The Effect of Fines Content on Strength of Granular Materials

Abstract

This study focused on the effect of fines (minus 75 microns) content and type (plasticity) on the strength of granular base materials. Four different sources of crushed rock Granular A materials were selected by the MTO for testing. Routine laboratory tests were carried out on the four Granular A materials including sieve analysis, standard Proctor compaction, Atterberg limits, California bearing ratio, resilient modulus and permeability tests. Fines from an additional source, to be plastic, were selected and used as substitute fines for the Granular A natural fines but were found to have a similar index of plasticity (PI). The influence of fines content on CBR varied for each Granular A source as some increased with increasing fines content and others decreased as the fines content increased. With the similarities in the PI of the two fines no clear trend in CBR values between samples with natural and substitute fines was observed. The resilient modulus (MR) values were by and large similar when the natural fines were replaced with substitute fines. The MR decreased as the fines content increased when the water content was below 5 percent but at 7 percent water content some of the tests failed (could not be tested) at higher confining pressures. Energy dissipation was calculated from the resilient modulus tests and it was observed that the energy dissipation decreased as the confining pressure increased, with lower dissipated energies corresponding to higher MR values. Given that definitive trends could not be identified with respect to the influence of fines and plasticity, the influence of the percent paste (sum of moisture and fines content at the time of compaction) was examined. As the paste increased the resilient modulus decreased and the damping ratio increased. Results from permeability tests showed that for specimens with no fines present the permeability was much higher than those with fines. There was no significant change observed in permeability for specimens in which natural fines were replaced by substitute fines or as the hydraulic gradient was varied.