Effects of Fines Content and Plasticity on Index Properties and Hydraulic Conductivity of Coarse-Fine Mixtures

Ten coarse-fine mixtures (CFM) (intermediate soils) having fines contents (FC) of 0%, 12%, 20%, 30%, 40%, and plasticity indices (PI) of 5% and 15% were prepared in the laboratory by mixing different fractions of gravel, sand, nonplastic silt, kaolin, and bentonite. Index properties, volume change during saturation, specific surface area (SSA), and saturated hydraulic conductivity of CFM were investigated. For compacted CFM, a FC=15% (for PI=5%) and FC=12% (for PI=15%) represent the transition between coarse- and fine-dominant behavior based on intergranular void ratio. The total SSA of CFM may be estimated from the weighted average of SSA determined individually for each soil fraction since the SSA of the cohesive portion was observed to be linearly proportional to the percent fractions of kaolin and bentonite. Hydraulic conductivity estimations based on the SSA of soils proposed by the Kozeny-Carman equation underpredicted the hydraulic conductivity of CFM by one to four orders of magnitude. Alternatively, the formulations from consolidation theory and modified Hazen's correlation utilizing effective particle size, D-10, and void ratio, e, estimated the hydraulic conductivity within an order of magnitude. Although hydraulic conductivity decreased with the increase of FC and PI, a smaller amount of decrease was observed in low plastic CFM. (C) 2022 American Society of Civil Engineers.