Hydraulic Behavior of Progressively Lengthened Mine-Waste Dumps

Abstract

Essentially, linear mine-waste dumps at open-pit mine sites represent expedient places to deposit large quantities of waste rock. Being placed in valleys in most cases, the effect of the water flowing through the deposit often raises questions about its hydraulic behavior and geotechnical stability, especially of the downstream toe. A general understanding of the hydraulic behavior of long embankment-like deposits of homogeneous coarse rockfill, whether proposed or existing, is presented in this study. Laboratory and analytical studies have resulted in better tools for assessing the long-term behavior of these deposits. First, the flow-field angle and effective hydraulic gradient theories were examined for predictive capability. Second, the accuracy of a class of methods originating with Casagrande, with and without incorporation of this flow-field angle, was compared. The common purpose is to predict the point of first flow-emergence on the downstream face. The flow-field angle approach was the best of the six compared. The linking of the upstream and downstream rating curve yields a simple relationship between the overall hydraulic gradient and the gradient that drives the flow out of the toe. Third, the relative depth coming out of the toe is found to be related to the porous media analogy of critical depth but with a restriction on the conveyance. Finally, the ideas of a hydraulically short (or long) condition and of an exit Froude number are introduced. These are used to circumscribe three paths of behavior. The resulting triangle is defined by first varying just flow, then both head and flow, and then just upstream head. The relative elevation of the point of first emergence of hydraulically long waste dumps was found to be fairly constant for a given upstream water level. The exit Froude number settles to a value of about 0.15 but with qualifications for dumps that are very long.