BHP is planning for closure at the Miami Closed Sites copper mines in Arizona (USA). A review of the original designs and the performance of the existing post-closure landforms identified long-term erosional stability of the cover systems as key to successful closure. In observing the 10 or more years of post-closure monitoring and maintenance of these cover systems, BHP conducted new field erodibility studies and modelling to re-assess the cost of new landform and/or cover systems with the benefit of reduced long-term maintenance.
In 2018, BHP commenced short-term erosion studies to quantitatively define the erosion potential of various cover systems. The 100m2 individual test areas included undisturbed native soil material (Gila), a reconstituted Gila cover, Gila with different sized quarried rock placed as rock armour on the surface, and placement of screened (coarse) Gila over the screened Gila fines. These test areas were subjected to simulated rainfall and overland flows, and measures of infiltration capacity, interrill and rill erodibility, and sediment size and density were made.
Gila was found to be quite erodible in its undisturbed state and when reconstituted. Application of smaller-sized quarried rock as surface armour reduced erosion potential. However, erosion rates were higher when the larger quarried rock was used. Use of screened coarse Gila reduced erosion potential.
The erodibility and sediment parameters were used within the WEPP erosion model to design batter shapes not prone to the high rates that lead to rill or gully erosion. These designs informed the design of large-scale erosion plots that will run over the medium-term (~4 years) and provide validation data from which the WEPP model’s accuracy can be confirmed or improved, increasing the confidence its long-term predictions and as a result the stability of the closure cover systems.
