Field Studies on Expansive Soil Stabilization with Nanomaterials and Lime for Flexible Pavement

Abstract

The long-term performance of pavement is greatly influenced by the subgrade soil-bearing capacity. The areas with lower bearing capability experience higher construction costs due to soil replacement. Soil stabilization is one of the engineering measures that may be used to improve soil properties. The improvement in the soil properties varies depending on the soil type and type and dosage of the stabilizer. The primary objective of this study is to determine the impact of the different types of stabilizers on different types of black cotton soil. In the present study, black cotton soil was treated with Terrasil (0.5, 0.75, and 1 kg/m³), Zycobond (0.5, 0.75, and 1 kg/m³), and lime (0, 2, and 3%). The influence of varying dosages of Terrasil, Zycobond, and lime showed a significant improvement in the FSI, CBR, and UCS. In this study, attempts were made to investigate the field performance of chemically treated black cotton soil. A 100 m trail section with chemical- and lime-treated subgrade was constructed and analyzed using the dynamic cone penetration test. Finally, the mechanical design indicated that the chemical stabilization layer could be helpful to reduce asphalt layer thickness by 30 mm and cost. It is anticipated that this study will be useful to perceive, visualize, and understand the advantages of chemically treated black cotton soil. Overall, it is a step toward sustainable construction, which will reduce the demand for natural materials by optimizing pavement design and the use of existing unsuitable materials (black cotton soil) in flexible pavement construction.

1. Introduction

The pavement performance generally depends on the properties of the materials and subgrade soil strength [1]. Each layer of the pavement must support and distribute the superimposed loads in order for the pavement to function correctly. Due to the heavy traffic and vehicle loads, roads will soon deteriorate without adequate resistance. Innovative techniques are required to make the expansion of the road network sustainable from economic and environmental perspectives. The subgrade’s soil stabilization improves the material’s stability to resist deformation from repeated transverse loads [2,3,4]. Soil stabilization is a technique used to prevent problems like potholes, rutting, and muddiness on roadways [5,6,7]. Black cotton soil has a dual nature when exposed to moisture imbalance, and this can cause significant degradation of the pavement surface (bitumen/concrete), which can lead to costly maintenance and repairs [8,9].

1.1. Literature Review

There are multiple techniques that may be implemented to reduce the level of damage caused by expansive soil due to volume change characteristics. Some of these techniques include expansive soils replaced with other suitable soils, controlling the moisture in the soil, and soil stabilization to improve the engineering properties of the soil [10].

There are two categories of soil stabilization techniques, i.e., (i) traditional stabilization (lime, cement, rice husk, and fly ash), and (ii) non-traditional stabilization (enzymes, polymers, sulfonated oils, and others). Over the past few years, researchers have examined the application and effects of traditional stabilizers [11,12].

Soil stabilization with the traditional stabilizers yielded positive results. However, these traditional stabilizers have some potential disadvantages [13]. Tang et al. [14] and Bell [15] reported that the plasticity index (PI) of the soil is increased with the use of lime as a soil stabilizer. Soil stabilization with cement has positive results, whereas during production, 1 ton of cement, emits 1 ton of CO₂. It potentially increases greenhouse gases and raises environmental concerns. In order to reduce the carbon footprint of the paving sector, researchers are exploring non-traditional soil stabilizing materials and techniques to improve carbon footprint and cost effectiveness.

Scholen et al. [16] based the mechanism of the non-traditional stabilizer, which is further divided as ionic, polymer, and enzyme, on its physical and chemical analysis. Enzymes are naturally occurring and concentrated liquid stabilizers that are formed from organic materials [17,18]. It is proven that bio-enzymes are non-toxic, cost-effective, and convenient to use as stabilizing agents [19].

Venkata Subramanian and Dhinakaran [20] concluded that with the use of a bio-enzyme the California bearing ratio increased 3 to 4 times, whereas the UCS improved 2 to 3 times depending on the soil type. Lacuture and Gonzalez [21] examined the effectiveness of the Terrazyme on the subgrade and sub-base soils. A progressive improvement in soil properties was observed, but early reports did not indicate any significant modifications. According to Hitam [22], the Terrazyme-built roads, which have had significant problems in the past due to rain, are performing well after two monsoons. Tingle et al. [23] concluded that the use of enzymes improved low plastic clayey soil strength by 4–6%.

Khan and Taha [24] studied the impact of three different bio-enzymes on soil stabilization. From the studies, a marginal enhancement of soil properties was observed. According to Sahoo and Sridevi [25], the CBR value was increased by 1.31 times when the soil was stabilized using Terrazyme compared to soil without a stabilizer. Li et al. [26] assessed the cost analysis of the soil stabilization with bio-enzymes and cement stabilization and reported a 25–30% reduction in cost with bio-enzymes. Kushwaha et al. [4] investigated the effects of three different Terrazyme dosages and curing times (7,14, 21, 28, and 45 days) on soil stabilization. The MDD improved by 1.6%, while the ideal moisture content (OMC) dropped by 9.59% with the usage of Terrazyme. The results show that when Terrazyme was used to stabilize the soil, the amount of work required to compact the soil decreased, and its workability improved. The CBR value was raised from 3.0% to 7.5% after curing for 45 days. This demonstrates that the curing time is essential for the development of strength in soil that has been stabilized by bio-enzymes.

1.2. Research Gap and Objective

Although several research studies have demonstrated that expansive soil performance can be enhanced by the addition of soil stabilizers, most of these studies have only examined the effects of a single type of stabilizer. They can only increase the engineering properties of the soil. The effect of water on the treated expansive soil is not considered in the durability study.

It is important to evaluate the effectiveness of combination stabilizers on the performance of expansive soils since the degree to which stabilization affects the strength parameters of the expansive soils varies depending on the type of stabilizer used. This study gave an insight into the effect of water on expansive soil. The treated soil becomes hydrophobic, resulting in neutralizing the reaction of water.

However, the vast majority of research was conducted in controlled laboratory conditions. Therefore, it is believed that further real-time field-based studies are required to obtain a better understanding of expansive soil subgrade construction with the use of soil stabilization techniques. Such insight could significantly improve engineering approaches to expansive soil stabilization.

The main object of this research was to evaluate the efficacy of lime and chemical (Zycobond and Terrasil) treatments on the subgrade strength characteristics of expansive soil. This study also examined the combined effect of chemical- and lime-treated expansive soil through field studies.

The specific objectives of this study are as follows:

• To evaluate the physical properties of expansive soil treated with different dosages of chemicals and limes using a CBR and UCS;
• To investigate and understand the combined effect of chemical and lime treatment through semi-field studies;
• To assess the influence of the combined effect of chemicals and limes on expansive soil field CBR measured by the dynamic cone penetrometer test (DCPT) and compare the cost analysis based on the mechanistic–empirical flexible pavement design;
• To develop an empirical CBR model for lime and chemically treated expansive soils.

2. Materials

2.1. Black Cotton Soil

In this study, the black cotton soil was collected from three different locations in Gujarat state, India, as shown in Figure 1. The lumps in the soil samples were pulverized with a wooden hammer after being dried in the oven. The soil samples performed a series of investigations to determine their geotechnical, mechanical, and index properties.

Table 1. Basic properties of black cotton soil.

Particulars	Soil-I	Soil-II	Soil-III	Standard
	(Lunawada)	(Rajpipla)	(Halol)
	%	%	%
Aggregates	0	0	0	IS 2720—Part 2 [27] and IS 1498 [28]
Sand	30.5	21.3	8.9
Silt and clay	69.5	78.5	91.1
LL	31.75	41.54	68.2	IS 2720—Part 5 [29]
PL	19.32	22.58	34.73
PI	12.43	18.96	33.48
FSI	30	40	70	IS 2720—Part 40 [30]
MDD	1.960	1.830	1.820	IS 2720—Part 8 [31]
OMC	9.6	14	13.7	IS 2720—Part 8 [31]
CBR	4.5	3.31	1.86	IS 2720—Part 16 [32]

lists the physical properties of the soil without stabilizers.

2.2. Terrasil

Terrasil is a water-soluble heat- and UV-resistant soil-modifying additive that is made entirely of organo-silanes. Its main purpose is to keep the soil waterproof. It has silanol groups, which modify the surface of the soil and give it long-lasting hydrophobic properties by interacting with the soil’s silicates. This will make the soil impermeable and avoid the problems that result from the presence of water. The basic properties of the Terrasil given in the

Table 2. Basic properties of Terrasil.

S. No.	Properties	Values
1	Color	Pale yellow
2	Odor	Slightly aromatic
3	Physical state	Liquid
4	Flashpoint (°C)	90
5	Density (g/mL)	1.05
6	Viscosity @ 25 °C, (cP)	100–500
7	Boiling point (°C)	211
8	pH	3.1–5.0

.

2.3. Zycobond

Zycobond is a co-polymer emulsion of acrylic nanotechnology with a lifespan of more than ten years. It works in conjunction with Terrasil to limit expansivity and bind, which strengthens and stabilizes the soil. Zycobond provides a watertight seal, preventing water from penetrating unpaved areas. The basic properties of the Zycobond given in the

Table 3. Basic properties of Zycobond.

S. No.	Properties	Values
1	Physical state	Liquid, dispersion
2	Odor	Faint odor
3	Color	Translucent
4	Flashpoint (°C)	>70
5	Density (g/mL)	1–1.02
6	Viscosity @ 30 °C, (cP)	20–200
7	Boiling point (°C)	100
8	pH	5.0–5.6

.

2.4. Lime

In the present study, quick lime (CaO) with a molecular weight of 55.89 and a purity level of 96% is used as a soil stabilizing agent, which satisfies the requirements of IS: 1514 (1990) [33].

Table 4. Physical properties of lime.

S. No.	Properties	Values
1	Physical state	Solid
2	Density (g/cc)	2.34
3	Specific gravity	3.1
4	Boiling point (°C)	2910
5	Melting point (°C)	2565
6	pH	12.35

lists the physical characteristics of lime.

3. Experimental Methodology

3.1. Method

Figure 2 shows the experimental plan for the design of expansive soil stabilization with chemicals and limes. The soil samples were collected from three different locations for the laboratory experimental studies. The chemical varied from 0 to 1 kg/m³, and lime was used at 0, 2, and 3% by the weight of soil. The soil was tested for the free swelling index, moisture–density relationship, unconfined compressive strength, and California bearing ratio (CBR). The laboratory test results were used to arrive at the optimum dosage of chemicals and limes. Field trials were carried out by constructing a 100 m stretch divided into two sections. The first 50 m of test track was designated as Section I and was constructed with 1 kg/m³ Terrasil + 1 kg/m³ Zycobond + 3% lime and compacted to a depth of 250 mm. The second test section (Section II) was constructed with lime stabilization and compacted to a depth of 250 mm. The dynamic cone penetrometer test and simple water affinity test were carried out on the test track. The filed CBR was measured from the DCPT data and was compared with the laboratory data. The mechanistic–empirical approach given in IRC 37: 2018 [34] was used to design the pavement thickness.

3.2. Sample Preparation

The black cotton soil is uniformly mixed with the different dosages of Terrasil (0.5, 0.75, and 1 kg/m³), Zycobond (0.5, 0.75, and 1 kg/m³), and lime (0, 2, and 3%). Prior to mixing with the soil stabilizers, the 20 mm IS sieve passed soil dried in the oven. Lime is added to the soil sample and mixed uniformly, followed by Terrasil and Zycobond. First, Terrasil is added followed by Zycobond in the water for spraying. Thereafter, the soil is mixed, uniformly compacted to the required density, and allowed to dry for four days.

3.3. Free Swelling Index

The free swelling index (FSI) measures the soil’s capacity to swell in accordance with IS: 2720 (Part 40) [30]. The 10 g of soil that passes through a 425 µm sieve is placed in two graded 100 m³ cylinders that are filled with kerosene and water. The volume difference between the two cylinders is measured, and the swelling percentage is calculated after 24 h.

3.4. Compaction

The optimum moisture content (OMC) and maximum dry density (MDD), which are compaction characteristics, are crucial soil sample parameters that have a significant impact on how well the soil performs in the field. The modified compactor test was carried out as per IS: 2720-1983 (Part 8).

3.5. California Bearing Ratio

The California bearing ratio (CBR) was used to determine the optimum stiffness of a subgrade layer in accordance with IS: 2720-1987 (Part 16). Prior to the test, the soil samples were submerged in water for 96 h. The CBR samples were prepared by the corresponding MDD and OMC.

3.6. Unconfined Compressive Strength

The unconfined compressive strength test (UCS) was carried out on black cotton soil with and without soil stabilizers in accordance with IS: 4332-1970 (Part 5) [35]. The MDD and OMC for the soil with the three different stabilizers were determined using the modified proctor test, which was used to produce the UCS samples. For each combination, three samples with identical measurements 3.8 cm in diameter and 7.6 cm in height were prepared. The samples were dried in a vacuum desiccator for seven days to prevent moisture loss. The sample was tested at a strain rate of 1.2 mm per minute.

4. Results and Discussion

4.1. Free Swelling Index (FSI)

The free swelling index (FSI) of Soil-I, Soil-II, and Soil-III with and without different additives is shown in Figure 3. The FSI of Soil-I, Soil-II, and Soil-III is 30, 40, and 70%, respectively. The minerals in montmorillonite clay are the primary cause of the expensive soil’s swelling [36,37]. The swelling potential of soil was gradually reduced with the inclusion of various proportions of Terrasil, Zycobond, and lime as additives. The FSI was reduced with the addition of 1 kg/m³ of Terrasil and 1 kg/m³ of Zycobond in Soil-I, Soil-II, and Soil-III by 50, 25, and 29%, respectively, without the addition of lime. On the other hand, the FSI of Soil-I, Soil-II, and Soil-III dropped by a significant amount once lime was added. The FSI values of Soil-I and Soil-II are completely reduced by the addition of 3% lime, whereas Soil-III reduced by 79%. This is because the water affinity of the soil decreased as a result of the pozzolanic reaction, which binds the flocculated soil particles [38,39].

4.2. Optimum Moisture Content and Maximum Dry Density

Figure 4 shows the MDD and OMC of Soil-I, Soil-II, and Soil-III with and without different additives. The OMC of Soil-I, Soil-II, and Soil-III is 9.6%, 14%, and 13.7%, respectively, without any additives. The OMC is reduced in three types of soils with the addition of Zycobond and Terrasil. After the addition of 1 kg/m³ of Terrasil and 1 kg/m³ of Zycobond, the amount of the OMC in Soil-I, Soil-II, and Soil-III reduced by 15%, 18%, and 29%, respectively. The use of Terrasil and Zycobond decreases the soil’s affinity for water, resulting in a lowering of the soil’s water content [40,41]. Similarly, the addition of lime along with Terrasil and Zycobond further reduces the OMC.

The concentration of electrolytes in the pore water increases and thinned the double layer with the addition of lime. As a result, van der Waals attraction prevails, and the clay particles draw closer together. This results in flocculation and the formation of a clay structure resembling a card house. This card house structure of the clay matrix successfully resists the effort to compact, leading to higher moisture and less density.

The MDD value of control samples was 1.961, 1.833, and 1.821, respectively, for Soil-I, Soil-II, and Soil-III, as shown in Figure 5. The addition of Terrasil and Zycobond improved the MDD value; however, the higher value was with 1 kg/m³ of Terrasil and 1 kg/m³ of Zycobond. Increased MDD is an indication of improved soil strength characteristics. Similar results were reported in a study conducted by Aboukhadra et al. [42].

On the other hand, a marginal decrement in the MDD with the addition of lime was observed. However, the MDD of the lime-added soil samples is higher than the control samples. The addition of lime to the soil may reduce the MDD because the soil particles are replaced by lime particles that are relatively low in density and the soil grading is changed.

4.3. California Bearing Ratio

Figure 6 illustrates the variation in the CBR with and without using soil stabilizers. From the results, it can be noticed that the CBR of Soil-I, Soil-II, and Soil-III is 4.5%, 3.3%, and 1.86%, respectively, after 4 days of soaking without any additive. The CBR value of Soil-I, Soil-II, and Soil-III increased with the addition of 0.5, 0.75, and 1 kg/cum of Terrasil and Zycobond. Out of three different proportions 1 kg/m³ of Terrasil and Zycobond have the higher CBR value. The results are in line with the other researchers, Kushwaha et al. [4] and Eujine et al. [43]. There is a drastic improvement in the CBR value with the addition of lime. After the addition of 2% lime, the CBR value improved 3 to 4 times compared with the Terrasil and Zycobond soil samples without lime. Similarly, the CBR improved 5 to 6 times with the addition of 3% lime, with the same level of curing period.

4.4. Unconfined Compressive Strength

Figure 7 also shows the compressive strength of Soil-I, Soil-II, and Soil-III at different dosages of soil Terrasil, Zycobond, and lime. In the figure, it can be observed that the compressive strength of Soil-I, Soil-II, and Soil-III is 0.75 MPa, 0.61 MPa, and 0.47 MPa, respectively, after 7 days of curing without any additive. The Soil-I soil has a higher UCS value compared with Soil-II and Soil-III. It was observed from the results that the inclusion of Terrasil and Zycobond improved the soil strength after 7 days of curing. Out of three different proportions, 1 kg/cum of Terrasil and 1 kg/cum Zycobond have the higher UCS value.

On the other hand, the inclusion of lime resulted in a drastic enhancement in the UCS values. This is because the rapid cation exchange process, which was previously enhanced by Terrasil and Zycobond, increases with the addition of lime. This improvement is attributed to the addition of lime to the soil, which increases the load-bearing capability and reduces the tendency of the soil to swell. Similar results were previously shown by Eujine et al. [42], Kushwaha et al. [4], and Renjith et al. [44].

5. Field Studies

Most of the research studies have been limited to experimental laboratory studies alone. However, it is necessary to implement the laboratory experiments in real field studies, at least through trial sections, to understand and overcome the challenges. Therefore, in this study, a 100 m long and a 7 m wide field test section were constructed to assess the performance of the chemically stabilized black cotton layer. Field trials were conducted on the Vadodara Expressway bypass road service road section between chainage 343 + 100 to 343 + 200 on the left-hand side to assess the performance of the chemical in the actual field. The trial section was divided into two sections (each with a length of 50 m and a width of 7 m). The first section (Section I) was treated with lime + chemical and the second section (Section II) was treated with lime. Figure 8 shows each stage of the trial section construction.

(a)

Section I (Chainage 343 + 100 to 343 + 150)

The first 50 m of the test track was designated as Section I and was constructed with 1 kg/m³ Terrasil + 1 kg/m³ Zycobond + 3% lime and compacted to a depth of 250 mm. The existing soil CBR value is 1.86%, and the optimum moisture content is 13.7%. The step-by-step procedure used to construct the test section is explained in the following sections.

• The tractor with a ripper and rotavator arrangement was used in this study to pulverize the black cotton soil and uniformly mix limes and chemicals to finish the work.
• The required quantity of lime (3%) was speared manually throughout the length of the test section, as shown in Figure 8b. Lime and soil were mixed using the rotavator for uniform mixing, as shown in Figure 8c.
• The required quantity of the chemical was mixed with the water. The optimum quantity of the water mixed with the chemical spread uniformly over the lime-mixed surface using the tanker to cover the entire surface. After sprinkling, water soil was thoroughly mixed using the rotavator for a uniform mixing of chemicals and limes, as shown in Figure 8d.
• The treated soil is compacted using a 10-ton vibratory roller with two layers, as shown in Figure 8e.
• After 7 days of curing, the density of the compacted layer using the core cutter apparatus was measures, as shown in Figure 8f. The degree of compaction achieved was 95%.

(b)

Section II (Chainage 343 + 150 to 343 + 200)

The second test section (Section II) was constructed with lime stabilization and compacted to a depth of 250 mm. Steps 1 and 2 followed to construct lime lime-treated test section (Figure 8a–c). The required quantity of lime was spread over the back cotton soil and mixed with the rotavator. The required quantity of water (OMC) was spread over the lime-mixed soil using the water tanker and then compacted using the 10-ton vibratory roller. After 7 days of curing, the density of the compacted layer using the core cutter apparatus was measured, as shown in Figure 8f. The degree of compaction achieved was 95%.

5.1. Testes on the Filed Section

5.1.1. DCPT Test

In situ soil strength can be characterized using the dynamic cone penetration (DCP) test. A pointed cone top, a driving rod, an anvil, a hammer, and a guide are typical DCP components. The DCP test was performed in accordance with ASTM D6951, and the setup is shown in Figure 9. A driving rod of roughly 1 m in length is attached to the cone tip, which has an apex angle of 60 degrees. The typical DCP can be penetrated by a hammer weighing 8 kg dropped from a height of 575 mm. In a standard DCP test, the depth of penetration per blow is used to calculate the strength index. The field CBR value was calculated using DCPT data based on the correlation provided in ASTM D6951-2018 (Equation (1)). Table 1 shows the calculated filed CBR values.

$$C.B.R=\frac{292}{{DPI}_{Index}^{1.12}}$$

(1)

where

• C.B.R—field CBR value
• DPI_Index—DCP index mm/blow.

DCPT readings were collected at regular 10 m intervals in both Sections I and II, immediately after a curing time and again after 3 months. The DPI values for both sections are listed in

Table 5. The DCPT test data from chainage 343 + 100 to 343 + 150 (Section I).

Sl. No.	Chainage in Mtr.	After 7 Days of the Curing Period		Measurement after 3 Months
		DPI Value	CBR Value	DPI Value	CBR Value
1	343 + 105	10.2	21.8	8.6	26.4
2	343 + 115	10.6	20.7	8.9	25.4
3	343 + 125	9.9	22.5	8.3	27.1
4	343 + 135	10.9	20.0	8.4	27.0
5	343 + 145	11.2	19.6	8.0	28.6
Average:			20.9		26.9

and

Table 6. The DCPT test data from chainage 343 + 150 to 343 + 200 (Section II).

Sl. No.	Chainage in Mtr.	After 7 Days of the Curing Period		Measurement after 3 Months
		DPI Value	CBR Value	DPI Value	CBR Value
1	343 + 155	47.4	3.9	46.8	3.9
2	343 + 165	45.7	4.0	45.2	4.1
3	343 + 175	44.4	4.2	43.8	4.2
4	343 + 185	46.4	4.0	45.3	4.1
5	343 + 195	47.5	3.9	45.8	4.0
Average:			4.0		4.1

, respectively. The DPI values were found to be lower in Section I compared to Section II. This demonstrates that the soil’s penetration resistance was enhanced as a result of the chemical addition in Section I compared to Section II. The DPI value of Section I reduced by 76% almost immediately after curing compared to the value of Section II.

On the other hand, in Section I, a 20% reduction in the penetration value after 6 months after the construction of the trial section was observed. The reason for a reduction in the penetration value treated with the chemical after the three-month curing period may be due to the evaporation of adsorbed water released from the diffused double layer around the clay particles. Renjith et al. [44] concluded that CBR increased after using a bio-enzyme to construct unpaved roads. However, there is no significant improvement in the penetration value of lime treated section.

5.1.2. Water Affinity

The affinity of black cotton soil toward water is neutralized by the addition of chemicals. A simple water-repellent test was performed on both chemically treated and untreated surfaces to check the effectiveness of the chemicals used in treatment. In Figure 10a, water absorption on the lime-treated surface can be observed. Figure 10b demonstrates that the poured water is not absorbed by the black cotton soil and does not penetrate the cracked surface.

6. Pavement Design

The present study investigated the effects of chemical-treated black cotton soil on flexible pavement design from the perspective of practical application using a mechanistic–empirical approach given in IRC 37: 2018. The pavement was designed for the 30 million standard axel, granular thickness is fixed in both cases as 450 mm (sub-base—250 mm, base—200 mm), and the bituminous layer varied. The measured CBR values for Section I and Section II are 20.9 and 4%, respectively. As per MoRTH 2013, soil with a CBR less than the design values shall be removed and replaced with suitable borrowed material. In the pavement design, the CBR value for the subgrade was considered 15% for Section I and 8% for Section II, which are the minimum and maximum values of CBR considered in IRC: 37-2018.

Table 7. Input parameters in IITPAVE for pavement design.

Pavement Layer	Modulus		Poisson’s Layer
Section I (BC soil + Chemical + lime)
Subgrade layer CBR = 15%	MRS = 17.6 × (CBR)0.64 (CBR > 5%) MRS = 17.6 × (15)0.64 = 99.6 MPa		0.35
Granular layer thickness 250 mm GSB—250; WMM—200	MRGRAIN = 0.2 (h)0.64 × MRS MRGRAIN = 0.2 (450)0.64 × 99.6 = 311 MPa		0.35
Bituminous layer	Mr = 3000 MPa		0.35
Section II (BC soil + lime)
Subgrade layer CBR = 8%	MRS = 17.6 × (CBR)0.64 (CBR > 5%) MRS = 17.6 × (8)0.64 = 66.6 MPa		0.35
Granular layer thickness 250 mm GSB—250; WMM—200	MRGRAIN = 0.2 (h)0.64 × MRS MRGRAIN = 0.2 (405)0.64 × 66.6 = 208 MPa		0.35
Bituminous layer	Mr = 3000 MPa		0.35
INPUT FOR IITPAVE
Tire pressure	0.56 MPa
Dual wheel single axle	80 kN (Load on each wheel—20 kN)
Center-to-center spacing	310 mm
STRAIN VALUES AS PER IRC 37:2018
	Actual	Section I	Section II
${\epsilon }_v$	237 µstrain	218 µstrain	223 µstrain
${\epsilon }_t$	416 µstrain	295 µstrain	357 µstrain
Asphalt layer thickness in mm		90	120

shows the parameters considered for the pavement design. An empirical equation to calculate the rutting and fatigue performance of pavement, as per IRC: 37-2018, is given below (Equations (2) and (3)):

$$N_R=1.4100\times {10}^{-08}\times {\left[\frac{1}{{\epsilon }_v}\right]}^{4.5337}$$

(2)

$$N_f=0.5161\times C\times {10}^{-4}\times {\left[\frac{1}{{\epsilon }_t}\right]}^{0.854}\times {\left[\frac{1}{M_{Rm}}\right]}^{0.854}$$

(3)

$$C={10}^{M}M=4.84$$

where

• N_R—Rutting life of the subgrade;
• N_f—Fatigue life of the bituminous layer;
• ${\epsilon }_v$—Vertical compressive strain at the top of the subgrade;
• ${\epsilon }_t$—Horizontal tensile strain at the bottom of the bituminous layer;
• M_Rm—Bituminous layer-resilient modulus;
• Va—Air voids in the bituminous mix;
• Vbe—Volume of effective bitumen in the mix;
• C—Adjustment factor.

The variance in strain that was computed for the various asphalt thicknesses is shown in Figure 11. An asphalt layer thickness of 90 mm in Section I satisfied both the rutting performance (218 µε < 237 µε) criteria and the fatigue performance (295 µε < 416 µε) criteria. Similarly, an asphalt layer thickness of 120 mm in Section II meets both the rutting performance (223 µε < 237 µε) and the fatigue performance (357 µε < 416 µε). From the pavement design analysis, it can be concluded that the chemically treated section requires an asphalt layer of 90 mm, whereas the section without chemicals requires an asphalt thickness of 120 mm for the given CBR value.

A cost comparison was performed for two methods adopted for soil stabilization using chemically treated lime and only a lime-treated layer. For cost analysis, the length and width of the stretch were considered to be 1 km and 3.5 m, respectively.

Table 8. Cost analysis.

		Rate (per kg)	Lime Treated	Chemical Treated
A	Test track dimension
1	Length (m)		1000	1000
2	Width (m)		3.5	3.5
3	Crust thickness (m)		0.570	0.540
B	Material cost
1	Cost of Terrasil (lakhs/km) (Dosage 0.75 kg/m3)	550	-	3.61
2	Cost of Zycobond (lakhs/km) (Dosage 0.75 kg/m3)	250	-	1.64
3	Asphalt (rate/cum)	10000	-	-
4	Cost of lime (Rs/kg) (Dosage 3%)	1.5	0.67	0.67
C	Test track cost
1	Improvement in CBR %		4%	20.9%
2	Asphalt layer thickness in mm		120	90
3	Total cost of material in lakhs per lane km	-	0.67	5.25
2	Cost of reduced asphalt thickness—lakhs per lane km (B3)	-	-	(11)
3	Cost of replacement in lakhs per lane km	-	5	-
4	Additional cost of construction in lakhs per lane km	-	5	(5.75)
5	Saving in lakhs per lane km	-	10.75

indicates the cost analysis, and it can be concluded that we can save approximately 10.75 lakhs per lane km when using chemical stabilization.

7. Development of the CBR Prediction Model

During pavement construction, road agencies deal with a large amount of data and conduct a series of experiments. Several measurements from similar stretches are typically included in a measurement series. The prediction process evaluates the usefulness of using mathematical equation(s) to make decisions based on limited field and experimental data. The Design of Experiments (DOEs) method is adopted to develop a prediction model for soil stabilization. Prediction models are a useful resource for making predictions about missing data and verifying the observed values. Empirical correlations are commonly utilized in geotechnical engineering to assess a wide range of engineering soil parameters. The CBR strength of the soil combination was affected by the different dosages of chemical and lime concentrations utilized, and the results were expressed through a non-linear regression equation (Equation (4)).

$${CBR}_{predicted}=14.91\times chemical+4.89\times +15.5\times MDD-31.07R^2-91.4$$

(4)

where chemicals—dosage of Terrasil and Zycobond (equal proportion—0.5, 0.75, and 1 kg/m³); lime—dosage of lime (0, 2, and 3%); and MDD—maximum dry density.

7.1. Assessing the Statistical Significance of the CBR Model

A statistical analysis was conducted using the Minitab software (2016) to obtain the goodness of fit of the CBR model. The goodness of fit of the CBR model was shown graphically in Figure 12a–d. The data set, as shown in Figure 12a, exhibits an almost normal distribution, low variance, and an OLS (ordinary least square)-like pattern, as seen in Figure 12b. Thus, the CBR model yielded a consistent estimate of the coefficient with a low standard deviation. In Figure 12c, the peak slightly shifted to the left, and the plot between residual vs. observation order indicated a small variation in the response from the predicted value. The residuals are uniformly dispersed and consistent with the centerline, as shown in Figure 12d. Overall, the variation from the predicted vs. actual is uniform and well within the boundaries of the expected value.

7.2. Comparison of Predicted and Observed CBR Data

The relationship between the experimental and predicted CBR values was examined using a linear model regression equation, and the coefficient of determination (R²) was 0.946, as shown in Figure 13. This demonstrated a strong correlation between the results of the CBR experiment and the predicted results.

8. Conclusions

It was observed all three types of soil particles become hydrophobic and nullify the effect of water after treatment. Therefore, the phenomenon of expansion and contraction due to the presence of water is eliminated, and the unsuitable soils are effectively modified to be suitable by adopting this technology. The engineering properties of the unsuitable soil permanently change with the addition of chemicals, and with time, its strength increases. This will help in optimizing the pavement design for reduced crust with increased performance, resulting in sustainable construction. Based on the result, the following conclusions were made.

• The FSI values of Soil-I and Soil-II are completely reduced by the addition of 3% lime, whereas Soil-III is reduced by 79%. This is because the water affinity of the soil decreased because of the pozzolanic reaction, which binds the flocculated soil particles.
• The addition of Terrasil and Zycobond results in an increase in the MDD and a decrease in the OMC. However, the addition of lime reduced the MDD and enhanced the OMC.
• The CBR and UCS increased by 7 to 8 and 3 to 4 times, respectively, with the addition of 3% lime, 1 kg/m³ of Terrasil, and 1 kg/m³ of Zycobond.
• The result from the UCS and CBR tests establishes that the optimum dosage is 3% lime + 1 kg/m³ of Terrasil + 1 kg/m³ of Zycobond for the treatment of black cotton soil.
• The measured field CBR value of the chemically treated soil significantly improved from 20% to 26.9% with a curing period of 3 months. Field-measured CBR is almost similar to the laboratory-measured CBR values in both sections.
• The pavement design indicated that a chemical stabilization layer could be helpful to reduce asphalt layer thickness by 30 mm, and the saving in cost is approximately 10.75 lakhs per lane km compared with the lime/untreated layer.
• An empirical model for lime and chemically treated black cotton soil was developed using the DOE method. The model is useful for predicting the CBR by considering lime %, chemical dosage, and proctor values of the soil to be treated. Further, numerical models can be developed using artificial intelligence suiting different types of unsuitable soil.