Strength and modulus evolution in laboratory prepared deep soil mix samples and implications in design for high plasticity clays

A comprehensive laboratory testing program was conducted to investigate the strength and modulus evolution of laboratory-prepared deep soil mixing (DSM) samples, focusing on inorganic high plasticity clays and peat soils. Unconfined compressive strength tests were performed on specimens prepared using different cement types and dosages, water-to-cement (w:c) ratios, and curing durations. Particular emphasis was placed on the influence of initial water content and soil consistency in high plasticity clays. The results demonstrated that the unconfined compressive strength (q u ) of high plasticity clays is strongly governed by initial consistency, quantified using the Liquidity Index (LI), rather than by the total water-to-cement ratio alone. To systematically capture this behavior, a Consistency Dependent Mixing Efficacy (CDME) framework is proposed, linking soil consistency, mixing efficiency, and achievable strength within a unified interpretation. The experimental data indicated that maximum strength development occurred within an intermediate consistency range, approximately LI ≈ 0.6–0.7, where measured strengths reached up to about three times the FHWA (2013) prediction. Peat soils exhibited limited strength improvement, with unconfined compressive strength values remaining low even at increased cement contents, particularly when the initial water content was high. Correlations between q u and secant modulus (E 50 ) were proposed for both soil classes providing practical input for serviceability-based design. Strength evolution with curing time was evaluated using a curing factor (f c ), showing continued strength gain with increasing curing duration. Mercury intrusion porosimetry tests conducted on selected clay samples revealed consistency dependent pore structure variations, supporting the observed macroscopic strength trends. Overall, the findings highlight the critical role of initial soil consistency and mixing efficacy in DSM performance and demonstrate how the proposed CDME framework can be used to extend conventional design approaches beyond total water-to-cement ratio based strength prediction.