AbstractRelationships between engineering properties and mineralogical and particle‐size variables were determined for samples from B and C horizons of selected soils. Measurements were made of internal friction (φ), California bearing ratio (CBR), and compression index (Cc). Particle size was determined by pipette analysis and mineral weight fractions were estimated on a clay, silt, sand, and whole‐soil basis using X‐ray diffraction and fluorescence, differential scanning calorimetry, and petrographic grain count analyses. Values of φ and CBR decreased with increasing whole soil phyllosilicate content (kg/kg) and decreasing particle size. Compressibility increased with increasing whole soil phyllosilicate content and decreasing particle size. Correlation coefficients between engineering parameters and most mineral weight fractions for isolated size fractions were low, but interactions between mineralogical and particle size variables were significant. Multiple linear regression models relating φ, CBR, and Cc to soil variables were highly significant, and R2 values for models were high. Interactions of transformed particle size and mineralogical variables were strong contributors in φ and CBR models. Signs of regression coefficients were consistent with the hypothesis that strength decreases and compressibility increases with decreasing particle size and increasing phyllosilicate content.