Investigations on Improvement of Expansive Soil Properties using Electrokinetic Method

Abstract

Expansive soils, rich in active clays with Montmorillonite mineral, undergo large swell–shrink volume changes with moisture variation, causing differential ground movements that damage civil infrastructure and get worsened by their low shear strength and high compressibility in saturated conditions. Conventional methods like compaction and lime or cement stabilisation work well near the surface but are difficult to apply at depth or beneath existing structures. The electrokinetic method of stabilisation offers a non-invasive way to improve deep, low-permeability soils. The fundamental principle of Electrokinetic stabilisation involves the application of a direct current (DC) across a soil mass via electrodes. This electrical field induces coupled flow phenomena, Electro-osmosis (fluid transport), Electromigration (ion transport), and Electrophoresis (charged particle transport), which can be manipulated to introduce chemical stabilizers into the soil matrix, when coupled with the injection of cationic fluids, such as Calcium Chloride, the process evolves into Electrochemical Stabilisation, facilitating profound changes in the physicochemical properties of the clay. This study presents a comprehensive analysis of a specific experimental investigation: the electrokinetic treatment of an expansive soil using Aluminium electrodes and Calcium Chloride Dihydrate (CaCl2 • 2H2O) solution as a stabilising agent. The study encompasses three distinct trials featuring increasing concentrations of Calcium Chloride: 0.5 M, 1.0 M, and 1.5 M, with a duration of 7 days for each trial. The primary objective is to evaluate the efficacy of these varying concentrations in mitigating swelling, reducing plasticity, and enhancing shear strength. A critical component of this analysis is the interpretation of the specific laboratory data obtained for untreated and treated soil for determining the efficiency of Electrokinetic stabilisation and identifying the optimal concentration of stabilising agent. From the results, it can be observed that Electrokinetic stabilisation is a viable, robust method for improving the geotechnical properties of soil. Based on the test results from compaction characteristics, Unconfined Compression Strength (UCC), and pH values, the optimal concentration for the selected expansive soil is 1.0M of Calcium Chloride (Dihydrate) achieved better results.

Introduction

Expansive soils exhibit undesirable properties such as high compressibility, low shear strength, high swell potential, dispersive behavior, and frost susceptibility, which can severely affect structures. Conventional remediation methods are effective but costly and time-consuming, especially for existing structures. Electrokinetic Stabilization (EKS) offers an alternative approach to improve soil properties with minimal surface disturbance by using electrical fields to enhance strength, shear resistance, and reduce swelling.

Mechanisms of Electrokinetic Stabilization:

1. Electro-osmosis: Water moves from anode to cathode, reducing moisture content and increasing soil density.

2. Electrophoresis: Charged soil particles migrate toward electrodes, rearranging soil structure.

3. Ion Migration and Cementation: Ions from added salts (e.g., CaCl?) or electrodes react with soil minerals, forming a cementing effect that binds particles and reduces swelling.

Literature Insights:

• Shear strength can increase by 127% near anode and 495% at cathode (Rustamaji, 2024).

• Ionic solutions such as CaCl? and Na?CO? reduce plasticity and swelling (Abiodunabiola, 2021; Jijo James et al., 2019).

• Improved electro-osmotic permeability correlates with ion concentration (Estabragh et al., 2020).

Materials Used:

• Soil: Samples collected at 2–3 m depth from APIIC Industrial Park, Tirupati, Andhra Pradesh; air-dried, pulverized, and sieved.

• Model Box: 0.5 × 0.3 × 0.3 m plywood box with separate compartments for anode, cathode, and soil.

• Electrodes: Aluminium anodes, widely used in electro-osmotic dewatering.

• Chemical Stabilizer: Calcium Chloride Dihydrate (CaCl?•2H?O), a water-soluble salt suitable for geotechnical applications.

Methodology:

1. Soil Tests Conducted: IS 2720 standards for natural water content, specific gravity, grain size, plasticity, free swell index (FSI), pH, electric conductivity, compaction, and unconfined compressive strength (UCC).

2. Sample Preparation: Soil compacted in layers in the model tank with geotextile barriers to prevent particle migration.

3. Electrokinetic Treatment:

   • Electrodes placed in soil; left for 24 h to equilibrate.

   • Calcium Chloride solutions of varying concentrations (0.5M, 1.0M, 1.5M) applied.

   • Regulated DC voltage of 25V applied for 7 days.

   • Post-treatment soil tested near anode and cathode for strength and swelling properties.

Results & Discussion:

• Untreated soil classified as Highly Compressible Clay with high FSI, indicating extreme expansiveness.

• Electrokinetic treatment with CaCl? improved shear strength, reduced plasticity, decreased swell potential, and enhanced compaction characteristics.

• Effectiveness varied with CaCl? concentration and proximity to electrodes.

Conclusion

The following conclusions are drawn based on the test results. 1) The native soil is Highly compressible clay with LL of 95.3 % and PI of 73.42 and an FSI exceeding 206 %, indicating high expansive nature and may not be suitable for engineering applications as the compressive strength (34 kPa) is low. 2) EKM is chosen for improving the strength of the selected expansive soil and swelling nature. 3) EKM was implemented in model box using Aluminium electrodes and CaCl2 (Dihydrated) as stabilizing agent. 4) Model test studies were conducted at various concentrations (0.5M, 1.0M, and 1.5M) of Stabilizing Agent to observe its influence on Plasticity characteristics, Compaction characteristics, Free Swell Index and UCC of soil. 5) The studies demonstrate the variation of Plasticity characteristics, Compaction characteristics, Free Swell Index and UCC with the varying concentrations of CaCl2. 6) The Liquid Limit reduced from 95.3 % to 33.9 %, and the Plastic limit increased from 20.96 % to 23.15 %. 7) The OMC decreased from 14.4 % to 9.9%, and MDD increased from 16.8 kN/m3 to 19.4 kN/m3. 8) Free Swell Index is reduced from 206% for untreated soils to 35% for soil treated with 1.5M of Calcium Chloride. 9) The Unconfined Compressive Strength (UCC) of soil improved from 34.13 kN/m2 to 78.6 kN/m2 at 1.0M concentration of Calcium chloride. 10) pH value for untreated soil is 9.7, which is a typical value for expansive clays. pH drops from 8.51 to 6.31 at the anode, and rises from 10.2 to 12.34 at the cathode, indicating a reduction in Diffusion Double Layer thickness and improving the strength of the soil. It can be concluded that the Electrokinetic Method of stabilization is a viable, robust method for improving the geotechnical properties of soil. It can be used as remediation of deficiencies underneath building foundations, roads, railways or pipelines. This technique involves an approach with minimum disturbance to the surface while treating subsurface contaminants and improving the engineering characteristics of subsurface soils.

References

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Copyright

Copyright © 2026 N. M. Purushotham, Dr. CH. Sudha Rani. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.