Hydraulic performance and stability of coarse rockfill deposits

Open-pit coal mining operations often generate enormous quantities of waste rock. This is dumped into nearby valleys and the coarsest fraction ends up at the bottom of the deposit; the final result is often referred to as a ‘rock drain’. These ‘flow-through’ rockfill drains can be 100 m high, thousands of metres long, and usually have a stream flowing through their base. When the stream that supplies the rock drain floods, the ability of the drain to convey water is exceeded by its supply, and a large but temporary pool develops at the upstream end of the dump. More importantly, the free surface within the rock drain becomes elevated, as does the point of exit at the downstream face. Because of historic instances of failures at the downstream face, the management of these buried streams and the hydraulic behaviour of the rock drains through which they flow are of interest. The behaviour of homogeneous embankments comprised of coarse rockfill has many similarities to the behaviour of long rockfill drains. Methods for assessing the hydraulic performance of either are presented herein. Definition of the complete water surface ‘envelope’ and the tendency toward unravelling of the downstream face receive primary focus. It is demonstrated that the hydraulic gradient directly beneath the seepage face and small variations in its direction play a far greater role than seepage-face overflow in promoting unravelling failure, and argues that all such deposits are, in effect, embankment dams that should fall under the applicable national dam safety regulations.