Physical and mechanical properties of a compacted silty sand with low bentonite fraction

Abstract

Compacted granular soils with small additions of bentonite have been used to build geotechnical structures such as impervious liners and cores of zoned earth dams. This paper presents a laboratory study showing how physical and mechanical characteristics of a silty sand are modified by a low percentage of bentonite. The effects of the addition of bentonite on the silty sand are reflected by an increase in the plasticity index, a reduction in maximum modified Proctor density, and a decrease in hydraulic conductivity. The most significant consequences on the mechanical properties are an increase of compressibility and secondary consolidation coefficients, and a reduction in shear strength. Different mixtures were either dynamically compacted at the optimum water content (compacted samples) or prepared after slurry consolidation from the minimum density (remoulded samples). Although the compacted and remoulded specimens show different isotropic compression lines, their critical-state lines in the v:p':q space are identical, where v is specific volume, p' is average effective stress, and q is deviator stress. Comparisons of the mechanical parameters with the existing literature database show that the compression coefficients of the remoulded mixtures are comparable to those of normally consolidated clayey soils of similar plasticity; nevertheless, those of the compacted mixtures are considerably lower. Also, the slopes of their critical-state lines in the q:p' plane are in good agreement with those predicted by empirical correlations for fine-grained soils.Key words: bentonite, silty sand, compaction, physical properties, compressibility, critical state.