THE EFFECTS OF ALTERATION AND WEATHERING ON MINE SLOPES AND WASTE ROCK PILES: IMPLICATIONS FOR LONG-TERM MANAGEMENT

The changes that can occur in pyritic mined waste rock or mined rock slopes, due to alteration and weathering, can have considerable consequences for the long-term stability of slopes and piles. Evaluating and predicting the geotechnical property changes resulting from such long-term physical and geochemical weathering has not been standard practice in the industry. Research into the long-term effects of such weathering is in its infancy. The extent to which material properties change is dependent on a number of factors, including the type and degree of alteration that occurred during mineralization, the sulphide content and reactivity, the alumino-silicate and clay make-up of the material, water-rock interactions etc. When mined, the material undergoes additional weathering as a consequence of exposure to air and water. Mining of altered and acid-generating sulphide containing waste rock increases, by several orders of magnitude, the surface area of the rock surface exposed to air and water resulting in increased rates of slaking (physical weathering) as well as geochemical weathering, hydrolysis, fragmentation and breakdown of the rock fabric, and an increase in the percentage of fines, including clays. The weathered rock can exhibit reduced strength properties. Alternately, there can also be cementation of particles as a consequence of precipitation of dissolved constituents. The mineralogical make-up of the cement and its solubility under the (often acidic) conditions in which they are placed will affect strength. Changes in particle grading and mineral composition over time result in changes in both the permeability and shear strength of the mine rock. These changes in turn can have large consequences for the long-term stability of mined slopes and waste rock piles formed from such altered and sulphide containing mine rock and need to be included in long-term predictions and planning for mined materials.