Effects of Clay-Sized Fines Content on the Flow Instability of Mixed Granular Materials: A Case Study of Red Mud

The particle size, morphology, and mineralogy of fines all contribute to the flow instability of mixed soils. However, few experimental studies have isolated the influence of particle size from the morphology and mineralogy when investigating the impact of clay-sized fines content on the flow instability of mixed soils. To fill this research gap, in this study, red mud—characterized by its clay-sized fine fraction and the consistent morphology and mineralogy of both its fine and coarse particles—was employed for such investigation and examined using monotonic undrained triaxial tests prepared via moist tamping. The results indicated that the changing fines content resulted in parallel translation and rotation of the critical state lines in the volumetric plane. Three regions of the critical state friction angle were identified with increasing fines content; the liquefaction potential, LP, first increased at lower fines content, then decreased as the fines content continued to rise, before increasing again at very high fines content. An analysis of the intergranular microstructure under the framework of critical state soil mechanics provided a rational explanation for the observations described previously. The sieve size adopted in moist tamping was also proven to influence the critical state due to the cementitious nature of the red mud.