Experimental and Numerical Investigation on Charcoal - Stabilized Black Cotton Soil

Abstract

This paper presents the use of charcoal powder as a sustainable stabilizer for black cotton soil. The natural soil is classified as CH soil.CBR of 3.0, and UCS of 3.17 kg/cm². Charcoal added at 5–40% improved soil behaviour, Strength parameters increased significantly, achieving a maximum CBR of 10 at 20% charcoal and a peak UCS of 8.5 kg/cm² at 10% charcoal. Slope stability analysis for earthen dams using PLAXIS 2D showed FoS values of 1.54 for natural soil, 1.57 for 20%, and 1.569 for 30% charcoal mixes. Overall, 10–20% charcoal provided optimum improvement in strength, workability, and stability, confirming its suitability as an eco-friendly stabilizer for embankments and pavement subgrades.

Introduction

Soil stabilization is crucial for improving the strength and stability of weak soils, especially expansive clays like black cotton soil, which exhibit high swelling and shrinkage due to montmorillonite minerals. Traditional stabilizers such as cement and lime are effective but costly and environmentally harmful. As a sustainable alternative, charcoal powder (biochar) offers eco-friendly, low-cost soil improvement by reducing plasticity, swelling, and moisture sensitivity while enhancing strength and drainage.

This study investigates the effects of adding 10%–40% charcoal to black cotton soil. Laboratory tests and PLAXIS 2D slope stability analyses show that charcoal increases cohesion, internal friction angle, and Factor of Safety (FOS), improving slope stability and load-bearing capacity. The main objectives were to evaluate geotechnical changes due to charcoal addition, identify the optimum mix proportion, and assess stability through numerical analysis.

Black cotton soil from Telangana was tested for basic properties, while industrial-waste charcoal served as the stabilizer. Soil–charcoal mixtures were prepared in various proportions (0–40%) and tested for Atterberg limits, compaction, CBR, UCS, and grain size distribution.

Results show that charcoal significantly modifies soil behavior. Plasticity increases up to 20% charcoal and then decreases, indicating reduced swelling at higher contents. Maximum Dry Density (MDD) decreases as charcoal content rises due to its lightweight, porous nature. Strength parameters (CBR and UCS) improve notably at 10–20% charcoal, where CBR reaches 10.4% and UCS peaks at 8.5 kg/cm². Beyond this range, strength declines because excessive charcoal reduces density and cohesion.

Conclusion

1) The black cotton soil used in this study was classified as CH (highly compressible clay) with poor engineering properties high liquid limit (52%) and plasticity index (21%) indicating significant shrink–swell potential. 2) The natural soil exhibited an Unit weight of 15.8 kN/m3, optimum moisture content (OMC) of 20%, CBR of 3.00, and UCS of 3.17 kg/cm², confirming its unsuitability for heavy load-bearing applications without stabilization. 3) Addition of charcoal powder modified the soil’s behaviour significantly, reduction in plasticity index beyond 20%, indicating improved workability and reduced swell potential. 4) The MDD decreased progressively from 15.9 kN/m3(5%) to 12 kN/m3 (40%) due to the lightweight, porous nature of charcoal, showing that excessive charcoal reduces soil compactness. 5) The CBR values improved notably, reaching a peak of 10 at 20% charcoal content, marking it as the optimum proportion for subgrade strength improvement. 6) The UCS values showed maximum strength (8.5 kg/cm²) at 10% charcoal addition, representing a 2.5times improvement over untreated soil; beyond this, strength declined due to reduced density and bonding. 7) Slope stability analysis yielded FoS values of 1.54 for natural clay, 1.53 for the 10% charcoal mix, 1.57 for the 20% mix, and 1.569 for the 30% mix. These results show that moderate charcoal incorporation (10–20%) provides stability levels comparable to that of natural clay, making such mixtures suitable as alternative core materials when good-quality soil is unavailable. However, higher charcoal content (30% and above) reduces both stiffness and cohesion, which in turn decreases the overall stability of the embankment.

References

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Copyright

Copyright © 2025 Thota Spandana, V. Padmavathi. 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.