3.1 Shear Strength of dump material
An increase in the proportion of coarse material in fine-grained granular soil results in an increase in friction angle (Holtz and Gibbs, 1956; Holtz, 1960). Typical friction-angle values for medium-dense sand can range from 32º to 38º, while typical friction-angle values for medium-dense sandy gravel can range from 34º to 48º (Das, 1983). Rockfill particles, similar to those found in mine rock piles have internal friction angles in the range of 40º to 50º, the lower end of the range corresponding to fine-grained material, and the upper end of the range corresponds to coarse-grained material (Leps, 1970).
Lewis (1956) concluded that the increase in friction angle with increasing size of particle sizes, was attributed to increase in interlocking particles, interference of particle shear, and increase in dilatational tendencies for the larger particles. For a given void ratio, the smallest sized material has the highest angle of internal friction, and the angle of internal friction decreases as the maximum particle size increases (Leslie 1961).
The angle of shear resistance increases with the size of the particles. This is due to the high stress build up at the contact surface of the angular particles with increasing particle size causing high particle breakage. This also accounts for the decrease in internal friction angle or shearing resistance with particle size.