Introduction

Construction of good quality roads will have many benefits such as reduced travel time; reduced wear and tear of vehicle parts, and reduced rates of accidents. Further, the impetuous failure of road pavements is a crucial hitch faced by many countries like India, and the reason for the same may change from case to case [1, 2]. The behavior of the road system relies majorly on its soil subgrade i.e., the foundations. Over the recent past, the speedy magnification of the world population has increased the demands for industrial and infrastructural growth substantially over unsuitable land spaces, which were considered as not-suitable in previous times. Nevertheless, the design and construction of economical and structurally sound pavements over these challenging subgrade conditions are a great confrontation for highway engineers. The most crucial layer that contributes to the stability of pavements overall will always be the soil subgrade course [3]. Adopting the ground improvement techniques i.e., stabilization methods for improving the engineering performance of such soil subgrades is generally the best choice under weak subgrade conditions [4,5,6]. The outcome of soil stabilization is improved engineering behavior in terms of the improved existing physical and chemical conditions of the original soil. Wherever poor ground conditions exist, the traditional forms of road construction will be expensive due to increased pavement crust thickness. Design engineers are always in search of economical methods to improve the engineering properties of the subgrade ground before pavement construction. It is majorly aimed at enhancing the soil density, increasing the shear strength, and reducing the plasticity index [4, 7, 8]. The use of locally accessible waste materials in stabilization works could lessen the burden on local governments' problems with solid waste disposal. The literature reveals the use of numerous stabilizing agents, the majority of which consists of the use of cementing agents, mechanical stabilizers, industrial by-product materials, chemicals, etc. Assadi‑Langroudi (2022) revealed the advancements in imitating forms, materials, generative processes, and functions found in natural systems for sustainable methodologies for ground modifications. These advancements are categorized into the advancements in "Materials, Models, and Methods" (3Ms), to assess the effectiveness of these 3Ms either individually or in combination [9]. Due to the growing volume of trash, the scarcity of landfills, the rise in transportation and disposal costs, and, most importantly, the growing worry about pollution and environmental deterioration, the management of industrial wastes is seen as a key problem by urban planners. According to the "Central Pollution Control Board" (CPCB) of India, the nation produces roughly 48 million tonnes of solid garbage annually, with 25% of that amount coming from the building industry [10, 11]. On the other hand, globally, India is the second largest producer of metal castings as reported by the foundry market in India (2018–2023). Around 40% of castings produced in India are consumed by the Automobile sector. Aluminum castings make up about 15% of the country's entire casting production as of 2018. Metal castings in India are expected to expand at a compound annual growth role (CAGR) of approximately 12.7% from 2018 until 2023. The production of metal castings will expand in the next years due to the rising demand for metals, which raises the production of industrial waste such as foundry sand and foundry slag. These waste products named "Waste foundry sand" (WFS) that are disposed in landfills after repeated utilization may impose several environmental issues due to their composition. Sustainable reuse of this large mass of waste products will meet the demand for green industrialization in the construction industry. Massive amounts of WFS generated by the casting industries require specific care in order to save landfill space and prevent adverse environmental effects. To reduce the use of raw resources like fine aggregates, green building technology is urgently needed in the construction sector (natural sand). Due to the limited supply of natural sand, it will be beneficial for sustainable development to replace fine aggregates with a portion of waste materials. Many studies reveal that the WFS is a valuable resource for geotechnical and pavement applications such as building embankments, retaining walls, hydraulic barriers, soil stabilization methods, subgrade, and subbase construction, etc. [12,13,14,15,16,17,18]. In the region of Karavali (coastal) Karnataka, the paramount types of soil are lithomargic soil (classified as Highly Compressible Silt i.e., MH type, locally called “shedi” soil), lateritic soils (SP i.e., Poorly Graded Sandy type soil), and the corresponding lithomargic-lateritic blend type soil (Silty Sand i.e., SM type) [3, 19]. In the present study, an effort was made to utilize WFS in the stabilization process of the lateritic soils in the coastal region of the undivided Dakshina Kannada district of Karnataka state. Many studies revealed the utilization of OPC as a fundamental additive to stabilize the locally available lateritic soils and successfully could able to implement the application to the desired levels [7, 20, 21]. The presence of OPC mainly leads to the binding of individual soil grains and thereby the attainment of chemical stabilization of the soil [4], whereas the use of WFS would help the soil to achieve improved gradation properties and thereby achieve mechanical stabilization of the latter soil [20]. Hence, the major purpose of this study is to examine the upshot of WFS in cement-stabilized lateritic soils through geotechnical investigations. Also, the application of the developed stabilized soil subgrade in the low-volume pavement design as per Indian Standards and the analysis of the developed pavement structure using the KENPAVE package were also have been carried out. Further, the strength developments in the UCS and soaked CBR were investigated by subjecting the un-modified and modified lateritic soil to sustained desiccator curing at different intervals up to 56 days. The durability tests were performed by subjecting the standard UCS specimens to alternate wetting–drying cycles and alternate freezing–thawing cycles. The test results were compared with that of the un-modified lateritic soils. Also, the effect of stabilization was assessed through the use of micro-structural studies by subjecting the sample collected from the selected WFS + OPC modified lateritic soil specimens upon 28 days of curing from the literature support [22, 23].

There are several researchers revealed the utilization of WFS in the stabilization, use of micro-structure studies, lateritic soil stabilization, durability studies, and the use of cement. The key findings from a few of the literature were presented in the following section.

Klinsky et al (2016) studied the assessment of Waste foundry soil (WFS) mixed with lateritic soil samples in pavement sub-bases and bases. The Soil mixtures with 60 and 70% of WFS showed lower Atterberg Limits than the soil sample. The results on compaction properties revealed an increment in MDD from 17 to 19 kN/m3 and a decrease in OMC for at same WFS contents. The addition of 70% WFS to soil decreased the swell factor from 0.5% to less than 0.1% [24]. Bhardwaj et al (2021) studied the improvements in the properties of clayey soil (CH) after treatment with WFS and molasses. The granular nature of WFS added in the varying amount of 10% to 30%, and the lucrative nature of molasses added in varying percentages from 5 to 15% to soil reduced liquid and plastic limit, thus reducing the overall plasticity index. Further, they reported an increment in UCS value from 246 to 659 kPa at the end of 28 days, which was attributed to the compact structure of the WFS [25]. Jiang et al. (2021) carried out the microstructure analysis on the cement-silica fume (SF) stabilized clayey soil, and revealed that the presence of SF could prevent the calcium hydroxide in forming weak zones and producing a stronger soil matrix with the increase in curing time, resulting in an increase in compressive strength [26]. Lekha et al. (2013) studied the influence of a chemical stabilizer named Zycosil on the locally available lateritic soil in Coastal Karnataka. The experiment study showed that the UCS of the soil increased to 636 kPa from 125 kPa upon stabilization. KENPAVE software is used in the study for the analysis of pavement models [27]. Kholghifard et al. (2014) investigated the effects of drying and wetting phenomena in the lateritic (MH) soil behaviors such as volume change and collapsibility. Scanning electron microscopy (SEM) analysis with EDX was conducted to analyze the effect of drying and wetting cycles on the test soil sample. This experimental study established that wetting–drying cycle has a significant effect on the collapsible behavior of residual lateritic soils [28]. Maichin et al. (2021) examined the use of cement-flue gas desulfurization (FGD) gypsum in improving a type of lateritic soil for the purpose of pavement construction. SEM analysis on the soil confirmed that the FGD gypsum could enhance the use of cement in soil stabilization to obtain a stronger stabilized material [29]. Cong et al. (2014) investigated the mechanical properties of OPC and CSCN-stabilized clay (CL-type). The UCS test was performed at regular intervals up to 90 days of curing. The SEM analysis verified that the higher degree of bonding and more compact microstructure resulted in higher compressive strength and secant modulus [30]. Pooni et al. (2019) explored the durability performance of the enzymatic stabilization of expansive soils in road pavements subjected to moisture fluctuation. The experimental studies on the mechanical and hydraulic behavior of stabilized soil subjected to cyclic moisture degradation were conducted at optimum, saturated, and residual moisture content [31]. Guney et al. (2007) conducted a comprehensive study on the effect of cyclic wetting and drying on the swelling behavior of lime-stabilized clayey soil [32]. Shivaramaiah et al. (2020) studied the suitability of using the Ground Granulated Blast Furnace Slag (GGBS) and alkali-activated cement stabilized lateritic soil of Dakshina Kannada district in Karnataka. The soil treated with 30% GGBS for 28 days was found to have improved UCS, fatigue, and flexural strength. The laboratory tests analyzed the freezing–thawing durability test, wetting–drying durability test, and micro-structural analysis of stabilized soil [33]. Ahmed and Ugai (2011) conducted an experimental study to investigate the effect of environmental conditions on the performance and durability of soil stabilized with recycled gypsum. The study showed increased compressive strength with an increase in the content of recycled gypsum. Studies on freeze–thaw, wet-dry cycles, and volumetric change were also conducted [34]. Marto et al. (2014) studied the effect of non-traditional additives (namely TX-85 and SH-85) on lateritic soil to study the engineering and micro-structural characteristics. Physicochemical changes were monitored via field-emission scanning electron microscopy (FE-SEM) and thermo-gravity analysis (TGA) [35]. Biswal et al. (2018) conducted a detailed laboratory investigation to determine UCS in the soaked and un-soaked state, flexural strength (FS) or modulus of rupture, indirect tensile strength (IDT), and flexural modulus (FM) of cement-stabilized granular lateritic soils (GLS) to be used as a structural layer of pavement. The mineralogical characterization of the stabilized lateritic soil has been carried out through SEM and X-Ray Diffraction micro-structural analysis, which revealed the presence of hydrated products such as "tobermorite" resulted in an enhanced strength of cemented treated lateritic soils. Peaks pertaining to quartz, kaolinite, goethitite, and hematite could be seen in the stabilized hydrated product. The SEM images revealed the presence of a "rod-like crystalline structure" such as "ettringite" which is created as a result of the hydration of cement-stabilized lateritic soils [22]. Chandu and Rao (2021) explored the strength and durability characteristics of Red Soil (RS) stabilized with WFS and OPC (C). The % loss of soil is almost insignificant for RS + 30%WFS + 7.5%C mix exhibited to 12 cycles of alternate wetting and drying. This clearly shows the impact of cement in lowering the plasticity properties by making it more moisture insensitive or creating an agglomerated structure of hydrated cement (i.e., CSH and calcium hydroxide) [23]. Lindh and Lemenkova (2023) studied the use of five different binders (lime, energy fly ash, bio fly ash, slag, and cement) as stabilizing agents in various proportions to improve the strength parameters of soil as required in engineering industry standards. The study revealed that the combinations of cement-energy fly ash—bio fly ash had the highest P-wave velocities of more than 3100 m/s. The enhancement in the strength of soil is attributed to the chemical reaction of soil structure with binders, which leads to the generation of a cementitious mixture which possesses a stronger resulting structure. This study also contributed to the laboratory investigation on soil strength for the construction of road beds [36].

Based on the literature works, it was clear that the use of cement and WFS can be made in enhancing the properties of the soil. However, there is no reported study on the effect of the WFS-OPC modification on the properties of lateritic soil from the coastal Karnataka region. Hence, there is a scope for extensive research work that has been identified as a gap in the literature and the current investigations were accordingly planned. The following are the specific objectives of the current research study:

  • To investigate the effect of WFS-OPC incorporation in the stabilization of lateritic soil of the coastal region of Karnataka.

  • To optimize the dosage of WFS-OPC content in the stabilization of lateritic soil.

  • To study the prolonged strength, durability, and micro-structural performance of modified lateritic soil with selected WFS-OPC dosages.

  • To analyze and design the structure of low-volume pavement by the use of WFS-OPC stabilized lateritic soil subgrades as per relevant standards.

Materials and Methodology

The lateritic soil for this research was collected from the vicinity of Kapu Town, Udupi District. The basic geotechnical features of the soil are indicated in Table 1. WFS required for the research work has been obtained from Lamina Foundries, Nitte, and Karkala Taluk. The gradation results of WFS indicate that it is classified under Zone IV as per Indian standards [37], and the specific gravity of the same was found to be 2.33. Further, Grade 43 OPC was used as a binding chemical stabilizing additive with WFS, whose specific gravity was found to be 3.13, initial and final setting time was found to be 47 min and 252 min, respectively, and Le-Chatelier’s soundness was about 0.92 mm. The 28-day compressive strength of cement was found to be 47.4 MPa. The tests to learn the effect of the cement stabilizer on the geotechnical features were done by following the appropriate procedure suggested by respective "Indian standard codes" of practice [38,39,40,41,42].

Table 1 Properties of lateritic soil
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Initially, grain size analysis tests were executed by mixing the various doses of WFS with lateritic soil. This attempt was made to understand the improvements in the gradation properties of the latter soil upon the incorporation of WFS content up to 100% at an interval of 10%. At 30% addition of WFS, a comparatively better gradation co-efficient was obtained; accordingly, "uniformity coefficient" (Cu) and "co-efficient of curvature" (Cc) of 5.50 and 0.97 was obtained. Hence, based on the results of analysis tests, the optimum dosage of WFS was fixed at 30% of the weight of lateritic soil and was maintained the same throughout further investigations. Further, the dosages of OPC were increased from 0 to 12% at a regular interval of 2%, and the compaction properties were studied. Nomenclature M indicates the mix, and the suffix number indicates the percentage of OPC concerning the weight of the soil; for an instant, M-4 indicates the soil mixed with 4% of OPC. For the selected mixes, the soaked "California Bearing Ratio" (CBR) Tests and "Un-confined Compressive Strength" (UCS) tests were executed, and the results were reported as an average of three consecutive sample test results. The sequence of the sample preparation, CBR, and UCS Testing of soil samples are typically presented in Fig. 1.

Fig. 1
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Preparation of soil, CBR testing and UCS testing of soil samples

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Further, the prolonged UCS and CBR tests were carried out at various curing periods from 0 to 56 days at a regular interval of 7 days. The UCS sample desiccator curing method suggested by ASTM was taken up for testing upon prolonged curing days [43], where the prepared soil specimens were kept in such a way as to ensure a constant “relative humidity” inside the desiccator atmosphere at laboratory temperature of 26–29 °C. The soaking of the CBR specimen and desiccator curing for the prolonged tests are shown in Fig. 2.

Fig. 2
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Glimpses of prolonged strength and durability testing of soil samples

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Due to both freeze–thaw and wetting–drying-related distress, the durability of subgrade material is a key concern in cold climatic areas and places with profound rainfalls. It is a well-established fact that any stabilization is acceptable only if the latter stabilized soil must be able to retain the gained strengths during the adverse seasonal variations of the weather. Thus, the stabilized soil must pass the relevant durability tests to satisfy the engineering needs. The durability tests in the present investigations were performed on the stabilized and referenced lateritic soil mixes. In the present study, alternate wetting and drying tests and alternate freezing and thawing tests were performed, as indicated in Fig. 2.

The tests were carried out following the guidelines per the available international standards [44, 45]. During each test, the standard UCS samples were prepared and subjected to repeated cycles for up to 12 days. The weight loss after each cycle is found separately and reported under each case as proposed in the literature [46, 47].

The pavement design suggestions were proposed based on the IRC recommendations for the low volume road applications using the IRC: SP-72 for the bituminous (flexible) pavement applications [48]. The contact pressure of 700 kPa, Spacing between the tires of dual wheels equals 31 cm, and the other parameters as per IRC 37 [49] were assumed for the analysis. Determination of maximum vertical displacement for the selected mixes and the vertical stresses at various radial coordinates were carried out using the KENLAYER package of KENPAVE software [50]. The "KENLAYER computer program" applies only to bituminous (flexible) pavements. The solutions are superimposed for several wheels, applied iteratively for nonlinear layers, and collocated at various times for visco-elastic layers. As a result, "KENLAYER" can be used for the systems under all loading arrangements ranging from single axle to multiple axle/wheels with together layer behaving diversely, either "linear elastic, nonlinear, elastic, or visco-elastic". Depending on the layer, it will counter in a diverse way. Each era has its unique material properties that enable damage analysis, and there may be a maximum of 12 periods every year. Whether there is one or more, each period has a maximum of 12 load groups. The effects of fatigue cracking and irrevocable deformation are summed together each time across all load categories to determine the design life.

Results and Discussion

Results of Compaction Properties, CBR, and UCS Tests

The outcomes from the IS light compaction tests for the OPC-WFS stabilized lateritic mixes are offered in Table 2 as the average of three experiments. According to the findings, the "maximum dry density" (MDD) of the mix increases as the dose of OPC increases up to a particular spot, and then an additional increase in OPC cause the MDD value to decrease. In contrast, the mix's "optimal moisture content" (OMC) value did not exhibit any observed trends. There was an irregular water need for the greatest compactness noticed during the compaction testing because the dosage of the WFS-OPC rendered the soil significantly drier, and the soil particles preferred to separate as single-grained structures. According to the findings of the compaction tests, the lateritic soil achieved the maximum MDD at 8% OPC in the presence of 30% WFS.

Table 2 Compaction properties of WFS-OPC modified lateritic soil
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When compared to lateritic soil mixtures without stabilizer doses, there was a 27% increase in MDD. For the UCS and CBR experiments, the chosen mixture containing a 30% dose of WFS and 8% of OPC was considered. Further, the CBR and UCS tests were conducted only for the selected three mixes. Reference mix with 0% OPC with 0% WFS, 0% OPC with 30% WFS, and optimum mix, i.e., 8% OPC with 30% WFS, were used for further analysis.

Table 3 illustrates the outcome of CBR and UCS tests. The test results reveal that the CBR value could be increased to 160% and UCS value to about 163% with the incorporation of 8% OPC with 30% WFS for the lateritic soil; just by the incorporation of 30% of WFS, there was an increment of "CBR and UCS value" of the lateritic soil. The determination of cohesion and angle of friction was made using UU-triaxial strength test, results of which revealed that the incorporating WFS could reduce the soil's cohesion intercept and increment the internal friction angle. Further, it was also observed an acute increment of cohesion intercept and reduction in the "angle of internal friction" value for the lateritic soil upon the WFS-OPC incorporation. The observations of improved engineering behaviour of lateritic soil may be accredited to the enrichment of the "post-peak strength" of the material upon reduction in the stiffness value as reported by the previous studies on a type of soil [51]. There was a reduction in the strength performance after 8% of the OPC dosage, which proves that the dosage of OPC cannot be increased beyond certain limits for the given soil under consideration.

Table 3 CBR and strength test results on selected WFS-OPC modified lateritic soil
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The unconfined compressive stress Vs compressive strain plots for 3 selected mixes are depicted in Fig. 3. The results noticeably indicate that the stiffness and the stress handling capacity of WFS and OPC stabilized lateritic soil are higher than that of the other test mixes. This may be due to enhanced binding ability in the presence of OPC in the stabilization process. The enhanced strength performances could be accredited to the transformations in its native structure towards more flocculated from a dispersed natal structure [52]. Nonetheless, it is thorny to generalize the reaction of soil stabilizations since the role of every stabilization technique acts especially with different soil and the actual mechanisms may be considered to be explicit to the soil under deliberation [8].

Fig. 3
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The unconfined compressive stress Vs strain plots for selected mixes

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Prolonged Strength Studies

To assess the prolonged strength performances, the selected soil specimens (Pure lateritic soil, i.e., M-R, M-0 mix, and M-8 mix) were prepared to Standard Proctor density are subjected to UCS and soaked CBR tests at intervals of 0 days, 7 days, 14 days, 21 days, 28 days, and 56 days of curing and the obtained results were analyzed. The results were depicted in Figs. 4 and 5 for UCS and soaked CBR values, respectively.

Fig. 4
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Prolonged unconfined compressive strength results for selected mixes

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Fig. 5
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Prolonged soaked CBR value test results for selected mixes

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The results of prolonged UCS and CBR indicate an apparent enhancement of the engineering performance of lateritic soil with WFS and OPC modifier intrusion. Concerning the un-modified lateritic soil, with the inclusion of only WFS, a minor increase in strength was observed, but with the inclusion of OPC along with WFS, there was a considerable-drastic improvement in the strength was noticed. The improvements in UCS value with WFS and OPC inculcations showed about 424%. The improvements in the UCS value up to 28 days of curing were observed to be about 393% when compared with the UCS values for similar mixes on day one. Further, after 28 days, it was observed that there were only slight improvements in the strength (UCS) performances when tested up to 56 days of curing. Similar results were observed even for soaked CBR results, where the improvements in the CBR value up to 28 days of curing were observed to be about 58% with only WFS inclusion; and 531% with both WFS and OPC inclusions in the lateritic soil. Similar to the trends observed in UCS improvements, after 28 days there were minor improvements in the CBR value noticed. In both cases, the retention of the achieved performances was witnessed to be guaranteed. The higher engineering performance of WFS-OPC incorporation can be attributed to the better packing capability of the resulting soil when particles of cement and WFS filled up the pore spaces of soil during compaction attempts at the preliminary phases; further, there was an improvement in the strength attributed to the pozzolanic reaction of hydration products of cement with the mineralogical constituents of the soil creating a dense cemented-aggregate soil mass-leading to the reduction in the net pore spaces as established by the literature [4]. Moreover, the development of hydration products from these OPC particles is also responsible for enhanced sustained strength for up to 28 days. However, a further increase in the dosage of OPC by more than 8% did not showcase any improvement, which indicates that the dosage of stabilizer will be effective until the optimum percentages for any kind of soil, as established by the pieces of literature [53].

Durability Studies

To assess the durability performances, the prepared standard UCS samples of three selected lateritic soil mixes were subjected to 12 cycles of alternate (i) freezing and thawing (F–T) and (ii) wetting and drying (W–D). For this purpose, three replicate standard UCS samples (dimensions of 3.80 cm diameter and 7.8 cm length) were produced for each selected lateritic soil blend. They were subjected to desiccator-moist curing for 7 days at laboratory temperatures before subjecting the specimens to testing. ASTM guidelines were used along with the literature support on similar soils [44, 45, 53]. The test results indicating the average weight reduction of the three consecutive samples were designated in Tables 4 and 5, respectively, for the investigations of "freezing and thawing" and "wetting and drying "cycles. It took about 7 weeks to complete the tests with the specified 12 durability cycles, respectively. It took about 7 weeks to complete the tests with the specified 12 durability cycles for W–D and F–T tests. Literature support recommends that, after the 12 cycles of durability tests, the % weight loss must not be more significant than 14% [53].

Table 4 Loss in weight for trial mixes subjected to alternate freezing–thawing cycles
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Table 5 Loss in weight for trial mixes subjected to alternate wetting–drying cycles
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The result of alternate F–T indicates that up to a certain number of cycles, there was a gain in weight. A "negative" sign has been indicated. After that, the loss in weight was noticed for all three mixes. The overall results clearly show that the loss in weight was higher in un-treated lateritic soil when compared with that of the corresponding modified lateritic soils, out of which the mix M-8, i.e., Soil + 30% WFS + 8% OPC mix, showed superior performances when compared to the other two selected test mixes. In the case of an optimized mix, the maximum loss in weight was around 8.50% lesser than that of the pure lateritic (un-stabilized) soil; hence it is evident that the use of WFS + OPC stabilizers in lateritic soils.

Similar trends were noticed when the selected trial mixes were subjected to alternate W–D cycles. Accordingly, the performances of the optimized mix, i.e., Soil + 30% WFS + 8% OPC showed a comparatively better outcome when compared with the other two trial mixes. The negative values of loss in weight at a few points of the test cycles indicate an increase in the weight due to the water absorption of the sample/s upon wetting; this observation is in line with the previous literature on similar kinds of soil [53]. The test results indicate that the lateritic soil specimens failed to retain even one cycle of W–D, whereas the sample with WFS incorporation could not retain more than 4 cycles of W–D. The optimized WFS + OPC stabilized lateritic soil mix (M-8) could remain intact for up to 12 cycles. Still, the percentage loss of weight at the end of the 12 cycles was reported to be more than 19%, which does not satisfy the requirement of 14% limits; nevertheless, the loss in average weight was reported as less than 20% which may be considered acceptable for the pavement subgrade applications.

Low Volume Road Structural Design Applications Using Stabilized Soil

The design of flexible pavement structures for low-volume roads is proposed per IRC: SP-72-2015 guidelines [48]. The "low-volume roads" are designed when the heavy vehicle load repetitions in terms of the “cumulative number of the standard axle” (CSA) is lower than 10,00,000, i.e., 1 million standard axles; however, the revised IRC guideline is extended for 2 msa. The pavement cross-section consists of a “subgrade layer” followed by “granular sub base” (GSB), “granular base layer” (WBM), and “premix carpet” (PC) with seal coat/ “mix seal surface (MSS)/surface dressing (SD)”. The carriageway for the low-volume roads is considered 3.75 m (general), which will be provided with a hard shoulder of 1.50 m on either side of the carriageway. As per IRC: SP-72 specifications, the design composition is mainly dependent on the “soaked CBR value” of the subgrade soil (5 categories from S1 for CBR < 2% to S5 for CBR ≥ 15%) and the traffic load in terms of “standard axle load repetitions” expressed in million standard axles (MSA) (9 categories from T1 for design repetitions 0.01 MSA to 0.030 MSA to T9 for design repetitions 1.5–2.0 msa). Most rural roads in the Dakshina Kannada region possess design traffic of greater than 0.5 msa repetitions and less than 1.0 msa repetitions [54, 55]. Accordingly, for the current research, the traffic classification was considered as T7 category as per IRC. The complete structural thickness design carried out as per IRC: SP-72 guidelines for the modified and unmodified soil is shown in Table 6.

Table 6 Pavement structure design composition for low volume roads as per IRC
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In each one of the cases, the designed structure of pavement composition consists of an Open Graded Premix carpet (OGPC) section of 20 mm thickness with Water Bound Macadam (WBM) followed by a "granular base course" layer (WBM or CRMB) and "granular sub-base" (GSB) section above the compacted subgrade soil layer [48, 55].

It is clear from the design table that, by using WFS and OPC, there is a drastic reduction in the total design thickness value of the pavement. While compared with the pure lateritic soil, a reduction in pavement thickness of about 50% was achieved. The typical design thicknesses of each layer provided over the compacted subgrade are shown in Table 6.

The inter-layer vertical displacements and the maximum stresses are determined for the pavement structure composition, modeled as per the design shown in Table 6 using the KENLAYER package of KENPAVE software. Since the design is for a rural road, three traffic axle loading conditions were used, i.e., "Single Axle Single Wheel" (SASW), "Single Axle Dual Wheel" (SADW), and "Tandem Axles with dual wheel" (TADW). The circular radius of the contact area was adopted as 15 cm with an inflation pressure of 700 kN/m2. The distance between the individual dual wheels (center-to-center) is 32.5 cm, and for the TADW, the distances between the individual axles were 142 cm [55]. The modulus of the subgrade, sub-base, base, and surface courses was determined individually per IRC code guidelines' recommendations, which are based on the CBR value of the supporting layer [49]. From the KENLAYER analysis, the interlayer-vertical displacements (∆ in mm) and the corresponding stress value (s in kN/m2) were determined and are revealed in Table 7 for the "SASW" loading condition. Further, Table 8 illustrates the maximum value of vertical displacements for both SADW and TADW loading arrangements. The results reveal that the surface deflection is considerably higher for the case of pure lateritic subgrades compared to the other two selected stabilization cases; this is undoubtedly due to the bad engineering behavior of the lateritic soil contrasted to the stabilized cases, which proves the effectiveness of combined WFS + OPC stabilization of lateritic soil in enhanced pavement performance.

Table 7 Vertical displacements and stresses for pavement underneath SASW loadings
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Table 8 Maximum vertical displacements for the pavement under TADW and SADW loads from KENPAVE analysis
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Microstructure Analysis of Modified Lateritic Soil

Thermo-Gravimetric Analysis (TGA)

For a better understanding of the chemical and physical properties of lateritic soil stabilized with 30% WFS and 8% OPC, thermal research was carried out. Figure 6 displays the temperature gravimetric profiles of modified lateritic soil. The decrease in weight of the treated lateritic soil observed during TGA was mainly brought on by the evaporation of water absorbed from the surface and soil particle interlayer. The breakdown of the organic components caused weight loss when the temperature was between 250 and 350 °C. Kaolinite underwent di-hydroxylation between the temperatures of 400 °C and 650 °C. According to previous studies, the constant weight loss at temps up to 400 °C was brought on by the combustion of organic matter and the loss of physically assimilated water [35, 56]. At 450 °C, an endothermic surge was ascribed to the structural water being lost by hydroxide ions. In the "Derivative Thermo-Gravimetric" (DTG) curve, four endothermic peaks were seen at temperatures 100 °C, 262 °C, and 497 °C. These peaks, similar to those seen by Bodian et al. (2018), were attributed to the loss of physically absorbed water, di-hydroxylation of the aluminum and iron hydroxides, conversion of kaolinite hydroxide into an amorphous phase of kaolinite (known as metakaolin), and de-carbonization, respectively [57].

Fig. 6
figure 6

TG-DTG curves for lateritic soil treated with 30% WFS and 8% OPC

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X-Ray Diffraction (XRD) Analysis

Figure 7a–c showcases the XRD results of WFS-OPC modified lateritic soil, which indicated that the main minerals present in the treated soil were kaolinite (2θ = 12.16°, 24.74°, 26.51°, 35.82°), quartz (2θ = 20.71°, 36.42°, 50.04°, 67.83°, 92.45°), and illite (2θ = 12.02°, 20.03°, 26.37°, 39.22°, 49.90°) [58]. Then the plot was exported to "match software", and the background noises were subtracted, after which, based on the figure of merit (FOM) values, the element composition will be matched. Then, the phase identification is carried out, and the essential reports will be generated where the angles corresponding to the mineralogical peaks are obtained. However, the results reveal that there were no significant differences between the XRD patterns of treated and untreated lateritic soil samples. The summits of kaolinite seemed to lose some of their sharpness upon the stabilization process. The stabilizer’s impact on the soil structure and the resulting weathering of the kaolinite particles were two potential causes of this decrease. Due to the stabilization process, the peak intensities for quartz stayed largely unaltered [59,60,61].

Fig. 7
figure 7

X-Ray Diffraction (XRD) results for lateritic soil treated with 30% WFS and 8% OPC

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Energy Dispersive X-Ray (EDX) Analysis

The quantified chemical elemental analyses of raw and cement-treated materials were presented from Fig. 8a to c. From Fig. 8a, the unprocessed lateritic soil used in this research had high weight percentages of oxygen (O), silicon (Si), and aluminium (Al), which were 48.35%, 24.14%, and 23.16%, respectively. Next up was iron (Fe), which is compatible with the lateritic character of the soil and had a high number (2.15%) in the chemical makeup of untreated soil [62]. Calcium (Ca) output resulted from the addition of cement to naturally lateritic soil, as shown in Fig. 8b and c. The weight proportion of calcium increases as cement volume increases [63]. For instance, the calcium (Ca) content of the 3% cement-stabilized soil was 3.8% (Fig. 8b); for 8% cement, it rose to 5.01% (Fig. 8c). The findings of earlier studies [63, 64] and the EDX testing results in this study were remarkably consistent. In cement-treated lateritic soil, a high weight percentage of calcium led to more significant gel formation (CAH, CSH, and CASH) [61]. As a result, the production of calcium may be responsible for the improvement in UCS that occurs with a rise in cement concentration. Additionally, stabilizing the soil increases the connecting and interlocking pressures between soil particles, which primarily gave the soil its improved engineered performance. Finally, denser soil was created due to the mineralogical improvements, which improved and strengthened the aggregate of resulting WFS-OPC stabilized soil particles.

Fig. 8
figure 8

Energy Dispersive X-Ray (EDX) results for unsterilized and stabilized lateritic soil. a EDX of un-stabilized Ssoil. b EDX of soil with 8% OPC. c EDX of soil with 8% OPC + 30% WFS

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Field Emission Scanning Electron Microscopy (FE-SEM) Analysis

The FE-SEM micrograph of untreated and treated (30% WFS + 8% cement) lateritic soil is shown in Fig. 9. Figure 9a shows the surface morphology of untreated lateritic soil particles coated with fine particles of kaolinite. Figure 9b indicates the presence of rod-like crystalline structures such as ettringite (Et) which is formed as a result of cement hydration reactions within the structure of treated soil [22]. The production of "Calcium Hydroxide" (CH) and "Calcium-Silicate-Hydrate" (CSH) in cement-stabilized soil directly induces enhanced engineering performances. Figure 9d is the microscopic image showing porous CSH, needle-shaped Et, and hexagonal plate-shaped CH products formed after the hydration reaction of cement. The lower strength generally exists in the pores, which may be attributed to the CH, the weakest pace of cement-based materials.

Fig. 9
figure 9

FE-SEM micrographs of unsterilized and WFS-OPC stabilized lateritic soil

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In contrast, the presence of CSH and Et contributes to the development of strength and its enhancement [26, 65]. Et crystals are elongated and thick, CSH and Et have increased and interwoven into an even framework, and the interweaving of the CSH and needle-like Et completely covers the soil particles. The strength of the porous material is further enhanced due to this reason. As shown in Fig. 9c, CSH has connected the soil particles into a whole, and the pores have been filled with hydration products; at the same time, the needle-like Et crystals are interwoven into a net, the soil particles are entirely covered by the interweaving of CSH and needle-like Et, and the strength of cement stabilized soil is further enhanced. A honeycombing matrix-like structure can be observed on an FE-SEM micrograph of cemented lateritic soil, as shown in Fig. 9c. The higher degree of bonding and more compact microstructure are believed to result in higher compressive strength and secant modulus [22, 30]. This matrix of the stabilized structure of soil provides mechanical strength against the sliding of the particles subjected to shear force. The higher degree of bonding and more compact microstructure is believed to result in higher UCS and improved secant modulus of the resulting soil.

Conclusions and Future Research Scopes

From the extensive experimental investigations carried out on the stabilization of lateritic soil by the use of WFS and OPC, the following significant conclusions can be drawn:

  • Utilizing WFS in any form is beneficial to the local disposal problems associated with the foundry industry; hence the outcome of this research project will give one of the alternate solutions to the significant local disposal concern.

  • From the preliminary geotechnical investigations, an effectively stabilizes the duplex blend of 30% WFS with 8% OPC effectively stabilizes locally available lateritic soil.

  • With the use of 30% WFS with 8% OPC, the maximum standard proctor density of 21.80 kN/m3 with the optimized water content of 15.70% has been achieved; also, there was a considerable increase in UCS and CBR value when compared with that of the un-modified lateritic soil.

  • There was an acute increment of "cohesion" and a reduction in the "angle of internal friction" value along with an improvement in stiffness and the stress handling capacity of the lateritic soil upon the WFS-OPC incorporation.

  • The prolonged strength tests on WFS-OPC incorporated lateritic soil indicate an increment of about 424% in UCS and 393% of CBR value at the end of 28 days of desiccator curing, upon which the gain in strength is not of the considerable remark.

  • Durability test results from alternative W–D and F–T cycles indicate a considerable improvement in the stabilization process upon WFS-OPC incorporation when compared with the associated results of stabilized and un-stabilized lateritic soil mixes.

  • The results from the pavement designed for the low-volume roads indicate a considerable decrease in total pavement thickness, a reduction in the total stress, and total deflection at vertical coordinates upon the WFS-OPC stabilization leading to economical pavement construction.

  • The microstructure and chemical profiling analysis made through the results of TGA, XRD, EDX, and FE-SEM revealed the mineralogical composition and mechanism of WFS-OPC stabilization of the lateritic soil.

The overall results from the current investigations were encouraging and would further lead to achieving sustainability with the economy in the construction of low-volume pavements. The outcome of this study also mainly gives a sustainable alternative to the disposal problems associated with the dumping of WFS, which otherwise could go as a land filler. Though, the explorations could be extended to investigate further the engineered characteristics such as tri-axial properties, permeability, compressibility characteristics, rutting performance studies on the pavement models, repeated loading studies, split tensile strength tests, retained strength, to gain more confidence in adopting WFS-OPC stabilizing combinations in the lateritic soil subgrades. Also, the pavement design for the actual traffic data and field conditions could be studied with the implementations for pavement design applications for -low- and high-volume pavements as per the relevant standards. Further, for proving the sustainability the carbon footprint analysis of the current research study could be carried out [66, 67]. Also, the effect of WFS incorporation with recently developed, sustainable stabilizing additives such as biopolymers and synthetic polymers can be tried for lateritic soil stabilization instead of OPC additive [68,69,70].