Abstract:

[Objective] Cohesive non-swelling soil (CNS) covering technology, when applied to the in-situ treatment of expansive soil foundations and slopes, frequently necessitates the modification of the expansive soil with traditional additives like lime to prepare suitable CNS materials. Research on the treatment of expansive soil using hydroxy-aluminum remains limited, and its application as an in-situ CNS material has not yet been reported. This study aims to ascertain the viability of using chemically stabilized soil (CSS) with hydroxy-aluminum solution as a CNS cushion layer material through laboratory experiments. [Methods] A series of basic physical-mechanical, chemical, and microstructural tests were carried out. Changes in particle size distribution, Atterberg limits, and compaction indices of soils were analyzed to evaluate the modification effect of hydroxy-aluminum on expansive soil. Subsequently, the permeability, shear strength, and swelling characteristics of the expansive soil (ES), CSS, and CNS were investigated under varying degrees of compaction (85%, 90%, 95%, 100%). Ion concentration analysis of soils and microstructural analyses (XRD, SEM) were also conducted to assess the overall performance of CSS and validate its potential as a CNS cushion layer material. [Results] (1) Following the addition of the hydroxy-aluminum solution, flocculation and agglomeration occurred between the hydroxy-aluminum and the clay particles of expansive soil. This process resulted in a reduction in the dispersibility and hydrophilicity of expansive soil, leading to denser particle packing. Consequently, the particle size distribution of expansive soil shifted, with an increase in silt content from 31% to approximately 46%, and a decrease in clay content from 65% to 51%, indicating a trend toward silty soil. (2) Plasticity index exhibited a substantial decrease, with a 43.5% reduction from 38.06 to 21.49. This decline resulted in a transformation of the soil classification from high-liquid-limit clay (CH) to low-liquid-limit clay (CL). These changes demonstrated a marked improvement in the basic physical properties of expansive soil. (3) Under varying degrees of compaction, the CSS exhibited substantial improvements in permeability, shear strength, and swelling characteristics compared to expansive soil. The permeability coefficient increased from 10^-8 to 10^-9 cm/s to the order of 10^-7 cm/s, reaching a level comparable to that observed in the CNS. The shear strength parameters were enhanced; notably, at high compaction degree (K=100%), the shear strength of CSS even exceeded that of CNS. The swelling potential of CSS was significantly reduced, with the development of swelling deformation following the same trend as CNS. The swelling percentage decreased from 16%-24% to 8%-15%, representing a reduction of 37.5%-50%, which was slightly higher than CNS but still within the range of non-swelling soil. [Conclusion] Overall, the comprehensive performance of CSS was found to be essentially equivalent to that of CNS. The modification of expansive soil by hydroxy-aluminum solution primarily involved physicochemical reactions, including adsorption, ion exchange, and flocculation-agglomeration. The concentrations of K⁺, Na⁺, Ca²⁺, and Mg²⁺ of CSS all showed a significant increase. The findings suggest that CSS has a better potential for the inhibition of the swelling behavior of expansive soil. The results demonstrate the feasibility of utilizing CSS as a CNS cushion layer material for expansive soil.

Key words: expansive soil, hydroxy-aluminum solution, chemically stabilized soil, cohesive non-swelling soil, cushion layer technology