Cause
An investigation into the disaster found that the dams were poorly maintained and the margin of safe operation was very small.
A pipe in the upper dam used to drain water had begun to sag under the weight of sediment, making the dam's drainage less effective. Meanwhile, water continued to be pumped into the reservoir behind the dam, which, coupled with the less efficient drainage, meant the pressure on the bank of the upper dam began to increase. Following the path of least resistance, water began penetrating the bank, causing the soil within to liquefy and weaken the bank until it failed. The water and tailings from the upper dam then flowed into the lower dam, which, under the immense pressure produced, failed thirty seconds later.
Read more about this topic: Val Di Stava Dam Collapse