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Case Studies in Construction Materials 19 (2023) e02508

Available online 24 September 2023

Effect of using Oil Shale Ash on geotechnical properties of cement-stabilized expansive soil for pavement applications

Samer R. Rabab ’ ah

^a^,^*

, Abdulla A. Sharo

^a^,^c

, Mudhaffer M. Alqudah

^a

, Ahmed M. Ashteyat

^b

, Haitham O. Saleh

^a

aDept. of Civil Engineering, Faculty of Engineering, Jordan University of Science & Technology, P.O. Box 3030, Irbid 22110, Jordan

bCivil Engineering Department, University of Jordan, Amman, Jordan

cAl Ain University, Civil Engineering Program, P.O. Box 112612, Abu Dhabi, UAE

A R T I C L E I N F O Keywords:

Expansive soil Soil stabilization Oil Shale Ash (OSA)

A B S T R A C T

Expansive soil is considered an engineering problem that may cause cracks and distresses in structures and roads. Due to its swelling potential and low unconfined strength, expansive soil causes failures in structures and leads to financial losses. Oil Shale Ash “OSA” is the byproduct of the combustion of the oil shale rock to produce electricity. Instead of dumping OSA materials into landfills, which has several negative environmental implications and cost burdens, utilizing these materials as building materials might alleviate the environmental concerns caused by their disposal. This research investigates the possibility of experimentally using the by-products Oil Shale Ash (OSA) and Portland Cement (PC) to enhance the geotechnical properties of problematic expansive soil. OSA and cement have been added to the soil, where OSA is used in four percentages by dry weight of soil (10%, 20%, 25%, 30%), and cement is used in three percentages (2%, 4%, 6%). A laboratory test program was implemented, including Atterberg limits, compaction test, unconfined compressive strength test (UCS), swell test, linear shrinkage test, and California Bearing Ratio (CBR) test. The results showed that OSA and cement have reduced the expansive natural soil’s swelling potential, plasticity index, and linear shrinkage. Also, the UCS and CBR values of treated soil have improved significantly. Pavement analyses demonstrated that OSA-cement-stabilized soil could be a suitable stabilization agent for the subgrade and base layers in constructing pavements.

1. Introduction

Expansive soil is a term used for soil that can expand and shrink, respectively, during wetting and drying. Chemical composition and particle size make expansive soil a hydrophilic material, meaning it has a high water affinity and can absorb and hold water [1].

Soil particles’ structure and properties affect the swelling potential of soil; for instance, flocculated clay and edge-to-face contacted particles tend to swell more than clay of dispersed and face-to-face contacted particles [2]. Also, with a high liquid limit, plastic soil absorbs more moisture and swells more than non-plastic soil. Roads, lightweight buildings, and sub-structures are in jeopardy of expansion/shrinkage strains that attack when the expansive clay layer gets wet or dry.

Remedial methods include chemical treatment by mixing soil with chemical admixture to improve its strength, mechanical

* Corresponding author.

E-mail address: [email protected] (S.R. Rabab’ah).

Contents lists available at ScienceDirect

Case Studies in Construction Materials

journal homepage: www.elsevier.com/locate/cscm

https://doi.org/10.1016/j.cscm.2023.e02508

Received 9 October 2022; Received in revised form 2 September 2023; Accepted 22 September 2023

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compaction by applying compaction effort on loose soil to get particles closer and reduce air void ratio, and soil injection by grouting material to increase soil strength and stability [3]. Chemical treatment of soil has been used for a long time; adding cement and lime is the most popular approach to chemical treatment. However, these chemicals are expensive to produce and can deliver environmental risks [4].

Cement is traditionally used to improve the mechanical properties of various kinds of soils. That is not just for its ability to enhance strength and physical properties but also for its quick reaction time. However, cement production is responsible for about 5–7% of CO2

emissions in the industrial sector worldwide [5]. Therefore, consuming cement in high percentages is not recommended to improve soil properties. For this reason, manufacturing by-products such as fly ash, Oil Shale Ash, silica fume, ground granulated blast furnace slag, rice husk ash, and many other materials are recommended to replace cement and lime in stabilizing expansive clay.

Oil shale is a sedimentary deposit rock containing a high content of solid organic material from which its oil production has been named hydrocarbons. In addition to the organic content, oil shale contains minerals such as calcite, clays, quartz, and calcium phosphate [6]. Oil Shale Ash “OSA” is the by-product of the combustion of the oil shale rock. After heating, oil shale decomposed into ashes with various chemical compositions depending on the oil shale rock’s combustion method and heating temperature. The chemical reactions below show the decomposition process of carbonate minerals during combustion [7]:

CaCO3 → CaO +CO2 (1) CaMg(CO3)2 → CaO +MgO +2CO2 (2)

Eqs. (1) and (2) are responsible for producing free lime (CaO), which plays a major role in the cementing properties of OSA.

Jordan has the eightieth-highest oil shale rock reserves globally, with 4000 million tons spread mainly in the central regions of Jordan [8]. Oil shale has not been combusted on a large scale yet. However, the Jordanian government studies the feasibility of producing power by combusting the oil shale rock, producing millions of tons of the by-product OSA. Several investigations have been conducted on OSA’s effect on soil’s geotechnical properties [9–13].

Road construction demands substantial-high-quality construction materials [14]. As an alternative to disposing of OSA materials through landfills, which have innumerable negative environmental repercussions and financial burdens, employing these byproduct materials in pavement construction could solve the environmental hazards caused by their disposal and decrease energy consumption [15].

In this study, the researchers sought to understand how introducing OSA alone and combined with cement could positively influence the engineering properties of expansive soil used as subgrade material for pavement construction. The effects of cement, OSA, and a combination of both admixtures were evaluated on the soil’s plasticity, swell potential, linear shrinkage, UCS, and CBR values.

2. Material and methods 2.1. Expansive soil

The expansive soil specimens in the study were collected from a (1–2) m excavation located in the western part of Irbid-Jordan. The properties of natural soil specimens were examined before being treated with OSA and cement. The basic properties of natural soil are summarized in Table 1, and the grain size distribution of natural soil is shown in Fig. 1.

From Table 1, the free swell percent of natural soil is 20.5%, the plasticity index is 42%, and the colloidal content is 40%. Therefore, according to Chen [16], the natural soil in this study is classified as highly expansive.

2.2. Cement

Cement is used for cementing properties and pozzolanic activity in most soil improvement applications. Also, it needs no mellowing time because the reaction is instant [17]—Type I Portland cement [18] was used for experiments performed in this study.

Table 1

Natural Soil basic properties.

Properties and Classification Values

Specific Gravity 2.72

Liquid limit (%) 72

Plastic limit (%) 30

Plasticity Index 42

Linear shrinkage (%) 17.1

% Sand (0.6–2 mm) 12

% Silt (0.002 mm to .06 mm) 43

% Clay (<0.002 mm) 45

% Of Material Finer than No.200 Sieve (0.074 mm) 88

Colloidal Content percent finer than 0.001 mm 41%

Unified Soil Classification CH

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2.3. Oil Shale Ash

Oil Shale ash is the by-product of the combustion of the oil shale rock. Usually, 50% of the rock mass turns into ash after combustion [19]. It can contain up to 40% by weight of lime (CaO) and 30% silicate (SiO2), which allows it to enhance expansive soil properties.

Because of its high content of free lime, the Jordanian OSA is very similar to the class C fly ash, which is usually called high lime ash (lime>20% by ash weight) [20]. Due to its chemical composition, OSA can impose pozzolanic activity and is a recommended stabilizer against swelling potential. The pozzolanic activity of Jordanian OSA was studied experimentally in a work by Khedaywi et al. [21]. The results showed that OSA had good pozzolanic activity, but not as high as other commercial pozzolans, but the OSA can be used as an additive to replace up to 20% of the cement by weight.

Specimens of oil shale rock were collected from the EL-Etarat region in south Amman-Jordan and combusted using a furnace at 10^◦C/min heating rate up to 800^◦C. Once the sample reached the desired temperature, it was held steady for 4 h. This extended duration ensured complete combustion, eliminating any hydrocarbons that may have been incorporated. An Energy Dispersive X-ray Fluorescence (EDXRF) spectrometer (NEX QC+QuantEZ manufactured by Rigaku) was employed to obtain the chemical composition of OSA. Table 2 shows the chemical composition of the OSA and cement used in this study.

2.4. Mixtures used for stabilizing soil

Various blends of OSA and cement were incorporated into the soil matrix, involving four different ratios of OSA based on the soil’s dry weight (10%, 20%, 25%, 30%) and three varying proportions of cement (2%, 4%, 6%). These combinations were systematically formulated to yield an optimal combination. Therefore, 17 combinations were studied, including the natural soil used as a reference point to determine the percent improvement. Table 3 summarizes the 17 mixtures of OSA and cement added to the soil.

2.5. Sampling and sample preparation

The soil used in this study was dried in an oven at 105ºC temperature and was sieved through sieve No.4 (4.75 mm); OSA was burned at 800 ºC, then milled and sieved on sieve No.200 (0.075 mm). Cement and OSA were added to the dry weight of the soil, and then water was added to the soil to achieve the optimum moisture content and maximum dry density of the molded soil. Specimens were prepared by gentle mixing until they reached a homogenous state.

The specific gravity of soil and OSA were determined in this study according to the ASTM D854 [23] specification. The specification of the ASTM D4318 (ASTM D4318, 2017) determined the soil’s liquid and plastic limits. All soil specimens used in this test passed sieve No.40. A series of different soil, OSA, and cement combinations were tested.

Fig. 1.Grain size distribution of natural soil.

Table 2

Oxide composition of OSA and Cement.

Chemical Composition OSA, Cement [22]

CaO 48.2 64

SiO2 20.1 20

Al₂O₃ 5.2 4.69

Fe₂O₃ 2.4 3.78

MgO 2.9 2.46

K₂O 1.2 0.4

Na₂O 1.2 0.02

SO3 4.5 2.27

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Gradation of coarse-grained soil, retrained on sieve No.200, was determined using a sieve analysis test depending on the ASTM D6913 [24] specification. Also, the fine-grained soil gradation was determined using the hydrometer apparatus, as per ASTM D7928 [25]. The maximum dry density (MDD) and optimum moisture content (OMC) of each mixture in Table 3 were determined using the standard compaction test (ASTM D698) [26].

The UCS for each mixture in Table 3 was determined following the ASTM D2166 [27] standard procedure. To study the effect of curing time on the UCS, specimens were wrapped in plastic sheets to prevent moisture loss and cured under controlled laboratory conditions for (7 and 28) days. Following ASTM D4546 [28], soil passing sieve No.40 was used in preparing specimens for the swell test, and the soil was compacted at the MDD and OMC. Soaked CBR tests were conducted on natural and stabilized soil following the specification of the ASTM D1883 (ASTM D1883, 2016). The linear shrinkage test was conducted following BS1377 (1990) (Methods of testing soils for engineering purposes - Soil classification tests)[29] [30]. The linear shrinkage test was conducted on mixtures of Soil:

OSA: cement ratios of 100:0:0, 90:10:0, 80:20:0, 70:30:0, 86;10:4, 84:10:6, 66:30:4, 64:30:6.

3. Results and discussion 3.1. Atterberg limits

OSA used in this study is fine materials passing sieve No.200, with a relatively high specific surface area. This indicates that OSA will affect the water absorbance of the treated soil and, subsequently, the Atterberg limits. Fig. 2 shows the effect of adding OSA on the liquid Limit, Plastic Limit, and plasticity index of natural soil, respectively. Fig. 2 showed an initial increase in the liquid Limit when 10% of OSA had been added to the soil, and then the values dropped by adding 20%,25%, and 30% of OSA. Similar behavior was observed with Mahedi et al. [31] for soil treated with cement and fly ash; the initial increase in liquid limit values was attributed to the cation exchange process, which resulted primarily from limited pozzolanic activities at lower OSA levels. With increasing OSA content, pozzolanic activities within the soil matrix increase, resulting in a lower liquid limit.

The plastic limit also showed a general increase when OSA was added. Fig. 2 shows a general trend when adding OSA, decreasing Table 3

Mixtures used in the study.

No.

Mixture Percentage (%)

Soil OSA Cement

1 Natural Soil 100 0 0

2 Soil+10% OSA+0% Cement 90 10 0

10 Soil+20% OSA+4% Cement 76 20 4

11 Soil+20% OSA+6% Cement 74 20 6

12 Soil+25% OSA+2% Cement 73 25 2

13 Soil+25% OSA+4% Cement 71 25 4

14 Soil+25% OSA+6% Cement 69 25 6

15 Soil+30% OSA+2% Cement 68 30 2

16 Soil+30% OSA+4% Cement 66 30 4

17 Soil+30% OSA+6% Cement 64 30 6

Fig. 2. Effect of different OSA contents on atterberg limits.

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the liquid limit, and increasing the plastic limit. Subsequently, the plasticity index is reduced by adding OSA. As shown in Fig. 2, adding 30% of OSA can reduce treated soil plasticity by 45% compared to the natural soil. This indicates that OSA can reduce soil plasticity, consistent with Attom et al. [9] results for soil treated using local OSA. This behavior is justifiable because when OSA is added, CaO in OSA initiates cation exchange processes within the soil matrix. This involves the replacement of monovalent cations with divalent cations. This exchange alters the electrostatic balance within the clay particles’ vicinity, reducing the thickness of the diffuse double layer (DDL) surrounding clay particles and influencing their behavior in the presence of water. The decrease in DDL thickness decreases clay particles’ capacity to absorb water. This chemical modification restricts the amount of water that clay particles can adsorb, contributing to the reduction in the overall plasticity of the soil. On the other hand, adding larger non-plastic particles (OSA) can lead to improved particle arrangement and reduced interparticle attraction, thereby affecting the consistency and plastic behavior of the soil.

On the other hand, some transportation agencies, such as The National Lime Association [32], presented a procedure to quickly determine the feasibility of using different chemical additives to modify or stabilize soils based on soil index properties such as minus No. 200 material and plasticity index [33]. Based on this preliminary assessment, cement treated is only recommended for soil with a plasticity index of less than 30. Therefore, the addition of OSA can be used effectively to reduce the plasticity of highly plastic soils, making them more workable to facilitate the addition of cement.

For studying the effect of adding cement on soil plasticity, the cement-soil mixture was cured for 7 days to allow cation exchange

Fig. 3.The effect of Cement-OSA on (A) MDD and (b) OMC for treated soil.

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and chemical reactions. Adding cement transformed the plastic clay into a fragile, non-plastic sand-like material (plasticity index=0);

therefore, plasticity tests were not performed.

3.2. Compaction test results

Fig. 3 shows the effect of adding OSA on the MDD and OMC of natural soil; the results show that OSA slightly decreases the maximum dry density and increases the OMC of the soil. In Fig. 3, maximum dry density (MDD) drops from 1.45 g/cm³for natural soil to 1.35 g/cm³(7% change) when 30% OSA was added. This can be explained by the fact that the specific gravity of OSA is 2.55 according to ASTM [23], which is lower than the specific gravity of natural soil [34]; this low value of specific gravity is consistent with Khedaywi [35] results which reported a specific gravity of 2.47 for the Jordanian OSA. Moreover, the OSA particle size (minus sieve No.200) is larger than the pore size between clay particles, which makes it hard for OSA to fill voids and increase density. As a result, the treated soils have lower specific gravity than natural soil and lower MDD. The results show that optimum moisture content ranges from (27–30)%, which indicates that OSA has a slight effect on OMC.

In Fig. 3, results show increasing MDD with adding cement because Portland cement’s specific gravity of about 3.15 [36] is higher than the soil’s specific gravity (equal to 2.72) [23]. Therefore, adding cement will increase the overall specific gravity of the mixture and consequently increase MDD [37]. For instance, as shown in the Figure, for a soil mixture with 30% OSA, 6% cement increased the MDD from 1.35 g/cm³to 1.38 g/cm³. However, increasing MDD corresponds with decreasing OMC, which has decreased from 32.5%

to 25.5%.

3.3. Effect of OSA on swelling potential

Fig. 4 shows the percent swell for natural and OSA-treated soil. Montmorillonite can result in a high swelling potential (20.5%) in untreated soil. The results showed that adding OSA reduced the soil’s free swell potential; this can be attributed to the effective reduction in soil plasticity achieved through the application of OSA. As shown in Fig. 4, adding 30% OSA can reduce swell to 10% of natural soil. This can be justified as when OSA adds the divalent and trivalent cations, concentrations will increase, and soil particles will floc together, reducing the specific surface area of soil. Subsequently, the soil’s ability to absorb water will decrease [38]. By adding non-plastic, non-swelling material to the natural soil, the plasticity characteristics of the soil will decrease, and the swell potential will almost diminish [39].

On the other hand, A significant reduction in free swell percentages for the combination of OSA-cement additives was observed.

This reduction is due to the pozzolanic reaction between soil and chemical additives. The pozzolanic activity of Jordanian OSA was studied experimentally in a work by Khedaywi et al. [21]. The results showed that OSA had good pozzolanic activity, but not as high as other commercial pozzolans, but the OSA Can be used as an additive to replace up to 20% of the cement by weight.

The pozzolanic activity reduces the soil’s ability to absorb water and, afterward, reduces the clay content by agglomeration and flocculation of soil particles [40]. Besides, OSA-cement introduces free lime (CaO) into the soil, a resource of Ca²⁺cations that replace univalent ions on the soil particles. With increased curing time, the pozzolanic reaction between the soil and cement becomes more pronounced, and the cementitious material binds the particles, reducing swelling to negligible levels.

3.4. Effect of OSA on linear shrinkage (LS)

Fig. 5 shows linear shrinkage results for stabilized soil by cement and OSA. OSA showed good abilities in decreasing linear shrinkage of expansive soil. As shown in Fig. 5, the linear shrinkage of the expansive natural soil decreased from 17% to 13.44% (21%

decrease) for soil treated with 10% OSA and 9% (47% decrease) for soil treated with 30% OSA. The percentage of reduction is 47%, which is a good indicator of the ability of OSA to improve the geotechnical properties of expansive soil. The results comply with Kariuki et al. [41], who studied the effect of the chemical treatment of expansive on the microstructural level; their results showed that pozzolanic reactions decrease the soil plasticity and increase bonding. Afterward, the shrinkage of treated soil is improved.

The addition of cement substantially decreased linear shrinkage, up to 91% for 30% OSA and 6% cement, whereas 30% OSA

Fig. 4.Swelling potential of natural and treated soil.

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decreased linear shrinkage to 47%. Even with the addition of 2% cement and 30% OSA, the LS was reduced by as much as 82%. This significant reduction indicates that the cement-OSA combination’s ability to reduce expansion/shrinkage of OSA-treated soil. Results in Fig. 5 are justifiable because the diffuse double layer of treated soil will diminish when OSA and cement are added. The reduction in the diffuse double layer is due to substituting metallic ions at the soil particles’ surface with cations like (Ca²⁺) [42].

3.5. Effect of OSA on compressive strength

Fig. 6 shows UCS results of natural and treated soil; results show the optimum UCS obtained at 20% OSA. UCS results for 28-day cured specimens are higher than 7-day cured ones; by adding 20% of OSA, the UCS of natural soil improved from 560 kPA to 750 kPa (33% increase) at one week and 1120 kPa (100% increase) at 28 days of curing time. The increase in UCS is due to an increase in the MDD, a decrease in the OMC, and an increase in the internal friction with the increase of OSA contents, which enhances the soil’s ability to withstand shear and compressive forces, resulting in higher UCS [43]. Nevertheless, when OSA content rises above 20%, OSA particles gradually replace clay particles, weakening the clay-to-clay contact. This alteration weakens the cohesive properties and consequently reduces the overall strength of the soil.

According to AASHTO 2008 pavement design guide [44], the stabilized layer should have a minimum compressive strength to adequately support the traffic loading. As a subbase layer in the pavement structure, AASHTO 2008 suggested, for flexible pavement, that the stabilized subgrade layer have a minimum compressive strength of 1.7 MPa [44] in order to sustain the structural pavement loads appropriately. Therefore, the subgrade soils can be stabilized using cementitious materials to achieve a higher strength for supporting traffic loads.

On the other hand, Federal Highway Administration (FHWA) recommends a typical UCS for subgrades at 7 days of cured soil of 1400 KPa [45]; the data presented in Fig. 6 indicates that the utilization of OSA for soil stabilization/modification falls short of achieving the recommended values. Consequently, the decision was made to incorporate cement to enhance the UCS of the soil treated with OSA.

Fig. 7 summarizes the UCS of treated soil and the effect of curing time on strength. Results showed that maximum strength could be attained at 28 days for 30% OSA and 6% cement, and natural soil strength increased from 560 KPa to 2780 kPa with over 500%

improvement. Also, it can be noticed that adding OSA helped improve UCS with less depending on cement as a stabilizer; For example, the introduction of 4% cement combined with 25% OSA yielded a UCS of 2124 kPa after a curing period of 28 days, representing a

Fig. 5. Linear shrinkage of natural and treated soil.

Fig. 6. Effect of adding OSA on UCS of natural soil.

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remarkable improvement of 380%. In contrast, the addition of 6% cement resulted in a UCS of 1831 kPa, marking a 326%

improvement. These results highlight the potential of OSA to partially substitute cement, leading to reduced cement consumption in soil stabilization processes.

A positive impact on the strength of the specimens was observed with the increase in the curing time. After 28 days of treatment, the UCS of the clay specimens treated with 10%, 20%, 25%, and 30% OSA increased by 35.8%, 49.3%, 28.8%, and 14.1%, respectively, when compared to the 7 days of OSA-clay treated specimens (Fig. 6). As an optimum case, the strength of the specimens containing 6%

cement - 30% OSA was 2375 kPa at 7 days of treatment, which increased to 2780 kPa (17% improvement) after 28 days of treatment (Fig. 7). The long-term increase in UCS values can be attributed to the pozzolanic reactions occurring between the additives and calcium hydroxide, as well as the formation of CSH gels within the clay-additives matrix [46]. These processes contribute to the improved strength and durability of the material as it evolves over time.

Fig. 7.Effect of OSA on UCS for cement-treated soil at (a) 7-day and (b) 28-day curing time.

Fig. 8.UCS failure mode of natural soil and treated soil by (10%, 20%, 25%, 30%) OSA.

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Fig. 7 shows a variation of optimum strength results with increasing cement content. For instance, maximum strength for 2%

cement is obtained with 20% OSA, while 4% cement gives higher strength results at 25% OSA, and 6% cement gives maximum strength at 30% OSA. This conclusion indicates that as cement content increases, the amount of OSA that can be added to the soil increases with no strength loss. Disposing of OSA is one of the outcomes of this study; therefore, it is recommended to use 4% cement and 30% OSA to replace as much soil as possible with OSA and get rid of the maximum available content of OSA, and at the same time to minimize the percent of cement to be used.

Fig. 8 shows the UCS failure mode of natural and treated soil specimens. As the OSA percentage increases, the soil becomes more brittle. This indicates that the soil’s stiffness has increased, and eventually, strength has increased.

On the other hand, the strain at failure for natural soil was 4.34%, 3.15% for 30% OSA-treated soil, 2.89% for 6% cement-treated soil, and 3.8% for 30%OSA-6% cement-treated soil. These results highlight a notable characteristic of cement-stabilized material, characterized by its tendency towards brittle behavior. This brittleness becomes particularly pronounced when the material is subjected to flexural and tensile stresses, which holds significant implications for certain applications such as pavement materials [47].

The angular structure and potentially more deformable characteristics of OSA particles alter the soil matrix’s mechanical response. The interactions between OSA particles and the cement-stabilized soil matrix introduce a certain level of flexibility, allowing for more ductile deformation under stress.

3.6. Effect of cement-OSA on California Bearing Ratio (CBR)

The soaked CBR results of natural and treated soil are shown in Fig. 9. CBR results have improved from 2% for natural soil to 42.5%

for 30% OSA added (about 20 times the natural soil value). For a good bearing capacity, the subgrade must have a minimum CBR value of at least 10% [48]. Due to its proven ability to increase bonding between soil particles and decrease soil plasticity, adding OSA can improve the CBR values of expansive soils [49].

The CBR test serves as a means to evaluate the subgrade quality during the construction of pavements. In the context of high-quality subgrades, a CBR value of 80 is deemed acceptable [50]. Therefore, cement has been added to the OSA to increase the CBR values. The maximum obtained value CBR is 225% (about 100 times the natural soil value) for soil treated with 30% OSA and 6% cement. The pozzolanic reaction of chemical additives with soil particles leads to flocculation and agglomeration of soil particles, eventually increasing CBR values [51].

3.7. Pavement performance evaluation

The Mechanistic-Empirical Pavement Design Guidelines (MEPDG) [44] is a pavement design method that combines mechanistic and empirical design approaches. The methodology of the MEPDG relies on computer-generated pavement responses, including stresses, strains, and deflections. These responses are computed by considering specific factors such as detailed traffic loading, material characteristics, and environmental conditions. A trial pavement design is then assessed to determine its suitability based on predefined performance criteria. The software analysis produces an output that predicts potential distress and smoothness levels while adhering to predetermined reliability thresholds [44].

The MEPDG calculates total rutting as a permanent deformation caused by vertical compression in all pavement layers along the wheel path. Alligator cracking is bottom-up fatigue cracking typically starts at the bottom of the asphalt layer, where the pavement is subjected to repeated and heavy loading, such as traffic loads and temperature fluctuations. The International Roughness Index (IRI) is a standardized measurement used to quantify and describe the roughness or smoothness of a road surface. Using an empirical function, the MEPDG forecasts the IRI, measured in inches per mile, as an average along wheel paths.

In order to ensure that materials possess the necessary strength to effectively support the expected loads and environmental factors throughout their service life within pavement structures, the local road specifications, The Ministry of Public Works and Housing,

Fig. 9. CBR values for chemically treated expansive soil.

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required a minimum soaked CBR value of 40% for subbase materials and 8% for subgrade materials, respectively [52]. Throughout this study, the natural expansive soil exhibited a CBR value of 2.1%, a high free swell percent of natural soil of 20.5%, and a plasticity index of 42%. These outcomes signify the soil’s problematic nature; it is necessary to implement stabilization measures before considering this soil for pavement construction.

This investigation evaluated the performance of a flexible pavement with an OSA-stabilized subgrade soil using the MEPDG software. Different runs using the software were performed on different pavement sections. All the sections of the examined pavement had the same structure consisting of 150 mm of asphalt concrete (AC) layer and a 300 mm base course layer.

With a 4% annual traffic growth rate, a 20-year design life was considered. An average annual daily truck traffic (AADTT) of 1200 vehicles per day was used in the analyses, with 50% of trucks in the design direction and 80% of trucks assumed to be in the design lane. The analysis used the MEPDG software’s default values for the number of axles per truck of each class, vehicle class distribution, and axle configuration.

Level III input was employed for the AC layer, along with a binder grade with the designation PG 64–10 utilized in most parts of the country [53]. For the unbound layers in the MEPDG Level-II design, the resilient modulus (MR) was calculated through correlations with CBR test values. Level II input was used for the base layer with a CBR value of 80% according to local road specifications. The results of the soaked CBR test for the OSA-cement stabilized soil obtained throughout this research were incorporated into the level II inputs for the subgrade layer. Table 4 summarizes the input parameters used in the MEPDG analyses.

The MEPDG software was used to evaluate the pavement distress parameters (IRI, alligator cracking, and rutting) for the sections analyzed in this study. Fig. 10 shows the resulting IRI values after 20 years of analysis. The Figure shows that the IRI values were slightly decreased using OSA-stabilized subgrade soil compared to the native non-stabilized soil.

Fig. 11 shows the predicted fatigue cracking for the MEPDG-analyzed sections. The same trend was observed here, where the alligator cracking of the pavement section slightly decreased with the increase in the OSA content. On the other hand, as shown in Fig. 12, the increase in the percentage of OSA utilized to stabilize the subgrade soil resulted in a significant reduction in the total rutting.

The above figures show that modifying the subgrade modulus will likely have the greatest impact on the total pavement rutting.

Other pavement performance indicators, such as fatigue cracking and IRI, are less affected by the amount of subgrade modulus change included in Fig. 12.

3.8. Scanning electron microscopy (SEM)

Scanning Electron Microscopy (SEM) tests were conducted to study the different soil mixtures’ micropores and morphology var- iations. Fig. 13 (a) shows that the natural soil has a discontinuous structure, with more apparent voids due to the lack of hydration products.

As shown in Fig. 13(b), the SEM demonstrated that OSA particles had an angular structure and a rough surface, resulting in higher mechanical interlocking and frictional resistance of stabilized soil mixture. As shown in Fig. 13(b), the SEM analysis of OSA particles revealed an interesting microstructural composition characterized by an angular shape and a surface exhibiting roughness. This rough surface texture of OSA particles plays a crucial role in amplifying frictional resistance by increasing contact points and interaction between adjacent particles within the stabilized soil mixture. This elevated frictional resistance translates into greater shear strength and improved load-bearing capacity of the stabilized mixture.

As shown in Table 2, the chemical composition of OSA has a high quantity of calcium oxide (CaO) and silicon dioxide (SiO2) as well as the oxide of aluminum and ferrite, which are very important compounds in the hydration reaction of cement to turn out new hydration products, such as calcium-aluminum-hydrate (CAH). The SiO₂would be consumed by the calcium hydroxide (CH), the main hydration product of cement, to form calcium-silicate-hydrate (CSH). As shown in Fig. 13(c), adding cement and OSA significantly enhanced the microstructure of the natural clay after 28 days of curing. The SEM showed ettringite resulting from sulfates (4.5%) in the OSA, increasing the OSA content. The unique needle-like structure of ettringite plays a crucial role in promoting crystal interlocking.

This characteristic promotes strong connections between individual crystals, enhancing the overall cohesion and strength of the material. The formation of CSH gels filled up the voids inside the soil and bound the soil particles with each other (Fig. 13(c)). The binding and aggregation of the soil particles improved the stress-strain response at the macro scale level. The resulting coherent mixture had a dense microstructure, greater shear strength (CBR and UCS), and swell/ shrinkage characteristics.

4. Conclusions

In this study, the influence of OSA on the behavior of cement-stabilized soils has been investigated in detail. The following conclusions can be drawn from the results of this study:

1. The addition of OSA has the remarkable potential to enhance the geotechnical properties of expansive soil. This enhancement is achieved through a dual mechanism that substantially improves the soil’s behavior. Firstly, OSA enhances the soil’s structural integrity and strengthens its ability to withstand applied loads and stresses. On the other hand, the addition of OSA is inherent in the expansive soil tendency for swelling and shrinkage.

2. Activating OSA by adding minor cement portions is recommended to increase the effectiveness and benefits of using OSA as a chemical stabilizer. The OSA-Cement combination showed good results in reducing soil’s liquid Limit and increasing plastic limit values, eventually decreasing soil plasticity.

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3. Among various combinations tested, the optimal blend is 30% OSA and 4% cement. This combination results in an outstanding performance, leading to elevated UCS and CBR values. Additionally, this combination effectively reduces swelling and shrinkage tendencies, resulting in diminished plasticity index values and, at the same time, minimizing the percentage of cement to be used.

Table 4

Inputs of MEPDG simulations.

Pavement Section Data Layer 1

(AC Layer) Design Level III

Material type Asphalt concrete

Layer thickness 127 mm

Asphalt grade PG 64–10

Layer 2

(Base Course Layer) Design Level II

Material type Crushed Stone

Maximum dry density 2040 kg/m³

Pass sieve #4 42.0%

Pass sieve #4 7.5%

PI NP

AASHTO Classification A-1-a

CBR 80%

Layer 3

(stabilized subgrade layer) Design Level II

Material type Stabilized soil

With 10,20%, 25%, and 30% OSA

Pass sieve #200 88%

PI Fig. 2

Maximum dry density Fig. 3

CBR Fig. 9

Fig. 10.IRI prediction using MEPDG analysis.

Fig. 11. Fatigue cracking prediction using MEPDG analysis.

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4. Based on this analysis, it can be concluded that pavement with an OSA-cement-soil mixture requires less AC thickness than pavement with non-stabilized subgrade soil. The reduction in AC thickness not only conserves materials but also leads to reduced energy consumption during both the construction and maintenance phases.

5. Despite the reduced AC thickness, pavements constructed on a stabilized subgrade with the OSA-cement-soil mixture can maintain excellent structural integrity. The stabilization process enhances the subgrade’s load-bearing capacity and resistance to deformation, ensuring the pavement can effectively handle traffic loads and maintain its performance.

Fig. 12.Total rutting prediction using MEPDG analysis.

Fig. 13.The SEM micrographs for (a) natural soil, (b) OSA particles, and (c) 30% OSA-6% cement-soil mixture.

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Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

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