Laboratory Study on the Geotechnical Behaviour of Expansive Soil Treated with Coconut Husk Ash and Silica Fume

Abstract

Extensive subgrades present severe structural challenges globally due to their inherent tendency to swell upon moisture intake and undergo severe volumetric contraction during dry seasons, causing structural distortion and excessive rehabilitation expenses. This experimental investigation appraises the stabilization performance achieved by combining an agro-waste residue, Coconut Husk Ash (CHA), alongside an industrial byproduct, Silica Fume (SF), on the index and strength parameters of problematic clayey soils. The testing scheme incorporated a fixed dosage of 6% CHA mixed with varying proportions of SF (0%, 2%, 4%, 6%, and 8% relative to dry soil weight), evaluating the matrix via Atterberg limits, Standard Proctor compaction, and Unconfined Compressive Strength (UCS) testing. Results demonstrate that co-supplementing CHA and SF significantly suppresses liquid limits and plasticity indices while enhancing maximum dry densities and compressive strengths. Peak geotechnical improvement—manifested by maximum dry unit weight and structural strength—was secured at a composite proportion consisting of 88% expansive soil, 6% CHA, and 6% SF, confirming its high viability for subgrade enhancement.

Introduction

1. Summary

   The text presents a geotechnical soil stabilization study aimed at improving the engineering properties of expansive black cotton soil (BCS) using Coconut Husk Ash (CHA) and Silica Fume (SF).

   1. Problem with Expansive Soil

   Expansive soils contain swelling clay minerals such as montmorillonite, smectite, and bentonite. They undergo significant expansion and contraction as moisture changes seasonally. This can cause:

   • Cracks in pavements and walls

   • Uneven settlement or heaving

   • Slope instability

   • Poor foundation performance

   • Low pavement-subgrade strength

   Therefore, stabilization is required before using such soil for foundations or pavement subgrades.

   2. Materials Used

   Coconut Husk Ash (CHA):
   CHA is an agricultural waste obtained by burning coconut husks. It contains a high amount of silica (58%) and alumina (9%), giving it pozzolanic properties. Its use can reduce agricultural waste while improving soil behavior, workability, and strength.

   Silica Fume (SF):
   Silica fume is an extremely fine industrial by-product containing approximately 95.8% silicon dioxide. Its reactive silica participates in pozzolanic reactions and helps form calcium silicate hydrate (C-S-H) gel, which densifies the soil structure and improves bonding and strength.

   3. Properties of Untreated Soil

   The natural soil contains approximately 83.10% clay, making it a highly expansive CH (high-plasticity clay) soil. Important initial properties include:

   • Liquid limit: 85.71%

   • Plastic limit: 28.51%

   • Plasticity index: 57.20%

   • Differential free swell: 100%

   • Maximum dry density: 1.50 g/cc

   • Optimum moisture content: 28.42%

   • Soaked CBR: 1.34%

   • Unconfined cohesion: 102.5 kN/m²

   • Angle of internal friction: 2°

   These values indicate that the untreated soil has high plasticity and swelling potential but poor subgrade performance, particularly its very low soaked CBR.

   4. Material Preparation

   Coconut husks were collected from Amalapuram, Andhra Pradesh, dried, controlled-burned, oven-dried at approximately 105°C, and passed through a 75 μm IS sieve.

   Silica fume was obtained from Aastra Chemicals, Chennai, and consisted predominantly of silicon dioxide. Both additives were dried before mixing.

   5. Methodology

   The experimental procedure consisted of:

   1. Collection and preparation of black cotton soil.

   2. Processing and characterization of CHA and SF.

   3. Mixing the soil with a fixed 6% CHA content.

   4. Adding SF at 0%, 2%, 4%, 6%, and 8% by dry weight of soil.

   5. Adding water gradually to reach the required optimum moisture content.

   6. Curing the mixtures for 24 hours before testing.

   7. Evaluating physical and engineering properties through laboratory tests.

   The tests included:

   • Specific gravity

   • Atterberg limits

   • Differential free swell

   • Standard Proctor compaction

   • Unconfined compressive strength (UCS)

   • California Bearing Ratio (CBR)

   • Direct shear testing

   CBR specimens were soaked for four days to represent severe moisture conditions experienced by pavement subgrades.

   6. Mix Proportions

   The main experimental combinations consisted of:

   Coconut Husk Ash	Silica Fume
   6%	0%
   6%	2%
   6%	4%
   6%	6%
   6%	8%

   This allows the researchers to determine how increasing silica fume affects soil already stabilized with 6% CHA and to compare the effects of individual and combined agro-industrial additives.

   Overall Summary

   The study investigates an environmentally sustainable method for stabilizing highly expansive black cotton soil. A constant 6% Coconut Husk Ash is combined with different percentages of **Silica Fume (0

    

2. ChatGPT said:

   Summary

   The text presents an experimental study on stabilizing expansive black cotton soil (BCS) using Coconut Husk Ash (CHA) and Silica Fume (SF). Expansive soils contain minerals such as montmorillonite and smectite that absorb water, causing significant swelling and shrinkage. This can lead to pavement cracks, foundation damage, slope instability, and poor bearing capacity. Therefore, stabilization is needed to improve their engineering performance.

   Materials Used

   • Coconut Husk Ash (CHA): Produced by controlled burning of coconut husks collected from Amalapuram, Andhra Pradesh. It is rich in silica and alumina and acts as a pozzolanic material. Its use also provides an environmentally beneficial way to reuse agricultural waste.

   • Silica Fume (SF): A very fine industrial by-product containing about 95.8% SiO?. It reacts with available calcium and other soil components to form calcium silicate hydrate (C-S-H) gel, which strengthens and densifies the soil structure.

Conclusion

The findings of this investigation demonstrate that pairing a constant 6% Coconut Husk Ash content with different proportions of Silica Fume represents a bearable then operative procedure meant for enhancing expansive BCS intended for subgrade applications. The incorporation of sub-micron-sized Silica Fume together with the agricultural ash brought about significant changes in the raw clay\'s physical, index, and strength parameters. At the ideal quantity of 88% BCS, 6% CHA, and 6% SF, the index chattels showed clear improvement — the LL diminished from 68.50% to 60.52%, to the PI declined since 38.60% to 24.01%, causing the soil to shift cataloguing since a highly plastic clay toward an intermediate-plasticity clay. The discrepancy allowed phantastic likewise dropped from 90% to 60%, reflecting a marked diminution trendy the loam\'s tendency near swell besides its sensitivity to moisture changes. The compaction response, load-bearing ability, and shear resistance all reached their highest values at the 6% SF dosage level. The MDD enlarged from a reference point of 1.45 gm/cc for the untreated soil to 1.57 gm/cc, convoyed by a decline in OMC to 20%. Soaked CBR values rose from 1.45% to 8.20%, more than five times the original figure, satisfying the threshold set by Indian Standard guidelines for a competent subgrade beneath flexible pavements. Similarly, the UCS increased from 78.50 kPa to 310.50 kPa. These gains in forte can be attributed primarily to two mechanisms: the physical packing of the soil\'s void spaces by ultra-fine silica particles, and secondary pozzolanic reactions that continuously produce reactions responsible for binding soil units together. Taken together, the results indicate that the 6% CHA + 6% SF combination provides a practical, eco-friendly, and cost-effective approach to stabilizing expansive subgrade soils. Subsequent research should build on these laboratory findings through field-scale pilot implementation and extended wetting–drying cycle testing to evaluate long-term durability.

References

[1] Sridharan, A., & Prakash, K. (2000). Classification procedures for expansive soils. Proc., Inst. Civ. Eng. – Geotech. Eng., 143(4), 235–240. [2] Rao, S. M., & Shivananda, P. (2005). Role of curing temperature in progress of lime-stabilization of an expansive clay. Geotech. Geol. Eng., 23(1), 79–85. [3] Phani Kumar, B. R., & Sharma, R. S. (2004). Effect of fly ash on engineering properties of expansive soils. J. Geotech. Geoenviron. Eng., 130(7), 764–767. [4] Cokca, E. (2001). Use of class C fly ashes for the stabilization of an expansive soil. J. Geotech. Geoenviron. Eng., 127(7), 568–573. [5] Phanikumar, B. R., Raju, M. J., & Raju, E. R. (2020). Silica fume stabilization of an expansive clay subgrade and the effect of silica fume-stabilised soil cushion on its CBR. Geomech. Geoeng., 15(1), 1–14. [6] Osinubi, K. J., & Nwaiwu, C. M. O. (2006). Design of compacted lateritic soil liners and covers. J. Geotech. Geoenviron. Eng., 132(2), 203–213. [7] Basha, E. A., Hashim, R., Mahmud, H. B., & Muntohar, A. S. (2005). Stabilization of residual soil with rice husk ash and cement. Constr. Build. Mater., 19(6), 448–453. [8] Koteswara Rao, D., Pranav, P. R. T., & Anusha, M. (2011). Stabilization of expansive soil with rice husk ash, lime and gypsum — an experimental study. Int. J. Eng. Sci. Technol., 3(11), 8076–8085. [9] Muntohar, A. S., & Hantoro, G. (2000). Influence of rice husk ash and lime on engineering properties of a clayey subgrade. Electron. J. Geotech. Eng., 5, 1–9. [10] Horpibulsuk, S., Rachan, R., Chinkulkijniwat, A., Raksachon, Y., & Suddeepong, A. (2010). Analysis of strength development in cement-stabilized silty clay from microstructural considerations. Constr. Build. Mater., 24(10), 2011–2021. [11] Al-Rawas, A. A., Hago, A. W., & Al-Sarmi, H. (2005). Effect of lime, cement and Sarooj (artificial pozzolan) on the swelling potential of an expansive soil from Oman. Build. Environ., 40(5), 681–687. [12] Puppala, A. J., Wattanasanticharoen, E., & Punthutaecha, K. (2003). Experimental evaluations of stabilisation methods for sulphate-rich expansive soils. Ground Improv., 7(1), 25–35. [13] Kolias, S., Kasselouri-Rigopoulou, V., & Karahalios, A. (2005). Stabilisation of clayey soils with high calcium fly ash and cement. Cem. Concr. Compos., 27(2), 301–313. [14] Amu, O. O., Fajobi, A. B., & Oke, B. O. (2005). Effect of eggshell powder on the stabilizing potential of lime on an expansive clay soil. J. Appl. Sci., 5(8), 1474–1478.

Copyright

Copyright © 2026 Mr. Dosapathni Venkateswara Rao, Dr. G. V. R. Prasada Raju. 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.