with steep slopes where it is not practical to place overliner. This is the case for the Creston Mascota gold mine (Phase 3), owned and operated by Agnico Eagle Mexico. The ore is crushed to a nominal size of 9.5 millimeters (mm) with a maximum fines content of 6.5%. A finite element seepage model was developed for Phase 3 of the heap leach facility to determine the hydraulic head created on the geomembrane for different scenarios. Two zones were evaluated: a bottom area with a “flat” (gentle) slope and a steep (2H:1V) side slope. The bottom area was designed with a 1-meter (m) thick overliner and the steep side slope was designed with no overliner, the crushed ore was in direct contact with the geomembrane. It was assumed that a single pipe at the toe of the slope would be sufficient to remove the solution without having an increase in the hydraulic head on the geomembrane on the slope. However, the results show that a series of solution collection pipes are required on the slope to lower the hydraulic head. The seepage model was run several times decreasing the pipe spacing until a hydraulic head less than 2 m was achieved. The results of the seepage model were compared with the results obtained using the conventional drain spacing equation used in the industry to calculate spacing of drain pipes on flat ground. The results of the seepage model indicate that solution collection pipes are required on steep slopes in order to maintain low hydraulic head on the geomembrane. 

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