Classification of expansive soils using void ratio index: an experimental study of bentonite-sand mixtures

Abstract

This study investigates the one-dimensional consolidation and swelling behavior of compacted expansive soils upon hydration. Four distinct expansive soil mixtures were prepared by blending fine sand with calcium bentonite at varying weight percentages (40%, 55%, 70%, and 85% bentonite) to model a wide spectrum of swelling potential. Oedometer consolidation tests were performed using two different loading pathways following the free swelling phase: Method 1 involved mechanically restoring the specimen to its original pre-swell height prior to consolidation testing, whereas Method 2 completed the consolidation process directly from the equilibrium state reached after full swelling. To characterize these structural volumetric transitions, a Void Ratio Index (VRI) was derived based on the initial void ratios (e_0) and those measured after free swelling (e_fs). The laboratory results revealed a clear distinction in compression behavior between the two methods; while the swelling index (C_s) remained practically constant across all mixtures, the compression index (C_c) exhibited a sharp increase under Method 2 due to the structural collapse of newly formed macro-pores. The proposed index (VRI) successfully classified the swelling potential into specific structural categories (ranging from low at VRI<25% to very high at VRI>60%), demonstrating a strong correlation with swelling pressure (22-113" kPa" ) and final water content. This newly developed index provides a practical, physics-based framework for predicting the compressibility of expansive soils without requiring lengthy laboratory testing cycles.