To quote my comment on the other linked thread about the Kennecott pit wall failure...... "I would point out that sloughs are part and parcel of open pit mines. I've been personally involved in one in Africa, seen the photos of Freeport in Indonesia, and now seen the photos of this one. All of them involved the movement of major league amounts of material (10+ million tonnes minimum). It's part and parcel of open pit mining. As one mine planning engineer once told me, the perfect mine plan for an open pit is after however many years of mine life that it has, the pit walls collapse just as the last truckload of ore leaves the pit. It's going back to the "funnel" I mentioned before, as you go deeper more waste has to be removed so you steepen up the walls of the pit as much as possible in order to reduce the amount of waste that has to be moved and so keep total costs down. The result of this is (to paraphrase Kevin Costner) - "Some days you win, some days you lose, some days it rains" .............