From our readers: Mark Smith

Dear Editor,

Kudos to Rick Thiel, Ed Kavazanjian and Xuan Wu on their excellent article, “Design considerations for slip interfaces on steep-wall liner systems” (February/March, 2015).
My consulting practice is principally heap leaching and most of my work now is reviewing designs for complex systems, and troubleshooting when things don’t work out as planned. The most complex heap leach pads are valley fills, which in many ways resemble the Lopez Canyon Landfill used as an example in this article. I routinely raise the issue of settlement-induced strains in the liner, yet I’m seeing a trend in the mining industry that is making things worse. Valley fill leach pads are getting more complicated—steeper terrain, deeper ore fills, more challenging climatic conditions. The largest of these will ultimately store crushed ore at depths approaching 200m (ore density is about twice that of municipal waste). The most aggressive slopes are often steeper than 2h:1v, and the slope distance between anchoring benches is often 100m. At the same time, designers are more frequently advocating double side textured geomembranes without providing either of the recommended protection measures (preferential slip surface or veneer-reinforcement layer).

The settlement potential for a leach heap is similar to a landfill, because the ore is stacked in its loosest possible state to encourage percolation of the leach solutions. I have measured settlements of up to 10% of the ore depth, and I’m sure that more is possible. For a valley leach pad with steep sideslopes, the resulting dragdown on the geomembrane can be several meters. This induces stress and strain in the geomembrane, which can and does create tears just below the mid-slope benches. I’m sure that for every tear I’ve seen, and thus been able to repair, there are many more that go undetected. Additionally, a liner in tension is more prone to puncturing and in my experience this is rarely considered in selecting the geomembrane/overliner combination.

I’d also like to thank the editorial staff of Geosynthetics for helping bring this issue to the attention of more designers and operators.

Mark E. Smith
Incline Village, Nev.