5.1 Bulk Gradation

The overall gradation of a mine rock has direct impact on the shear strength and the permeability characteristics of dump slopes. In general, coarser materials with few fines, have higher strength and hydraulic conductivity than materials with appreciable fines. Where mine rock contains less than about 10% fines, the most important factors controlling gradation are hardness and compressive strength of the rock fragments. Coarse dump materials generally derive their strength from interparticle contacts and exhibit engineering properties similar to rockfill.

The gradation of mine rock depends on a wide variety of factors, including: lithology, durability, frequency and character of discontinuities, blasting and excavation technique, handling and transportation, placement methods, and other factors. Gradation may also change with time, due to mechanical or chemical breakdown (e.g. freeze thaw, swelling of clay minerals, oxidation, etc.).