Dynamic properties of fine-grained soils engineered with a controlled organic phase

Soils with high organic content are frequently encountered beneath earthquake sensitive infrastructure, such as bridges or levees. Historically, the dynamic properties of these organically rich soils have been difficult to predict due to the heterogeneity of the natural organic matter that is found in natural soils, even though their response to dynamic loading remains critical to assessing the ongoing stability of the infrastructure. In this study, an experimental investigation was performed on a montmorillonite soil that was modified with a controlled organic phase. Quaternary ammonium cations were exchanged onto the soil particle surfaces through cation exchange with the clay′s naturally occurring cations (e.g., Na⁺, Ca²⁺). Quaternary ammonium cations with a variable structure were chosen, which allowed control on the cation′s size and length of alkyl chain, as well as a control on the density of organic loading on the clay surface. The dynamic properties of organoclays were then quantified experimentally using resonant column and bender element tests. This study demonstrated that the increase in the total organic carbon content of the soil increased the shear wave velocity and stiffness of the soil (Gₘₐₓ) due to a reduction in the void ratio of the organically rich soil. Cation structure did have a measureable impact on the soil stiffness, with organic cations with carbon concentrated primarily in a single tail demonstrating higher stiffness than those soils engineered with a branched cation structure. When compared to inorganic soils, the presence of the organic cations in the soil increased the range of linear elastic behavior of that soil, with the organoclays having a threshold strain of 0.024% or higher. The soil samples with the largest percentage of total organic carbon and the lowest void ratio demonstrated the largest damping ratio (ratio between dissipated and stored energy) during cyclic loading at small strain. Regression analysis of the dynamic test results demonstrated that the total organic content and the void ratio were the most dominant factors in determining Gₘₐₓ for the high organic content clays.