Experimental Study on the Stabilization of Expansive Soil using Gypsum Hemihydrate

Abstract

An expansive soil is a type of soil that expands or contracts as a direct consequence of variations in water volume. Expansive soils have low bearing capacity, large settlement, and excessive water absorbability as their physical properties. They have inadequate bearing capacity due to high water content, and causes consolidation settlement. This makes them problematic for civil engineering construction. Various issues in the buildings or construction projects may arise as a result of expansive soil’s swelling and contracting behaviour such as crucible damage to roads and walkways and other lightweight structures, wall and ceiling cracks, excessive settlement etc. Hence, before beginning any construction projects, checking for the presence of expansive soil and implementing an appropriate treatment strategy are critical. The consequence of swell- shrink nature of expansive soils can be reduced by using a number of techniques such as replacement of soil by controlling compaction, moisture controlled; surcharge loading; use of geosynthetics. The best way for controlling expansive soil volume changes is to stabilise the soil with the addition of admixture which can restricts volume changes or alters characteristics of expansive soil. The goal of soil stabilisation is to enhance soil shear strength, decrease permeability, increase durability of soil mass, and strengthen the load carrying capacity of foundation soils. In this study, gypsum hemihydrate (Plaster of Paris) is used as a stabilising agent for stabilisation of soil. Gypsum hemihydrate is added to the soil in proportions of 3 to 12 percent. Effect of gypsum hemihydrate on Atterberg’s limits, unconfined compressive strength, free swell index, pH and conductivity, consolidation properties are determined. The use of gypsum hemihydrate successfully strengthens the soil while minimizing the swelling

Introduction

This study focuses on the stabilization of expansive clayey soil using gypsum hemihydrate (Plaster of Paris, Ca?SO?·0.5H?O) as a stabilizing agent. Expansive soils are problematic in construction due to their high water absorbency, low bearing capacity, and tendency to swell and shrink with moisture changes. Soil stabilization enhances its strength and usability using mechanical or chemical methods. Gypsum hemihydrate was chosen as a sustainable alternative to cement for chemical stabilization.

Objectives of the Study:

1. Analyze plasticity characteristics of gypsum-treated soil.

2. Investigate swelling characteristics.

3. Evaluate curing effects on soil strength.

4. Assess chemical properties (pH, conductivity).

5. Study consolidation behavior.

Literature Review Highlights:

• Multiple studies show gypsum (and in combination with agents like calcium chloride, fly ash, and crumb rubber) improves soil's strength, compaction, plasticity, and bearing capacity.

• Gypsum hemihydrate enhances soil by cation exchange, reducing swelling and improving compressive strength.

Materials & Methodology:

• Soil Sample: Expansive CH-type clay soil from near Guwahati.

• Additive: Gypsum hemihydrate collected locally.

• Testing Plan: Soil was treated with 0%, 3%, 6%, 9%, and 12% gypsum hemihydrate and tested for:

  • Atterberg’s limits

  • Unconfined Compressive Strength (UCS)

  • Free Swell Index (FSI)

  • pH & Conductivity

  • Consolidation behavior

Key Findings:

1. Untreated Soil Properties:

   • Liquid Limit: 76%

   • Plastic Limit: 32.79%

   • Plasticity Index: 42.21%

   • Free Swell Index: 157.14%

   • UCS: 111.82 kN/m²

   • pH: 5.92 (acidic)

   • Classification: CH (High plastic clay)

2. Atterberg’s Limits:

   • Liquid and plastic limits decreased with gypsum addition.

   • Shrinkage limit increased due to particle aggregation.

3. Compaction Characteristics:

   • Maximum Dry Density (MDD): 1.51 g/cc

   • Optimum Moisture Content (OMC): 28%

4. pH and Conductivity:

   • pH increased from 5.92 to 12.3.

   • Conductivity increased from 2.56 to 5.43 ms/ppt with higher gypsum content due to Ca²? release.

5. Unconfined Compressive Strength (UCS):

   • UCS increased with gypsum content and curing time.

   • Maximum UCS: 910.31 kN/m² at 12% gypsum for 14 days curing.

6. Free Swell Index:

   • FSI decreased with more gypsum.

   • Indicating reduced swelling due to cation exchange.

7. Consolidation Test:

   • Treated soils showed improved consolidation behavior with increasing gypsum content.

Conclusion

The coefficient of consolidation (C v ) obtained from the consolidation test for untreated and treated soil with the addition of 0%, 3%, 6%, 9% and 12% increases from 0.91 mm 2 /min to 21.03 mm 2 /min for pressure range 0-100 kPa and 0.74 mm 2 /min to 20.34 mm 2 /min for pressure range 100-200 kPa. This is due to the fact that the increase content of gypsum hemihydrate decreases the plastic characteristics of the blended sample. As the result the compressibility of blended sample are decreases and hence time required for consolidation is also decreased. The increase in the coefficient of consolidation value is enormous from 6% to 9% but the increase is less from 9% to 12%. So, the optimum percentage is 9 %. The entire conclusion is drawn from the experimental study performed on expansive clay soil and scope for future work. Different percentages of gypsum hemihydrate is added to the soil to evaluate plasticity characteristics, swell characteristics, chemical characteristics and curing effect on strength of gypsum hemihydrate treated soil. The experiments conducted led to the following conclusions: 1) With the addition of 0%, 3%, 9%, and 12% gypsum hemihydrate on treated and non- treated soil, the liquid limit, plastic limit, plasticity index the values reduces with rise in the percentage addition of gypsum hemihydrate. The shrinkage limit rises with rise in addition of gypsum hemihydrate. 2) The result obtained from the pH test for soil with the addition of 0%, 3%, 6%, 9% and 12% increases from 5.92 to 12.3 due to release of Ca 2+ ions which is basic in nature. 3) The result obtained from the conductivity test for soil with the addition of 0 %, 3%, 6%, 9% and 12% increases from 2.56 to 5.43. 4) The compressive strength obtained from the UCS test for soil increases with the addition of 0%, 3%, 6%, 9% and 12% due to the fact that the soil when mixed with gypsum hemihydrate, the alkaline cation exchange process occurs. The highest compressive strength is 910.31 KN/ m 2 at 12 % addition of gypsum hemihydrate for 14 days curing. 5) With the addition of 0%, 3%, 6%, 9%, and 12% gypsum hemihydrate on soil, the swelling characteristics decreases with rises in the percentage addition of gypsum hemihydrate. The decrease in the swelling potential is enormous (23.81) from 6% to 9% but the reduction potential is less (4.76) from 9% to 12%. So, the optimum percentage is 9 %. 6) With the addition of 0%, 3%, 6%, 9%, and 12% gypsum hemihydrate on soil, the coefficient of consolidation increases with rise in the percentage addition of gypsum hemihydrate. The increase in the coefficient of consolidation value is drastic (7.42mm 2 /min for 0-100kPa and 5.77 mm 2 /min for 100 to 200kPa) from 6% to 9% but the increase is less (1.07 mm 2 /min for 0-100kPa and 2.13 mm 2 /min for 100-200kPa) from 9% to 12%. So, the optimum percentage is 9 %. From the above conclusion, the expansive clayey soil can be stabilized with the stabilizing agent gypsum hemihydrate (Plaster of Paris). The use of gypsum hemihydrate successfully strengthens the soil while minimizing the swelling.

References

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Copyright

Copyright © 2025 Malemleima Oinam, Arunav Chakraborty. 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.