A Comparative Study on Soil Stabilization Relevant to Transport Infrastructure using Bagasse Ash and Stone Dust and Cost Effectiveness

Soft ground improvement to provide stable foundations for infrastructure is national priority for most countries. Weak soil may initiate instability to foundations reducing their lifespan, which necessitates the adoption of a suitable soil stabilization method. Amongst various soil stabilization techniques, using appropriate admixtures is quite popular. The present study aims to investigate the suitability of bagasse ash and stone dust as the admixtures for stabilizing soft clay, in terms of compaction and penetration characteristics. The studies were conducted by means of a series of laboratory experimentations with standard Proctor compaction and CBR tests. From the test results it was observed that adding bagasse ash and stone dust significantly upgraded the compaction and penetration properties, specifically the values of optimum moisture content, maximum dry density and CBR. Comparison of test results with available data on similar experiments conducted by other researchers were also performed. Lastly, a study on the cost effectiveness for transport embankment construction with the treated soils, based on local site conditions in the study area of Assam, India, was carried out. The results are analyzed and interpreted, and the relevant conclusions are drawn therefrom. The limitations and recommendations for future research are also included.


Introduction
Reducing long-term settlement of infrastructure and providing cost-effective foundations with sufficient load-bearing capacities are national priorities for infrastructure development in most countries. In particular, transport infrastructure build on soft soil can cause excessive settlement, initiating the undrained failure of a super-structure if proper ground In some cases, particle segregation and decreased soil strength were reported, especially when the addition of the stone dust as admixture was beyond 20-30% of the virgin soil's dry weight, although its influence on the CBR values were not studied [19][20][21]. Zaika and Soeharjono [22] found that reduction in the value of soaked CBR took place while using bagasse ash alone as an admixture and suggested blending lime, Portland cement, and gypsum to enhance the CBR. Chen et al. [23] stated that the use of 2% lignosulphonate improved the shear strength of sandy silt and its ductile behavior. Blending the existing poor soil sub-grade with hydrated lime and bagasse ash managed environmental concerns through waste reduction [24]. Application of fly ash and quarry dust as admixtures exhibited significant decreases in void ratio, plasticity index, and swelling potential with shear strength increments of virgin soil [25].
The use of stone slurry containing lime as an admixture at a proportion of 4 to 5% enhanced the shear strength of virgin soil, although its influence on the soil's bearing capacity was not studied [26,27]. Ogila [28] investigated the decrease in swelling pressure and heave with the addition of ornamental limestone dust in samples of expansive soils, although alterations in the treated soil's strength parameters were not observed. Hasan et al. [29] found that the presence of montmorillonite clay in soil treated with lime and bagasse ash was likely to initiate shrinkage cracks; in such cases, the use of geomembrane or an emulsified cushion was recommended.

Significance of the Research
For transport corridors, compaction and penetration characteristics are the two vital sub-grade soil properties to support transport infrastructure, such as the highways and railways. Although various admixtures to improve soft soil are available, as per the literature, the use of bagasse ash and stone dust have been quite effective, owing to local availability in large quantities, as well as low cost, besides satisfactory performance in soft soil stabilization. However, the available literature has yet to provide insight into a comparative investigation on the suitability of bagasse ash and stone dust as admixtures in terms of the compaction and penetration characteristics of treated soft soils, or a study on the cost effectiveness of such a soil stabilization technique, specifically for transport infrastructure. This present study aims to bridge this knowledge gap by conducting a comprehensive laboratory experimental program, followed by cost computations.

Experimentations
In this section, the materials used, their engineering properties, and the experimental approach and plan are described briefly in sequence.

Materials
The soft soil sample was stabilized by applying admixtures, i.e., bagasse ash and stone dust in target quantities. The material properties are described below.

Soft Soil
The soil sample used in this study for the laboratory tests was collected from Guwahati, Assam, India by means of an auger boring technique from a depth of about 1-2 m below the ground's surface. The natural moisture content of the soil was measured about 31%. The sample was air-dried, and thereafter, used in the laboratory for investigation. The particle size distribution (PSD) performed by sieve analysis and hydrometer tests indicated that the soil could be classified as well-graded silty clay; the PSD curve is presented in Figure 1. The geotechnical properties are shown in Table 1. The soil may be classified as CL, after the unified soil classification system.
Limited research was carried out previously by other researchers with virgin soil at the study area around the Deepor Beel at Guwahati, Assam, India, including subsoil characterizations and groundwater quality assessment [30,31], although any investigations on chemical stabilization with the virgin soil is yet to be conducted.

Bagasse Ash
The dry pulpy fibrous residue of sugarcane after juice extraction is termed as bagasse. It is extensively used as a building material, as well as for manufacturing biofuel [32]. The raw bagasse collected from sugar mills is oven dried, and thereafter burnt to ashes, which are used as an admixture for soil stabilization. The bagasse ash is dark black in wet conditions, and gray in dry conditions, consisting of Silica, as well as oxides of Magnesium, Calcium, Iron, Sodium, Potassium and Aluminium [22]. The bagasse is locally available in bulk quantities for utilization in ground improvement for transport infrastructure. The physical and chemical properties of the bagasse ash used for experimentation were obtained from the laboratories, and presented in Table 2. A representative photographic view of the bagasse ash is shown in Figure 2

Stone Dust
Stone dust, which is used as a civil construction material, is a waste material generated while crushing stones in a stone crusher that produces angular aggregates in different sizes. Stone dust is mostly reduced into powdered form after the breakdown of boulders and rocks and appears grayish in color. It is largely available in Guwahati and in other parts of Assam, India. A representative photographic view of the stone dust used in the experiments is shown in Figure 2(b). The particle size distribution curve (see Figure 3), obtained from sieve analysis data, indicated sand and gravel contents of 90 and 10% respectively. The geotechnical properties of the stone dust obtained from laboratory tests are given in Table 3.

Test Approach and Plan
The virgin soil collected was first oven dried for 48 hours, and thereafter, manually ground and uniformly mixed with the above-mentioned admixtures at selected proportions by weight. Two different categories of stabilized soil samples (remolded), one with bagasse ash and the other with stone dust, were separately tested and comparative studies were carried out. While a standard Proctor test is suitable for ordinary transport infrastructure, pavements for heavier traffic loading, especially aircraft and frequent truck traffic, demand a modified Proctor test, following the procedure included in ASTM D1557 [33]. In the study area in Assam, India, the measured traffic loading is much lighter [34]. Hence, the standard Proctor test was followed.
The compaction characteristics of the stabilized soil samples were determined by a standard Proctor test following the procedure described in ASTM D698 [35]. On the other hand, the penetration characteristics of treated soil samples were determined by CBR tests for un-soaked and soaked samples, as per recommendation of ASTM D1883 [36]. To carry out the tests, the proportion of admixtures was varied between 2-10% by weight of the dry virgin soil sample and mixed separately with the soil. A total of 33 sets of tests were performed, including the untreated soil, as detailed in Table 4. To minimize the experimental error, three separate experiments were conducted for each set of tests, and the average values of the results were taken for analysis and interpretation. In the laboratory, the CBR specimens were prepared at the optimum moisture content for each test category. It is acknowledged the field CBR values may differ if the field moisture content is different from the optimum moisture content. Both the bagasse ash and stone dust are largely available in bulk quantities around the entire study area in Assam, India at cheap rates because of the large number of sugar mills and rock quarries existing in the region [37,38]. Previous studies revealed that the optimization of compaction and penetration characteristics was achieved when their quantities vary between 4-12% of the dry soil's weight [10,21,29]. In order to ensure adequate enhancement of the treated soil's compaction and penetration characteristics, while limiting the transport cost to ensure cost-effectiveness, the maximum quantity of admixtures was restricted to 10% in this paper.
It is true that based-on previous studies the bagasse ash needs to be treated with lime or cement as an activator, especially when the soil is expansive or very soft compressible clay [39]. In the present study, the virgin soil is silty clay. Thus, additional activators might enhance the cost significantly, compared to the relative benefits in the enhancement in the compaction and penetration characteristics. Hence, considering the local soil's characteristics, an activator was not used for soil stabilization.

Results and Discussions
The plot of maximum dry density versus moisture content for untreated soil (i.e., without any admixture), obtained from the standard Proctor test data, is shown in Figure 4, from which the optimum moisture content and dry density were evaluated as 19.2% and 15.95 kN/m 3 , respectively. For the CBR test for un-soaked and soaked untreated soil, the plot of applied plunger load versus the penetration is shown in Figure 5. After incorporating correction in the load-axis, the CBR values for un-soaked and soaked specimens were evaluated as 4.92 and 2.66 %, respectively. The Proctor test results for the treated soil are presented in Figure 6. The CBR test results for un-soaked and soaked treated soil are shown in Figure 7 (with corrected load axis). The optimum moisture content, maximum dry density, and CBR values for the untreated and treated soils are included in Table 5.

Main Findings: Analyses and Interpretations
To study the influence of admixtures on the compaction and penetration characteristics of the virgin soil, the optimum moisture content, maximum dry density, and CBR value were normalized as follows (Equations 1 to 3): where, αo, αm, and αc are the normalized values of optimum moisture content, maximum density, and CBR, respectively.

Optimum Moisture Content
The variation of normalized optimum moisture content (αo) with bagasse ash content is portrayed in Figure 8(a). As the bagasse ash content increased from 2 to 10%, the parameter αo was observed to increase from 1.12 to 1.47. The pattern of variation was found to be curvilinear with a descending slope. Figure 8(b) depicts the variation of αo with increasing the content of stone dust. The normalized optimum moisture content was observed to increase in the range of 1.09 < αo < 1.61 as the content of stone dust increased from 2 to 10%, the pattern of variation being relatively linear. 14 The value of αo greater than unity indicated increases in the optimum moisture content due to the addition of admixtures, the value being slightly higher in the case of stone dust. The above observations may be justified by the possible occurrence of ion exchange between the admixtures and soil particles [40]. In addition, the admixture particles probably reduced the free silt and clay fractions in the soil, thereby occupying larger void spaces for water retention.   Figure 9(a) presents the variation of normalized maximum dry density with bagasse ash content. As observed, the parameter αm decreased fairly linearly in the range of 0.95 < αm < 0.99 as the bagasse ash content increased from 2 to 10%. In the case of stone dust, on the other hand, as shown in Figure 9(b), αm decreased following a curvilinear pattern with a descending slope with increasing stone dust content; the range of variation being 0.97 < αm < 1.0. The values of αm less than unity indicated a reduction in the value of maximum dry density due to admixture addition. This is advantageous in terms of the decrease in the self-weight of the subgrade with stabilized soil at optimum moisture content. Considering the findings of Kaniraj and Havangi [41], the above observation may be justified with the possible accumulation and flocculation of virgin soil particles with ion exchange between the admixture molecules, which probably initiated the weight-volume ratio reduction.

3.1.3.California Bearing Ratio
The variation of the normalized CBR against increasing bagasse ash content is plotted in Figure 10(a). The increasing bagasse ash content from 2 to 10% initiated the parameter αc to increase in the ranges of 1.0 < αc < 2.2 and 1.25 < αc < 3.25 for the un-soaked and soaked tests, respectively. The patterns of variation in both the cases were observed to be curvilinear with ascending slopes; for the un-soaked tests, a reverse curvature was noted with a point of inflection at a bagasse content of about 5%. Figure 10 (b) depicts the variation of the parameter αm with the stone dust content. The range of variation of αc was found to be 1.0 < αc < 2.7 and 1.25 < αc < 2.3 for the un-soaked and soaked tests, respectively. The pattern of variation was observed to be curvilinear with ascending slopes.

Soil stabilized by stone dust
For both of the above cases, the value of the parameter αc was found to be greater than unity, indicating enhancement in the CBR with respect to the untreated soil, due to addition of admixtures. Furthermore, the values corresponding to those with the stone dust were higher compared to those with the bagasse ash, which implies that the soil stabilized with stone dust produced lower penetration-susceptibility. Considering the findings of Mousavi and Karamvand [42], the above observation may be justified with the possible chemical reaction between the soil particles and admixtures. The admixtures probably attributed to cementing effects on the soil, increasing their penetration resistance, thereby increasing the CBR; such cementing efficiency appeared to be more in the case of stone dust. The untreated virgin soil had a specific gravity of 2.64, whereas the bagasse ash and stone dusts had specific gravities of 2.51 and 2.71, respectively. Therefore, mixing the virgin soil with the admixtures at various proportions altered the specific gravity of the treated soil. This factor attributed to the alteration in the treated soil's CBR values [43,44].

Comparison with Previous Studies
The test results obtained from the present study were compared with the previous experimental results of Sharma and Kaushik [10] and Zaika and Soeharjono [22] for bagasse ash test data, and Agarwal [20], Kumar and Bishnoi [26] and Venkateswarlu et al. [27] for stone dust test data, as shown in Figures 11 and 12.

Implications and Explanations
In the case of the standard Proctor compaction tests, the parameter αo was observed to vary following a random pattern with increasing admixture content, in the case of previous test data, as opposed to a regularized manner for the current tests, as observed from Figure 11(a). The parameter αm, on the other hand (see Figure 11b), was observed to decrease with increasing admixture content in the case of previous test data; for bagasse ash, the pattern of variation was regular curvilinear, whereas for stone dust, it was random. In the case of previous test data relevant to the CBR tests, the parameter αc was observed to vary in a random pattern (see Figure 12) against a regularized pattern for the current tests, as discussed above. The difference in magnitudes, as well as the pattern of variation for the parameters αo, αm, and αc in the case of the previous test data compared to the current test results may be justified with the fact that the soil types were different, collected from various sites, in the cases of previous tests by other researchers. Moreover, the properties of the bagasse ash and stone dust used were also of different properties, compared to the present experiments.

Cost Effectiveness
Highways are considered as nationally important and require periodical maintenance. In rural areas of Assam, India, constructing a pavement requires higher thicknesses of base course and sub-base course to provide adequate drainage facilities, which undoubtedly increases the construction cost. Basack et al. [45] investigated the cost effectiveness of fly ash in pavement construction, and it was observed that using fly ash reduced the cost significantly. The present study is an attempt to estimate the cost of pavement construction with and without additives. From the analysis, it is observed that using soil treated with bagasse ash in pavement construction is more economical in comparison to untreated soil and soil treated with stone dust (see Tables 6 and 7, as well as Figures 13 and 14). The analysis reveals that the cost, compared to the embankment constructed with untreated virgin soil, is reduced by 14.43 and 9.67 % in the cases of bagasse ash and stone dust at 10% proportions, respectively.
For designing the proposed pavement, the project requirements were established by analyzing the soil properties. The dimensions of the road and design parameters, such as design life and traffic estimations, were considered as per Indian standard technical specifications [46]. The embankment's trial geometry was finalized following the recommended guidelines available [47]. This research is intended to stabilize soil along the stretches of Deepor Beel, a freshwater lake that forms a channel to the Brahmaputra River in Guwahati, Assam, India. As per the sample analysis, as well as the transportation and storage facility available at site, an optimum of 10% bagasse ash and 10% stone dust can be used for securing the banks of Deepor Beel significantly to stabilize the soil. Local availability of both bagasse ash and stone dust in bulk quantity is a major advantage. In addition, the optimum percentage of additives conforms to economic stabilization of pavement sub-grades, thereby curtailing the construction cost significantly.
The components of costs related to bringing additives to the site, time of treatment, and mixing costs are added in the analysis. The optimum time and temperature of the calcinations process to produce bagasse ash with high pozzolanic activity is three hours and a 600°C at 10°C per minute heating rate, respectively [48]. While there are various procedures of mixing of materials at the site, the mix-in-place method, which should be equipped with rotavator and compacting with a smooth wheel roller to achieve the desired density, is recommended from the available information based on the local conditions [49][50][51][52].

Summary and Conclusions
A laboratory-based investigation was performed with the objective of stabilizing soft soil by the addition of admixtures, namely bagasse ash and stone dust; the admixture content was varied from 2-10%. The compaction and penetration characteristics of the treated soil were studied through a series of standard Proctor and CBR tests.
The paper presents a comprehensive study on the suitability of using bagasse ash and stone dust as admixtures for soft ground improvement, in terms of compaction and penetration characteristics. Through extensive laboratory experimentations, the variation of optimum moisture content, maximum dry density, and soaked and un-soaked CBR with admixture content were studied in detail. Through appropriate costing analysis, based on local conditions, the proposed soil stabilization technique was found to be quite cost effective in the case the soft ground supports for transport infrastructure. However, it is essential to conduct a generalized study on the improvement of soft soil in terms of strength, stiffness, and durability [57]. Moreover, the movement of vehicles via transport corridors initiates cyclic loading on the soil sub-grade, altering its strength and stiffness [58]. This study aspect was not covered in the paper.
The study revealed that the optimum moisture content of the stabilized soil increased, in comparison with that of the untreated soil; the increment is up to 47 and 61% in the cases of bagasse ash and stone dust, respectively. While the variation pattern of the optimum moisture content with the admixture content is curvilinear for bagasse ash, the same is observed to be linear in the case of stone dust.
The maximum dry density of the stabilized soil decreased, compared to that of the untreated virgin soil, up to about 5% for bagasse ash and 7% for stone dust. The pattern of variation is linear for bagasse ash and curvilinear in the case of stone dust. The addition of admixtures produced significant enhancement in the soil's CBR; the increment being as high as 120 and 225% of that of the untreated virgin soil, for un-soaked and soaked samples, respectively, with bagasse ash used as the admixture. In case of stone dust, the increment was observed to be 170 and 120%, respectively. The pattern of variation was observed to be curvilinear.
A comparative study to justify the suitability of the two different admixtures implies that bagasse ash produced relatively less increment in the optimum moisture content, and almost an identical decrease in the maximum dry density, compared to stone dust. This indicates comparatively lower water requirements for initiating compaction in the case of bagasse ash. Additionally, the use of bagasse ash enhanced the soaked CBR significantly, thereby implying a higher penetration susceptibility, compared to the stone dust. As far as cost effectiveness is concerned, the use of bagasse ash and stone dust can reduce costs significantly. Hence, the appropriate admixture choice would depend on several other factors, including local availability and site treatment procedure costs.

Recommendations for Future Research Directions
As discussed above briefly, future research should be directed towards a generalized study on the utility of bagasse ash and stone dust as admixtures for chemical soil stabilization. Research should also consider the short-term and longterm enhancement of strength, stiffness, and durability of the treated soil through in-situ and laboratory tests, including plate load tests, vane shear tests, unconfined compressive and tri-axial tests, consolidation tests, etc. In addition, shrinkage and swelling tests and ductility tests should also be conducted for expansive soils. To ascertain the cyclic characteristics of the treated soil in the case of transport infrastructure, cyclic direct shear or tri-axial tests should be conducted. Lastly, a more general cost analysis involving other types of structures may be conducted as well. Such generalized study is currently under progress by the authors, and interesting results are expected.

Data Availability Statement
The data presented in this study are available in article.

Funding
Minor financial support to procure materials, as well as carry out the laboratory experiments, data collection, and infrastructure support was received from the Scholar's Institute of Technology and Management, Elitte College of Engineering, and University of Nevada, Las Vegas. However, no formal funding sources were received by the authors to conduct the research; hence information such as, Funder, Award Number, and Grant Recipient are not available.