Introduction

Expansive soils, such as black cotton soil, pose significant challenges in construction and civil engineering due to their unique physical properties and behavior in response to moisture variations. These soils are characterized by high clay content, which gives them the ability to undergo significant volume changes with changes in moisture content. This phenomenon, known as shrink-swell behavior, causes black cotton soils to expand when they absorb water and shrink when they dry out. The expansive nature of these soils can lead to severe structural problems for buildings and infrastructure. During wet periods, black cotton soils exert pressure on foundations, causing them to heave and potentially crack. Conversely, during dry periods, the soil contracts, leading to foundation settlement and structural instability. These cycles of expansion and contraction can result in costly damage to structures, affecting their durability and longevity. In regions where black cotton soils are prevalent, such as certain parts of the United States, Australia, and India, addressing these soil challenges is essential for successful construction projects so as to minimize the economic losses. Traditional approaches to mitigate the effects of black cotton soils include using deep foundations, moisture barriers, and chemical stabilizers1,2. However, these methods can be costly, time-consuming, and may not always provide long-term solutions. Given the environmental and economic impact of black cotton soil-related issues, there is a growing interest in innovative solutions that can effectively stabilize these soils while also being cost-effective and sustainable3.

One such solution that has shown promise is the use of industrial byproducts like calcium carbide residue (CCR) for soil stabilization3. CCR is an alkaline waste product generated in large quantities during the production of acetylene gas from calcium carbide. The disposal of CCR has traditionally posed environmental challenges, as it is often landfilled, leading to land resource wastage and pollution. The chemical composition of CCR, primarily calcium hydroxide, makes it highly effective for soil stabilization. When mixed with problematic soils like black cotton clays, CCR initiates pozzolanic reactions that result in the formation of cementitious compounds. Research and field applications have demonstrated the efficacy of CCR in enhancing the geotechnical properties of various soils. Past studies have shown significant improvements in soils’ bearing capacity, compressive strength, and reduced permeability when CCR is used2,4,5,6,7,8,9. Lateritic soil–fly ash (FA) geopolymers containing up to 30% calcium carbide residue (CCR) achieved soaked 7-day unconfined compressive strength (UCS) that met pavement requirements. However, the maximum 90-day UCS occurred at a 20% CCR replacement, while excessive CCR (30%) resulted in spongy and cracked FA particles due to premature aluminosilicate gel precipitation2. CCR-stabilized clayey soil reached a California Bearing Ratio (CBR) of 8.5% and a resilient modulus of 70 MPa, outperforming quicklime-stabilized soil, which had a CBR of 6.0% and a resilient modulus of 55 MPa4. CCR markedly improved the physical and mechanical properties of clayey soils, with UCS increasing by up to 50% and California Bearing Ratio improving by 30% compared to quicklime-stabilized soils5. The effects of CCR, lime, and fly ash on a bentonite-sand mixture (75–25%) were compared, and it was found that adding 15% CCR improved unconfined compressive strength to 420 kPa, cohesion to 35 kPa, and internal friction angle to 25°, while reducing swelling pressure to 12 kPa. CCR outperformed lime and fly ash, which achieved UCS values of 400 kPa and 350 kPa, respectively6. The addition of CCR to red soil significantly reduced permanent strain, with improved performance across various moisture contents and stress levels7. A mix of CCR and FA significantly reduced soil plasticity and increased strength, with optimal performance achieved at specific water contents and CCR levels8. An amount of 9% CCR in bentonite and 12% CCR in kaolin resulted in unconfined compressive strength (UCS) values of 320 kPa and 410 kPa, respectively, along with improved microstructural properties10. Group A specimens with a CCR ratio of 40:60 achieved peak unconfined compressive strength (UCS) at 12% binder content, while Group B specimens with a CCR ratio of 60:40 performed best at 15% binder content. SEM images of Group A at 12% binder content revealed a more integrated soil matrix with reduced voids, significantly enhancing soil strength9.

Other waste material from the agricultural sector is rice husk ash (RHA), which has emerged as a promising material in soil stabilization applications, offering both engineering benefits and environmental advantages. Derived from the combustion of rice husks, RHA is rich in amorphous silica, which exhibits pozzolanic properties when activated with calcium hydroxide (lime) or other alkalis. Various studies have demonstrated the effectiveness of stabilizing soil using rice husk ash in different proportions11,12,13,14,15,16. A mixture of 20% RHA and 2% lime reduced the plasticity index and free swell of black cotton soil by around 90% and 70%, respectively, and increased the CBR value by 800%, making it suitable for S2 pavement subgrade design12. Adding 6–8% cement and 15–20% RHA reduced the plasticity index and maximum dry density (MDD) while increasing optimum moisture content (OMC) and strength13. The optimal quantity of RHA with lime decreased dry density, plasticity index, and deformability, while enhancing OMC, shear strength, and California bearing ratio (CBR) of clayey soil14. Adding 9% RHA to local clay soil reduced the plasticity index by 80% and increased unconfined compressive strength by 8%, though it lowered the MDD value15. A decrease in liquid limit, free swell index, and cohesion value for both alluvial and clay soils when an optimal amount of RHA was added; MDD increased in alluvial soil but decreased in clay soil, with both soils showing increased soaked CBR value and internal friction angle16.

The review of past literature clearly indicates that adding CCR and RHA enhances the geotechnical properties of soils with challenging characteristics. Despite the clear benefits of adding calcium carbide residue (CCR) and rice husk ash (RHA) to enhance the geotechnical properties of soils, past studies often focus on their individual effects rather than on their combined synergistic impact. Additionally, there is limited research addressing the optimal mix ratios and specific conditions that maximize the stabilization effects of CCR and RHA in various soil types, particularly expansive soils like black cotton soil. This gap indicates a need for comprehensive studies that investigate the interaction between CCR and RHA, their effects on a wider range of soil properties, and the development of practical guidelines for their use in geotechnical applications.

Keeping in view, the present study focuses on adding CCR and RHA together to black cotton soil to assess their suitability as a stabilizer material for subgrade soils. The need to add both calcium carbide residue (CCR) and rice husk ash (RHA) together to stabilize black cotton soils is driven by their complementary chemical properties, which effectively mitigate the challenges associated with such soils. CCR is rich in calcium hydroxide, which reacts with soil particles to reduce plasticity and swelling potential. RHA contains high levels of silica, which, in the presence of calcium from CCR, undergoes pozzolanic reactions to form additional cementitious compounds. This combination will not only enhance the immediate stabilization effects provided by CCR but will also ensure strength through the continuous pozzolanic activity of RHA.

Materials

Soil

The black cotton soil was procured from Adoni, kowthalam, and Kurnool villages in Andhra Pradesh State, India, for the purpose of this study (Figs. 1 and 2). The soil sample was initially sieved using a 4.75 mm sieve to eliminate larger aggregates and achieve a uniform particle size distribution. Following sieving, the soil was subjected to oven drying for 24 h to ensure complete moisture removal, thus facilitating precise laboratory analyses. The processed soil was subsequently analyzed using wet sieving and hydrometer methods to ascertain its granulometric composition and particle size distribution as per17, as shown in Fig. 3. The wet sieving technique enabled the separation of coarser particles, while the hydrometer analysis provided detailed information on the finer fractions of the soil. The comprehensive assessment of the soil’s physical properties and chemical composition is tabulated in Tables 1 and 2, respectively.

Fig. 1
figure 1

Sample Collection site.

Full size image
Fig. 2
figure 2

Black cotton soil.

Full size image
Fig. 3
figure 3

Particle size curves for various materials.

Full size image

The A-line chart represents the plasticity characteristics of black cotton soil (BCS) on a plasticity chart, which is a standard tool used for soil classification in the Unified Soil Classification System (USCS). The x-axis denotes the liquid limit (LL) in percentage, while the y-axis represents the plasticity index (PI) in percentage (Fig. 4). The chart is divided into various soil groups based on the A-line and other boundaries. The plotted point for BCS is situated well above the A-line, in the zone designated as “CH,” which corresponds to inorganic clays of high plasticity. The high PI and LL values of BCS indicate that it has a significant swelling and shrinkage potential, characteristic of expansive soils dominated by montmorillonite clay minerals.

Fig. 4
figure 4

A-line chart for black cotton soil.

Full size image

Calcium carbide residue

For the current study, Calcium Carbide Residue (CCR) was sourced from a gas welding shop located in Chandigarh, India (Fig. 5). Upon acquisition, the CCR underwent an initial sun-drying process lasting 3 to 4 days to reduce its elevated moisture content. Subsequently, for experimental purposes, the residue was subjected to further drying in an oven set at 110 °C for a duration of 24 h. Table 1 comprehensively details the physical properties of the CCR, and Table 2 offers an extensive analysis of the CCR’s chemical composition.

Fig. 5
figure 5

Calcium carbide residue.

Full size image

Rice husk Ash

The rice husk used in this study was sourced from Raja Rice Mill industry located in Ludhiana, Punjab, India, as Punjab is one of the leading states in rice production in India, contributing approximately 10% to the national total (Fig. 6). The rice husk underwent controlled combustion in a furnace at the mill, where temperatures were maintained between 500 °C and 600 °C for approximately 1 h. This controlled burning process was designed to maintain the amorphous state of silica within the husk and to produce rice husk ash. Following combustion, the rice husk ash was carefully sealed in air-tight packaging and transported to the geotechnical engineering laboratory for thorough evaluation of its geotechnical properties. The analysis of the gradation curve of the rice husk ash revealed that a significant portion of the particles have sizes below 75 μm (Fig. 3). Comprehensive geotechnical and chemical properties, provided by the industry, are detailed in Tables 1 and 2 respectively.

Fig. 6
figure 6

Rice husk ash burnt at 600 0C.

Full size image
Table 1 Physical properties of various materials used in the study.
Full size table
Table 2 Chemical properties of various materials used in the study.
Full size table

Methodology

A systematic approach was employed for soil stabilization to ensure uniformity and enhance geotechnical properties. The black cotton soil was sieved through a 4.75 mm sieve to remove larger particles and air-dried for a minimum of three days. The specific gravity tests for various materials were conducted as per18 and then differential free swell tests were conducted on soil and various percentages of CCR (5, 10, 15, 20, and 25%) as per19. The selected percentages also account for the unique properties of the black cotton soil under investigation, allowing for a comprehensive evaluation of the optimal CCR content. The optimum content of CCR was again then mixed with black cotton soil and RHA was added in varying amounts (5–25%) to evaluate swelling behaviour. For carrying out compaction tests, three repetitive tests were conducted for compaction characteristics under modified Proctor energy as per ASTM20, and the optimum moisture content and maximum dry density were determined for black cotton soil, black cotton soil mixed with varying content of CCR (5–25%), and CCR stabilized black cotton soil (optimum content) mixed with RHA. The liquid and plastic limits of the CCR-stabilized samples and CCR + RHA stabilized samples were also determined immediately after mixing as per21. These compacted samples aimed to evaluate the impact of CCR on strength development and enhancement zones. The unconfined compressive strength (in accordance with22 for a period of 3, 7, 28, and 56 days and soaked CBR tests as per23 of black cotton, black cotton soil mixed with varying content of CCR (5–25%), and CCR stabilized black cotton soil (optimum content) mixed with RHA. Based on CBR values, the resilient modulus was noticed for various combinations, and finally, the thickness of the flexible pavement was designed for all combinations as per24. Table 3 presents the various combinations of CCR and RHA selected for the study.

Table 3 Various combinations of CCR and RHA.
Full size table

Results and discussion

Differential free swell

The differential free swell value for black cotton soil is 73%, revealing a higher degree of expansion and may pose problems to substructures built over it (Fig. 7). In order to reduce the DFS value, CCR is added in varying amounts from 5 to 25%, and the differential free swell is noticed. Initially, the differential free swell of BCS soil decreases when 5% and 10% calcium carbide residue CCR is added because CCR, with its lower specific gravity and volume, helps reduce the BCS soil’s overall swelling potential. The CCR particles fill the voids within the soil matrix and potentially alter the soil’s structure, thus mitigating its ability to absorb water and swell. This results in a decrease in the differential free swell index at these lower percentages. However, as the percentage of CCR increases to 15%, 20%, and 25%, the differential free swell index tends to stabilize or revert to levels closer to that of the original soil (Fig. 7). The increase in differential free swell (DFS) observed beyond 10% CCR addition results from a shift in the soil’s structural and chemical stabilization balance. Up to 10% CCR, the pozzolanic reactions improved soil particle bonding, filling voids, and reducing swelling. However, at higher CCR levels, excess unreacted CCR began to act as an inert filler, disrupting the soil matrix and retaining moisture, which increased swelling potential. This unreacted CCR impedes the soil-cement matrix, leading to a less cohesive structure that enhances DFS under moisture exposure. As a result, the differential free swell approaches the initial values, reflecting the combined influence of the high CCR content and the inherent swelling properties of the BCS soil. From the previous study it was indicated that the swelling of black cotton soil (70%) was significantly reduced with the addition of CCR, making the soil suitable for subgrade applications7.

When RHA is added in small amounts (5–15%) in 10% CCR stabilized soil, the DFS typically decreases (Fig. 7). This is because RHA, like CCR, contributes to reducing the soil’s swelling potential by filling voids and modifying the soil matrix. The lower specific gravity and the reactive silica content in RHA work to further stabilize the soil, thus decreasing its ability to swell when exposed to moisture. As the proportion of RHA increases to intermediate levels (20–25%), the DFS reduction slows down. The combined effects of RHA and CCR in controlling soil swelling start to balance out, leading to less pronounced changes in DFS. This may be due to the effectiveness of RHA and CCR in reducing swelling, and also the increased volume of RHA may alter the soil’s structure in a way that reduces the overall stabilization effect. Additionally, the higher moisture retention capacity of RHA might contribute to increased swelling potential, counteracting the benefits of the CCR stabilization. A combination of 10% CCR with 15% RHA addition provides the optimum value of DFS as 14%, thus lying under low swelling soil.

Fig. 7
figure 7

Differential free swell of soil mixed with 10% CCR and varying RHA content.

Full size image

Compaction characteristics

Figures 7 and 8 depict typical compaction and strength curves for CCR-stabilized clay with varying CCR contents, respectively. The maximum dry density (MDD) decreases as CCR content increases (Fig. 8), which may be primarily due to its lower specific gravity. This reduction in MDD correlates with an increase in optimum moisture content (OMC). With higher CCR content, the water sensitivity of the stabilized clay diminishes, meaning that a significant increase in water content results in minimal changes in dry unit weight. From the previous study it was noticed that increasing CCR content in stabilized clay reduces maximum dry unit weight and increases optimum water content while decreasing water sensitivity25.

Fig. 8
figure 8

Compaction curves for black cotton soil comprising varying amount of CCR.

Full size image

When rice husk ash (RHA) is added to 10% calcium carbide residue (CCR)-stabilized BCS soil in quantities ranging from 5 to 25%, the optimum moisture content (OMC) and maximum dry density (MDD) are affected in distinct ways. As RHA is incorporated to 10% CCR stabilized BCS soil, the OMC generally decreases from an initial value of 24–18.8% at 15% RHA content. This reduction occurs because RHA, being a lightweight material with relatively low water absorption capacity, does not require as much moisture to achieve optimal workability compared to the soil and CCR mixture. Simultaneously, the MDD typically increases from 1.551 g/cc to 1.597 g/cc with the addition of varying amounts of 15% RHA in 10% CCR stabilized BCS soil. This increase in MDD is due to RHA’s pozzolanic properties, which enhance soil compaction and reduce voids within the soil matrix, thus improving overall density. Therefore, the addition of RHA to a CCR-stabilized soil generally improves compaction efficiency, resulting in a higher MDD while reducing the OMC needed for optimal compaction (Fig. 9). However, in some combinations the increase in maximum dry density (MDD) despite the lower specific gravity of CCR and RHA may be due to enhanced particle rearrangement and densification from pozzolanic reactions. CCR provides calcium hydroxide, which reacts with the silica in RHA, forming cementitious compounds that improve soil structure by reducing voids and increasing interparticle bonding. Additionally, the altered soil texture and improved workability due to stabilization may allow for better compaction efficiency, leading to a denser soil matrix despite the lower specific gravity of the stabilizers.

Fig. 9
figure 9

Compaction curves for black cotton soil comprising varying amount of CCR.

Full size image

Consistency limits

Figure 10 illustrates the evolution of index properties in CCR-stabilized BCS soil. With increasing CCR content, there is a substantial rise in the plastic limit (PL) of the stabilized clay, accompanied by a slight decrease in the liquid limit (LL), resulting in a reduction of the plasticity index (PI). This decline in PI signifies the flocculation of clay particles, driven by the adsorption of Ca²⁺ ions through the cation exchange process. However, beyond a CCR content of 10%, the change in PI becomes negligible (Fig. 4). This suggests that the BCS soil reaches its maximum capacity for Ca²⁺ ion adsorption at 10% CCR, defining this threshold as the “CCR fixation point.” Earlier research revealed that as CCR content increases, the plastic limit of stabilized clay slightly rises, while the liquid limit decreases, reducing the plasticity index due to clay particle flocculation. The change in plasticity index stabilizes after 7% CCR, indicating the “CCR fixation point” where maximum Ca²⁺ ion adsorption occurs7.

Fig. 10
figure 10

Consistency limits of CCR blended soil.

Full size image

Further, the addition of RHA in varying amounts from 5–25–10% CCR stabilized black cotton soil reduces the liquid limit (LL) of the composite from an initial value of 45–25% at 15% RHA content, and further addition of RHA beyond 15% decreases the liquid limit but at a very constant rate (Fig. 11). Initially, the abrupt decrease in LL with the addition of 15% RHA occurs because RHA significantly alters the soil’s moisture retention properties. At this level, RHA effectively disrupts the soil matrix by filling voids and reducing the soil’s capacity to hold water. Its high specific surface area and low moisture content mean that it significantly lowers the amount of water required for the soil to reach a liquid state. This dramatic reduction reflects a substantial change in the soil’s consistency and fluidity.

However, as RHA content increases beyond 15%, the effect on LL becomes more gradual. This is due to the fact that the soil mixture reaches a new equilibrium with the increased RHA content. Beyond 15%, additional RHA has a diminishing impact on further lowering the LL because the soil’s ability to change its moisture characteristics becomes limited. The soil matrix has already been modified significantly, and the capacity for further alteration decreases. As a result, the decrease in LL becomes less pronounced with higher RHA concentrations, reflecting a stabilizing effect where additional RHA contributes less to further reducing the liquid limit.

The contrasting behavior of CCR and RHA in affecting the plastic limit (PL) of soil likely results from their differing chemical properties and their interactions with soil particles. When CCR is added to soil, its high calcium hydroxide content promotes cation exchange reactions with clay particles. This exchange alters the soil structure by increasing flocculation and reducing particle mobility, leading to an increase in the PL as the soil becomes more resistant to deformation under moisture. In essence, the CCR alone stabilizes the soil by binding particles more firmly, resulting in higher cohesion and plasticity.

However, when RHA is introduced in combination with CCR, its high silica content encourages additional pozzolanic reactions with the calcium from CCR. These reactions form cementitious compounds that strengthen the soil matrix, leading to a denser, more stable structure with reduced plasticity. The resulting effect of these pozzolanic reactions is a reduction in the PL, as the soil particles become more tightly bound and less prone to changes in consistency with moisture content. This combined action effectively counters the increase in PL caused by CCR alone, resulting in an overall decrease in PL when both materials are used together.

Fig. 11
figure 11

Consistency limits of varied RHA to CCR (10%) blended soil.

Full size image

Unconfined compressive strength

Fig. 12
figure 12

Strength development of CCR blended soil at varying contents.

Full size image

Figure 12 depicts the strength development of CCR-stabilized black cotton soil at its OMC corresponding to maximum strength. The evolution of strength can be delineated into three distinct phases: Initially, with increasing CCR content, there was a notable rise in strength, defining this phase as the active zone. Beyond this range, the rate of strength improvement diminished, exhibiting a gradual increase with nearly zero incremental gain, characterizing the inert zone (CCR content range of 8–12%). Subsequently, a decline in strength became apparent when the CCR content exceeded 15%, marking the onset of the deterioration zone. Based on comprehensive test results, the optimal CCR content that yields the highest strength was determined to be 10%, coinciding with the CCR fixation point where maximum strength was also achieved. Past studies indicated that the strength of CCR-stabilized samples increases with CCR content up to 7%, beyond which it stabilizes, indicating the CCR fixation point25.

When CCR is added to black cotton soil in proportions ranging from 5 to 25%, the UCS of the soil exhibits notable changes (Fig. 13). Initially, at 5–10% CCR addition, there is a significant increase in UCS due to the initial stabilization and the onset of pozzolanic reactions between CCR and the soil particles. This reaction reduces soil’s plasticity, resulting in increased strength. As the CCR content is increased beyond 10%, the UCS starts decreasing for all curing periods. This optimal range marks the most effective stabilization, with the pozzolanic reactions fully contributing to the strength enhancement of the soil. However, when CCR content exceeds 15% and goes up to 25%, the UCS tends to decrease abruptly. This may occur due to the excessive amount of CCR, which may lead to incomplete reactions or an imbalance in the soil’s composition, thereby reducing the effectiveness of stabilization. Consequently, the addition of 10% CCR may be considered optimal for maximizing the UCS of black cotton soils.

Fig. 13
figure 13

Unconfined compressive strength of CCR blended soil.

Full size image

When rice husk ash (RHA) is added in proportions ranging from 5 to 25% to black cotton soil already stabilized with 10% calcium carbide residue (CCR), the unconfined compressive strength (UCS) of the soil undergoes significant changes (Fig. 14). Initially, with the addition of 5–10% RHA, the UCS increases substantially due to the pozzolanic reactions between the silica in the RHA and the calcium hydroxide from the CCR. These reactions produce additional cementitious compounds, which enhance the soil’s bonding and strength. As the RHA content is further increased to 15%, the UCS reaches its peak, indicating an optimal balance between the CCR and RHA, which maximizes the pozzolanic activity and the resultant strength. Beyond this point, when RHA content is increased from 20 to 25%, the UCS tends to stabilize or slightly decrease. This can be attributed to the excess RHA, which may not fully participate in the pozzolanic reactions, leading to a dilution effect (refers to the reduction in the concentration of reactive compounds when excessive CCR or RHA is added, leading to incomplete pozzolanic reactions and limited strength gains) and potential issues with the mix’s homogeneity. Thus, the addition of RHA in the range of 10–15% to CCR-stabilized soil is generally considered optimal for enhancing the UCS of black cotton soils. Several past studies have explored the effect of adding rice husk ash (RHA) in optimal quantities (typically 10–15%) to expansive soils and have observed significant improvements in various soil properties, particularly the unconfined compressive strength (UCS). RHA, a pozzolanic material, reacts with the clay minerals in expansive soils, leading to the formation of cementitious compounds (which occurs as a result of the pozzolanic reaction between the silica-rich RHA and the calcium hydroxide (Ca(OH)₂) present in CCR and leads to the formation of key cementitious compounds such as calcium silicate hydrate (C-S-H), calcium aluminate hydrate (C-A-H), and calcium aluminosilicate hydrate (C-A-S-H), which significantly contribute to strength development and soil stabilization. C-S-H is primarily responsible for enhancing the binding properties and improving the soil’s load-bearing capacity, while C-A-H and C-A-S-H further enhance durability and resistance to environmental degradation that enhance the soil’s strength and stability.

Fig. 14
figure 14

Unconfined compressive strength of varying RHA in CCR (10%) blended soil.

Full size image

California bearing ratio

To assess the effect of CCR on the CBR of black cotton soil, CCR was incorporated in varying proportions from 5 to 25%, and the soaked CBR values were evaluated. The initial addition of 5% and 10% CCR significantly enhanced the CBR from 1.98 to 4.52% and 8.56%, respectively. This improvement is attributed to the enhanced compaction characteristics and reduced plasticity of the soil, leading to increased load-bearing capacity (Fig. 15). However, beyond 10% CCR, the rate of improvement diminishes, likely due to excessive CCR causing adverse effects such as reduced workability and alterations in soil texture, which limit further strength gains.

When RHA is introduced into black cotton soil stabilized with 10% CCR, a continuous increase in CBR values is observed as RHA content increases from 5 to 15%. RHA, being silica-rich, reacts with the calcium hydroxide from CCR, forming additional pozzolanic compounds that enhance soil stabilization and strength. The optimum RHA content for maximum improvement is found at 15%, yielding the highest soaked CBR value of 12.4%. Beyond this threshold, the rate of improvement plateaus or increases marginally, likely due to excessive RHA causing workability issues or an imbalance in the pozzolanic reactions. This indicates that an optimal CCR-RHA combination is critical for achieving superior soil stabilization and load-bearing performance.

Fig. 15
figure 15

Soaked CBR values for various combinations.

Full size image

Resilient modulus

Determining the resilient modulus (Mr​) from California bearing ratio values is essential for designing flexible pavements, as it provides insight into the material’s ability to recover its shape after stress application. The resilient modulus is used to characterize the elastic behavior of the soil or pavement layer under repeated loading conditions, which is particularly relevant for traffic loading scenarios. Based on the CBR values of various mix proportions, resilient modulus is calculated as per guidelines given in24, and values are tabulated in Table 4.

The following formulae are used to determine resilient modulus based on CBR values for various combinations:

For CBR ≤ 10%, \(\:{M}_{R}\)= 10 x CBR

For CBR > 10%, \(\:{M}_{R}\)= 17.6 x \(\:{CBR}^{0.64}\)

Table 4 Resilient modulus values for various combinations.
Full size table

The CBR value of subgrade soil plays a crucial role in the design of pavement thickness as it represents the strength and load-bearing capacity of the underlying soil. A higher CBR value indicates a stronger subgrade, requiring a thinner pavement, whereas a lower CBR suggests a weaker subgrade, necessitating a thicker pavement to distribute traffic loads effectively. In flexible pavement design, the CBR value is used as an input parameter in empirical and semi-empirical methods24. It helps determine the required thickness of pavement layers (sub-base, base, and surface course) to ensure structural stability, durability, and performance under expected traffic loads.

To determine the thickness of various layers of flexible pavement, the thickness values are designed using the plates provided in24, as shown in Table 5. This table illustrates the thickness requirements for different million standard axles (msa) ranging from 5, 10 and 20 msa, based on the CBR values of different soil composites. The data reveals that incorporating optimal proportions of various waste materials, either individually or in combination, effectively reduces the pavement thickness across all msa values. For instance, at 50 msa, the total pavement thickness of a flexible pavement is a minimum of 630 mm when black cotton soil is not replaced with any material. When the soil is replaced with 10% CCR, the thickness decreases to 600 mm, and further addition of 15% RHA to 10% CCR stabilized black cotton soil reduced the pavement to 590 mm, respectively. This trend of reduced pavement thickness with increased waste content is consistently observed across all msa values, as detailed in Table 5.

Table 5 Pavement thickness for flexible pavement for various combinations24.
Full size table

Economic analysis

The cost analysis for a single-lane National Highway having a width of 3.75 m with a flexible pavement design for 50 msa was conducted using the Punjab Public Works Department (PWD - Border & Roads) 2023 rate handbook. In this study, a road length of 1000 m was assumed with a side slope of 1:2, with the pavement structure comprising several layers: Bituminous Course, Dense Bituminous Macadam (DBM) Course, Water Bound Macadam (WBM) Course, Sub-base Course, and Subgrade, and taking thickness of each material combination from Table 5. Each layer’s materials and associated costs were analyzed to determine the most cost-effective combination for the pavement structure. Table 6 presents a cost analysis comparing three subgrade material combinations for road construction: 100% BCS, 90% BCS with 10% CCR, and 75% BCS with 10% CCR and 15% RHA. The use of alternative materials in the second and third combinations significantly reduces costs due to the low prices of CCR and RHA, which are industrial and agricultural byproducts, as clearly seen from Table 6. For example, while the 100% BCS configuration costs ₹33,274,130, introducing 10% CCR reduces costs to ₹30,257,830, saving ₹3,016,300. Further adding 15% RHA reduces costs to ₹28,985,156.25, yielding savings of ₹4,288,973.75. These savings result from the lower material costs of CCR and RHA and their ability to enhance soil stability, reduce plasticity, and minimize moisture-related expansion issues common with BCS. The improved engineering properties contribute to a more strengthened pavement structure with lower maintenance needs, offering long-term economic benefits and aligning with sustainable construction practices by repurposing waste materials. The reduction in material usage, such as lower raw material consumption, reduced carbon emissions from material production and transportation, and minimized construction waste, inherently leads to sustainability advantages, aligning with established principles in pavement design and green construction practices.

Table 6 Cost analysis of flexible pavement for various mixes.
Full size table

Statistical analysis

The Spearman correlation heatmap analysis (Fig. 16) of unconfined compressive strength (UCS) and key soil properties—maximum dry density (MDD), optimum moisture content (OMC), rice husk ash (RHA), and calcium carbide residue (CCR)—in black soil provides critical insights into soil stabilization mechanisms. A strong positive correlation between UCS and RHA (0.8278) suggests that RHA significantly enhances soil strength, likely due to its pozzolanic properties and ability to improve particle binding. Similarly, a moderate positive correlation between UCS and MDD (0.5239) indicates that well-compacted soil structures contribute to strength enhancement. Conversely, the moderate negative correlation between UCS and OMC (−0.5) highlights the detrimental impact of excessive moisture on compressive strength, as higher OMC reduces soil compaction efficiency. The strong negative correlation between MDD and OMC (−0.9977) further supports this finding, indicating that increased moisture content reduces dry density and compromises structural integrity. Additionally, the strong positive correlation between MDD and RHA (0.8669) suggests that RHA enhances soil densification, whereas its strong negative correlation with OMC (−0.8476) implies that RHA reduces the moisture requirement for optimal compaction. The weak positive correlation between UCS and CCR (0.2478) indicates a limited effect of CCR on strength, though it may still contribute to stabilization in conjunction with other additives. The moderate positive correlation between OMC and CCR (0.5998) suggests that CCR increases soil water demand, potentially due to hydration-related chemical interactions. The weak negative correlation between RHA and CCR (−0.1486) indicates minimal interaction between these two additives in stabilization. These findings, as illustrated in Fig. 16, underscore the effectiveness of RHA in improving UCS, the critical role of MDD in achieving optimal soil strength, and the necessity of managing OMC to prevent strength reduction. The results provide a robust framework for optimizing black soil stabilization strategies in geotechnical engineering applications.

Fig. 16
figure 16

Correlation of UCS with various additives and their properties.

Full size image

Conclusions

The utilization of calcium carbide residue (CCR) and rice husk ash (RHA) offers a promising approach for sustainable soil stabilization, particularly for black cotton soils. Based on the experimental testing, the following major conclusions have been drawn from the study:

  1. 1.

    The study demonstrates that using 10% CCR and 15% rice husk ash (RHA) reduces the differential free swell index of black cotton soil from 73 to 14%, effectively classifying it as low-swelling soil. This reduction is attributed to CCR’s void-filling capacity and pozzolanic interactions between RHA’s silica and CCR’s calcium.

  2. 2.

    Increasing CCR content up to 25% results in a reduction in maximum dry density from 1.597 g/cc to 1.551 g/cc and a corresponding rise in optimum moisture content due to the lower specific gravity of CCR. When RHA is added in varying proportions (5–25%) to 10% CCR-stabilized soil, the OMC decreases from 24 to 18.8%, and MDD increases, indicating enhanced densification and moisture efficiency for field compaction.

  3. 3.

    The addition of CCR increases the plastic limit and decreases the liquid limit, reducing the plasticity index. The maximum PI reduction is observed at 10% CCR, attributed to clay particle flocculation and Ca²⁺ ion adsorption from CCR, resulting in effective stabilization.

  4. 4.

    Unconfined compressive strength improves notably within the 5–10% CCR range, peaking at 10% CCR as the most effective stabilization point. Adding 15% RHA to CCR-stabilized soil further elevates UCS due to additional pozzolanic reactions that enhance cementitious bonding. Higher CCR or RHA contents yield limited further strength gains, likely due to incomplete reactions or dilution effects.

  5. 5.

    California Bearing Ratio (CBR) shows significant improvement, rising from 1.98% (untreated soil) to 8.56% with 10% CCR and peaking at 12.4% with 15% RHA addition. This improvement results from particle reorganization and cementation, making the soil suitable for high-load applications in flexible pavement subgrades.

  6. 6.

    Based on CBR-derived resilient modulus values per IRC:37–2018 guidelines, the required pavement thickness reduces with the CCR-RHA mix. For instance, a pavement thickness of 630 mm for untreated soil is reduced to 590 mm with the optimal CCR-RHA combination, offering both economic and environmental benefits in pavement design.

  7. 7.

    The cost reduction for the final combination (BCS: CCR: RHA::75:10:15) compared to the original one (BCS::100) is approximately 12.89%. This demonstrates significant savings while maintaining or improving soil stabilization and pavement performance. This approach not only achieves cost-effectiveness but also promotes sustainability by utilizing industrial and agricultural byproducts.

  8. 8.

    The analysis confirms that RHA enhances UCS, while MDD plays a vital role in achieving optimal soil strength. Proper OMC management is essential to prevent strength reduction and improve black soil stabilization.

The findings of this study illustrate the successful stabilization of black cotton soil using calcium carbide residue (CCR) and rice husk ash (RHA), achieving significant improvements in geotechnical properties while addressing environmental concerns associated with their disposal. This combined approach not only offers a sustainable solution for enhancing subgrade soil performance but also demonstrates its economic viability for large-scale infrastructure projects.

Future recommendations

Future studies may focus on conducting comprehensive field trials to confirm the laboratory findings regarding the use of calcium carbide residue (CCR) and rice husk ash (RHA) for soil stabilization. Researchers may investigate the long-term stability of treated soils, especially under varied environmental conditions like moisture and temperature changes, to understand durability better. There may also be value in exploring the applicability of CCR and RHA stabilization for different soil types, potentially benefiting regions with similar soil challenges. Additionally, large-scale studies on economic feasibility may provide insights into cost savings and benefits for infrastructure development, which may guide policy and industry practices.

The future studies may focus on conducting comprehensive field trials to confirm the laboratory findings regarding the use of calcium carbide residue (CCR) and rice husk ash (RHA) for soil stabilization. Researchers may investigate the long-term stability of treated soils, especially under varied environmental conditions like moisture and temperature changes, to understand durability better. There may also be value in exploring the applicability of CCR and RHA stabilization for different soil types, potentially benefiting regions with similar soil challenges. Additionally, large-scale studies on economic feasibility may provide insights into cost savings and benefits for infrastructure development, which may guide policy and industry practices.