Swelling soil, also known as expansive soil, is a type of soil that changes its volume significantly due to fluctuations in moisture content [18] . When this soil absorbs water, it expands and increases in volume, and when it dries, the soil shrinks considerably, leading to cracks on its surface [19] . This dynamic behavior makes it one of the most detrimental types of soil for engineering structures. Expansive soils often feature high clay content, which increases their ability to expand and shrink. Black cotton soil, for example, is a classic example of swelling soil, being rich in clay minerals that show visible cracks during dry periods and appear expanded during wet seasons [17] . This continuous change in volume leads to significant problems for foundations and structures built upon them, such as cracks in foundations and settling of walls [20] .

Swelling soils are a major concern in many parts of the world, especially in countries with fluctuating climates that significantly affect soil moisture levels. During periods of high moisture, the soil expands, and during dry periods, it shrinks, placing immense stress on engineering structures such as roads, buildings, and foundations, causing rapid structural deterioration [21] . Therefore, scientifically and accurately managing these soils is crucial for ensuring the long-term stability of structures. This has been pointed out by several researchers, such as Ahmed Sharif in his study on the effects of expansive soils in Sudan, where he emphasized that these soils pose a significant threat to construction works [22] .

Soil stabilization is a process aimed at improving soil properties to increase its stability, either through mechanical or chemical modification. These techniques include a variety of methods designed to enhance the mechanical strength of the soil and reduce volume changes due to moisture variations [13] .

Mechanical stabilization involves several methods such as compaction, replacing the soil with non-swelling materials, or reinforcing the soil with additional materials. Compaction is an effective method for increasing soil density and reducing air voids, thereby enhancing its load-bearing capacity [2] . Additionally, replacing expansive soil with non-swelling materials like sand or gravel is another solution. However, this may be impractical in some cases, especially in areas with large quantities of expansive soil [21] .

In contrast to mechanical stabilization, chemical stabilization focuses on improving the geotechnical properties of the soil using chemical agents, such as lime, cement, and fly ash, which chemically react with the soil’s components. Chemical stabilization is often more effective in treating expansive soils, as the chemical agents alter the soil’s internal structure and reduce its potential for expansion and shrinkage [23] . One of the most well-known chemical techniques is the addition of lime to clayey soil, where lime helps to strengthen the soil and reduce its expansion. It also helps to decrease plasticity and improve the soil’s compressive strength.

Using lime is one of the most effective methods for stabilizing expansive soils. Lime significantly modifies the geotechnical properties of the soil, with both chemical and mechanical effects that contribute to reducing the soil’s capacity for expansion and shrinkage. When lime is added to expansive soil, a cation exchange reaction occurs, where the positive ions from the lime replace the negative ions associated with the clay minerals. This reaction improves the molecular structure of the soil, reducing the plasticity index and consequently decreasing the soil’s expansion when it absorbs water [24] .

Lime also contributes to the formation of new chemical bonds between soil particles, enhancing its strength and stability. These chemical reactions allow lime to reduce the effects of moisture fluctuations, leading to more stable soil during both dry and wet periods, thus reducing the risks of cracking and shrinkage that can damage foundations and other structures [14] . A study by Zumerway (2016) confirmed that lime improves the resistance of soil to expansion and shrinkage, thereby enhancing the stability of foundations built upon it.

Lime significantly increases the mechanical strength of expansive soils. Through the chemical stabilization process, the soil’s ability to withstand heavy loads improves, which contributes to enhancing the stability of structures built on it. Lime also helps to improve the bearing capacity of the soil, making it more resistant to stress caused by loads or climatic changes. These improvements make the soil more resilient to pressure from loads or moisture fluctuations, contributing to the stability of the structures and reducing structural risks [25] .

Using lime for expansive soil stabilization is an excellent environmental and economic option. Lime is widely available and relatively inexpensive, making its use in soil treatment a cost-effective choice. Additionally, the use of lime reduces the need for soil replacement or costly modifications to foundations, promoting the sustainability of construction projects and reducing the environmental impact of excavation or replacement operations [25] .

Expansive soil samples were collected from the Shambat area in Khartoum State, Sudan. After drying and grinding, the samples were prepared for testing. The soil was mixed with different percentages of quick lime (CaO) (0%, 4%, 6%, and 8% by weight of dry soil). The lime used in this study was derived from heating limestone, a material commonly used in soil stabilization due to its ability to enhance the soil’s strength and reduce its swelling and shrinkage properties.

The lime-treated samples were left to cure under controlled conditions for 7 days at a temperature of 25˚C and a relative humidity of 55% to ensure the pozzolanic reactions between the lime and clay minerals in the soil.

The following geotechnical tests were conducted on both treated and untreated soil samples:

1) Grain Size Analysis

To assess the effect of lime on the soil structure, grain size distribution was determined. The soil samples were washed, dried, and passed through a series of sieves. The percentage of fine particles passing through sieve No. 200 (0.075 mm) was calculated using the following Equation (1)

$\text{\%fine}=\frac{\text{Weight of particles passing through sieve}}{\text{Total weight of the sample}}\times 100$(1)

2) Specific Gravity Test

Specific gravity test was conducted under a control room temperature, pycnometer was filled with soil passing through sieve No 40 then, weighed. distilled water was added to fill about half to three-fourth of the pycnometer. Then the sample was soaked for 10 minutes and weighted. The pycnometer filled again with water and weighted and Gs was calculated using Equation (2)

$\text{Gs}=\frac{W_2-W_1}{\left(W_2-W_1\right)-\left(W_3-W_4\right)}$(2)

3) Atterberg Limits Test

The liquid limit (LL) and plastic limit (PL) of the soil were determined to assess its plasticity. The Plasticity Index (PI) was calculated as the difference between the liquid limit and the plastic limit, using the following equation Equation (3)

$\text{PI}=\text{LL}-\text{PL}$ (3)

4) Compaction Test (Proctor Test)

The Proctor compaction test as shown in Figure 1 was conducted to determine the maximum dry density (MDD) and the optimum moisture content (OMC) for each lime-treated sample. This test is essential for determining the optimal compaction conditions for the soil. The Maximum Dry Density (MDD) was calculated using the formula Equation (4)

$\text{MDD}\left(\text{g}/{\text{cm}}^2\right)=\frac{\text{Dry weight of soil}}{\text{volume of soil sample}}$(4)

5) Bearing Capacity Test

The soil sample was thoroughly mixed and then placed in five layers within a CBR (California Bearing Ratio) mold. Each layer was compacted using a hammer weighing 25.5 N. The prepared soil sample was then soaked for several days to simulate field conditions. After the soaking period, the CBR test as shown in Figure 2 was conducted to evaluate the soil’s load-bearing capacity. The California Bearing Ratio (CBR) was calculated using the formula Equation (5)

$\text{CBR}\left(\%\right)=\frac{P_{\text{sample}}}{P_{\text{standard}}}\times 100$ (5)

The experimental data collected from all tests were processed using statistical tools to ensure a precise evaluation of the results. Mean values and standard deviations for each tested property were calculated, providing a clear understanding of the data variability. A comparative analysis was performed between the results of untreated soil samples and those treated with varying lime percentages (0%, 4%, 6%, and 8%). Additionally, the lime-treated samples were compared with data from previous studies on alternative materials such as granite powder.

The sieve analysis test conducted on the natural soil revealed a predominance of fine-grained particles, with 97.33% passing through sieve No. 200. The soil was classified as A-7-6 (high plasticity clay) according to the AASHTO classification system. The basic properties of the collected soil are presented in Table 1 .

As shown in Table 2 , the addition of lime significantly reduced both the liquid limit and plasticity index. With the increase in lime percentage from 0% to 8%, the liquid limit decreased from 65.8% to 46%, while the plasticity index decreased from 35.97% to 14.63%. This reduction is attributed to the chemical interactions between lime and clay particles, particularly the ion-exchange process, which reduces the soil’s swelling potential and enhances its stability under varying moisture conditions as shown in Figure 3 .

As shown in Table 3 the results from the compaction test showed that the addition of lime significantly improved the compaction characteristics of the soil. The dry density increased with higher lime percentages, with the maximum dry density (MDD) reaching 1.62 g/cm³ with the addition of 8% lime, and 1.6 g/cm³ with 6% lime. Additionally, the optimum moisture content (OMC) decreased gradually with the increasing lime content, indicating that lime effectively reduces the water required to achieve the maximum soil density as shown in Figure 4 .

As shown in Table 4 , the addition of lime had a significant positive effect on the soil’s load-bearing capacity. The CBR values dramatically increased from 1.7% for untreated soil to 50.7% with the addition of 8% lime. This significant improvement is attributed to the pozzolanic reactions between lime and clay minerals, forming strong and durable bonds within the soil.

Chemical reactions such as ion exchange between calcium ions from lime and less-charged ions on the surface of clay particles contribute to a reduction in plasticity and an increase in soil stability. Additionally, the process of flocculation-agglomeration helps fine particles cluster together to form larger, more stable masses, which improves the soil’s mechanical properties and reduces its potential for expansion and shrinkage. Pozzolanic reactions occurring between lime, silica, and alumina in the soil lead to the formation of cementitious compounds, which enhance the soil strength and contribute to the significant increase in CBR values.

The study results demonstrated substantial improvements in the engineering properties of expansive soil upon the addition of lime. This enhancement was reflected in a significant reduction in the Liquid Limit (LL) and Plasticity Index (PI), thereby mitigating the soil’s potential for swelling and shrinkage.

Furthermore, compaction characteristics improved considerably; the Maximum Dry Density (MDD) increased to 1.62 g/cm³ at 8% lime content, while the Optimum Moisture Content (OMC) decreased, indicating better soil stability. The California Bearing Ratio (CBR) test provided evidence of a massive increase in the soil’s load-bearing capacity, with the CBR value soaring from 1.7% for untreated soil to 50.7% at 8% lime content, representing a substantial gain in strength.

Although continuous improvement in CBR and plasticity values was observed up to 8% lime content, prior research literature suggests that exceeding this concentration may lead to diminishing returns. Based on technical analysis and economic feasibility considerations—since higher additions increase project cost without commensurate performance improvement—the optimal lime content for stabilization was determined to be between 6% and 8%. This range provides the optimal balance between superior performance and cost-efficiency for stabilization applications.

1) Field Trials: It is recommended to conduct field trials on lime-treated soils in various regions of Sudan to evaluate the long-term effects of lime treatment on soil stability under natural conditions.

2) Exploration of Alternative Stabilizing Agents: While lime proved effective in stabilizing the soil, further research should explore other locally available stabilizing agents, such as fly ash or cement, to enhance the stabilization process.

3) Adopting Lime Stabilization in Construction: Lime stabilization techniques should be implemented in construction projects in areas with expansive soils, such as Shambat, to improve the stability of foundations and reduce the risks associated with expansive soils.

4) Economic Feasibility Studies: Conduct economic feasibility studies on the use of lime for soil stabilization in large-scale construction projects, particularly in urban areas, to ensure its cost-effectiveness and promote sustainable infrastructure development.