Stabilization of expansive soil with agricultural waste additives: a review

Soil is one of the most important construction materials in the world. It has been effectively exploited for various purposes, such as building social infrastructures, growing agricultural products, and promoting many other important activities that can be useful for human life. The nature of the soil is uncertain from one place to another due to its physical, chemical, and mechanical properties. The expansive type of soil is the most problematic soil and causes damage to the foundations of roads and buildings. On the other hand, solid agricultural waste is widely available and is also a serious problem for the environment and its ecosystem. Therefore, improving the property of problematic soil by using sustainable, locally available, and low-cost agricultural waste materials is required. This paper aims to review the existing knowledge and practices from the recently published state-of-the-art journals related to expansive soil stabilization by agricultural waste additives and to support the findings with scientific data analysis. The effect of using agricultural waste additives such as coffee husk, rice husk, sawdust, wheat straw, cornhusk, sugarcane bagasse, and bamboo powder was carefully evaluated in terms of geotechnical characteristics, and strength parameters. As a result of the review, agricultural waste additives improved California Bearing Ratio (CBR), Plastic Index (PI), and Unconfined Compressive Strength (UCS) values, significantly lowered Optimum Moisture Content (OMC) and increased the Maximum Dry Density (MDD) of the soil. Furthermore, the effects of microstructural composition, morphology, and changes in expansive soils treated with agricultural waste additives were analyzed based on the XRD test results and SEM image analysis.


Introduction
Soil is one of the most important construction materials in the world.It has been effectively exploited for various purposes, such as the building of social infrastructures, growing agricultural products, and promoting activities that can be useful for human life [52,67,105,115].Hence, the selection of suitable soil sites is an important engineering requirement for building and constructing new infrastructures [26,105].Thus, investigating locally available materials, equipment, and cost-effective methods of soil improvement is an indisputable approach to optimizing the technique more efficiently and sustainably [113].Some soils with weak engineering properties cause major damage to civil engineering infrastructures and result in a massive maintenance and repair costs.Hence, several methods have been used to improve weak soil properties to construct stable foundations and avoid unfavorable conditions [40].
Soil improvement is one of the oldest introduced and simplest methods for improving the bearing conditions of soils.It can be done by either replacing the poor soil with more competent materials or altering the characteristics of the in-situ soil by using additives, considering obtaining good quality materials taking into consideration of local availability, environment friendliness, and low cost [20,35].This promotes the incorporation of industrial by-products and other waste materials, mainly from agricultural sources as construction material in terms of [39]; -sustainability and exploiting convenient ways to produce new materials from waste.
-eliminating the problem of waste treatment.
-avoiding the use and depletion of new natural resources.
-developing innovative and smart composite materials.
Expansive soil is known as a problematic soil that retains water in the wet season and shrinks in the dry season [36].In some regions, expansive soil is called vertosolic soil based on its unique morphological and volumetric change characteristics [16,57].Expansive black cotton soil has long been known as very problematic for construction; is considered as an unsuitable soil [16,41].
Nowadays, enormous attention is given by the researchers and geotechnical engineers to use agricultural waste as an alternative soil improvement material.Based on the assessment done by the researchers, agricultural waste additives have been used to improve the weak properties of subgrade soil.Moreover, agricultural waste is sustainable, low cost, and can be considered a potential alternative material for soil stabilization because of the availability of large quantities of waste [5,127].Hence, utilizing waste materials for soil stabilization is viewed positively for several reasons, such as improving the bearing capacity of the soil, increasing the shear strength, and reducing the permeability [46,127].In many countries, people living in rural areas, rarely have access to all-weather roads for daily transportation needs.The roads are often poorly constructed or have been damaged [15].Additionally, it is important to solve the problem of the environment and satisfy the rising demand for access to roads and transportation services [46].Hence, this research work aims to review and closely scrutinize the various techniques of agricultural-based soil additives for expansive soil stabilization in terms of their effectiveness, sustainability, cost, and engineering performance.Several comparisons of soil stabilization mechanisms were studied, analyzed, and discussed in the article.
This critical review of the stabilization of expansive soil with agricultural waste additives has immense significance in terms of the utilization of agricultural wastes, reducing environmental pollution that could harm human safety and health, advocating the geotechnical researchers to widely consider the use of it for soil stabilization and disseminate the efforts of other researchers work to the academic and scientific community.Moreover, the most recent research works and publications, and scientific discussions have been thoroughly reviewed with the focus of the expansive soil stabilization using agricultural wastes.

Principles of soil stabilization
The process of soil stabilization was introduced at the start of human settlement in ancient times.Ancient civilizations of the Chinese, Romans, and Incas utilized various methods to improve the strength of weak soil, some of these methods being so effective that their buildings and roads still exist today [37,113].The unsuitable soil was stabilized with local materials by blending one another and modifying the properties to improve the strength and acquire the strength of stable structures [40,41,43].As a result, different stabilizing materials and techniques have beenpracticed to achieve the better soil stabilization process [1,54,81].In Fig. 1, the highly expansive soil seems to be impassable in the wet/ rainy season due to the collapse of the road.Indeed, it's not only a collapsing problem but also the issue of maintenance is remained a major challenge.Figure 1 is a typical example of the failure and damage of road infrastructure built on highly expansive soils.
Several studies have been done to enhance the different methods of problematic soil stabilization since expansive soils are problematic soils and could cause serious damage to the infrastructures and buildings built on it [54,109].The mineralogical formation of this type of soil is from the smectite group, such as predominantly montmorillonite or some illite groups, characterized by extreme volume changes when the soil is in contact with moisture variation [16,37,109].On hand, some of the studies have focused on increasing the soil-bearing capacity or decreasing the expected soil settlement as well as taking into consideration the environmental and cost issues [40,60].According to [50], sometimes, soil reinforcement is interchangeably used for soil improvement or the stabilization process.The most common soil stabilization techniques are contextualized in the following chapters from conventional or chemical stabilization /resource-intensive methods to sustainable, robust, and most importantly stabilization of expansive soils with agricultural plant-based soil additives.

Conventional/traditional methods
Traditional/conventional stabilizers such as Portland cement and lime, bitumen, and fly ash are well-known, widely used, and suitable for the stabilization of expansive soils Fig. 1 Damaged expansive black cotton soil due to moisture variation (left) and collapsed road infrastructure in Ethiopia (right) (self-study) [122,123].This type of stabilization can be classified into mechanical, chemical, or electrical.The most important process of conventional stabilization is blending, mixing, densification, and changing the soil mass conditions.The key benefits of these stabilization techniques are increases in soil strength and durability, stiffness, and decreases in soil plasticity and swelling/shrinkage [37,97,123].Lime has been used to improve the workability of the soil and to increase compaction density.A pozzolanic reaction occurs when lime is mixed with clay soil particles [37,76,123,127].Nevertheless, [43] and [72] stressed that the use of cement and lime consumes many resources and energy, resulting in the release of substantial amounts of carbon dioxide, and consequently, causing harm to human health.
In addition, fly ash is also one of the chemical stabilizers of soil.Hence, many researchers consider these stabilizers to be not sustainable, less environment friendly and harming the health of humans [40,76,102].In Fig. 2, the various techniques of soil stabilization are described in terms of methods and technical aspects.The techniques described in Fig. 2 are a diagrammatic scheme to enhance which type of soil stabilization techniques is suitable in terms of cost, construction methods, and ability to improve the bearing condition of the soil.Some of the conventional/chemical stabilizers mainly lime, cement, and fly ash are discussed in the sub-sections and Table 1.

Stabilization with lime
Lime is a proven stabilizer and has been widely used to reduce the swell-and-shrink potential of clay soils, in particular expansive soils which cause volume changes and differential settlement [9,112,122].Lime can be found in a couple of ways namely, hydrated lime and quick lime.Hydrated lime is suitable and efficient when compared with quick lime [14,84].The content of lime in the stabilization process should be taken into consideration and it is not recommended to use higher content of lime-mostly in the range of 7-10%.The chemical process that occurs in the stabilization of expansive soil with lime is agglomeration and flocculation, pozzolanic reaction, and cationic exchange.This process directly affects the soil mineralogy, the soil chemistry, and the soil-water relationship [19,38,109].The lime stabilization technique improves the hydro-mechanical parameters of soil properties like shear strength, higher resistance to cracks, permanent deformation, and fatigue [9,14,90].Hence, higher CBR, higher UCS, and increased Fig. 2 Basic soil-stabilization techniques and methods [50] MDD and OMC were substantially found in lime-stabilized expansive soils [9,19,83,122].

Stabilization with cement
The soil stabilization of expansive soil with cement went back a century.According to the studies by [3,30,45] using cement as a soil stabilizer is creating cemented soil with high stiffness, and rapid strength gain, potentially allowing for a reduction in pavement thickness, and enhancing the performance of the soil mass.The cement-stabilized soil alters the unsuitable makeup of existing in-situ soil.The hydration, cation exchange, flocculation, and soil aggregation would take place at cement soil stabilization.It has both advantages and disadvantages [38,93].Advantages of cement-stabilized soil in terms of time and cost, it saves time to construct/maintain roads, requires less construction time, and facilitates smooth traffic flow, and disadvantages could be the high cost of production, environmental pollution, and greenhouse gas emissions [19,30,109].It is also found to be a reliable engineering alternative to satisfy the requirements of sustainability and availability in the market.The content of cement addition into the soil must be optimized and advised to use 12-15% by mass of the soil [45,77].As a result, the CBR, PI, and, UCS values have significantly improved for cement-stabilized expansive soil.

Stabilization with fly-ash
Fly ash is a by-product of coal-fired power generation facilities and can be freely available.It has a different chemical composition, mainly consisting of SiO 2 , Al 2 O 3 , and Fe 2 O 3 [48,99].It has been used to improve the stability of subgrade embankments and reduced the settlement and volume change of the soil.Fly ash with higher content (40% of soil by mass) has increased MDD and OMC and improved the PI of fly-ash soil mixtures [61,69].Fly ash must be mixed with other additives to achieve significant improvements [19,99,112].

Unconventional /non-traditional methods
In both methods, soil stabilization can be defined as modifying the soil property to increase its engineering performance.Unconventional or non-traditional stabilizers cover various material sources such as wastes such as industrial or agricultural, solid, or liquid, agricultural waste additives, geotextile or geosynthetic, fiber reinforcement, and enzymatic stabilizers can be used as a potential and efficient treatment for problematic soil stabilizers [10,38,80,90].These stabilizers could be less toxic and locally available and do not significantly harm the environment.Non-traditional stabilizers are mostly mixed with traditional stabilizers to create a composite cementing soil mass and increase the bearing capacity of the treated soil [23,25,42,91,94].

Stabilization with enzymes
In the past two to three decades, enzymatic soil stabilization has gained attention as an alternative to soil stabilization in terms of cost and environmental concerns.The enzymes being used as soil stabilizers are common products of the fermentation process [46].The idea of using enzymes for soil stabilization was developed from the horticultural application [81].However, there are no standards set for the use of enzymes as stabilizers, and thus, stabilization is the process being performed by empirical guidelines based on previous experience and manufacturer guidelines [17,41,81].Enzymes are liquid solutions, unlike traditional stabilizers, and these liquid-concentrated substances have been demonstrated to improve the stability of subgrade soil for pavement structures in road construction [119].
When applied in a suitable amount and compacted properly, the treated soil can be changed into a dense, firm-to-hard, water-resistant bound layer [81,100].The enzymatic reaction results in an improvement in the chemical bonding of the soil particles which creates a long-lasting structure that is more resistant to weathering, water penetration, and wear and tear [81,110,114,119].The most used commercial enzymes for soil stabilizers, such as permazyme, terazyme, ecozyme, renolith, etc., are among the proven bio enzyme products [46,79].Therefore, bio enzymes are non-toxic, environment-friendly, biodegradable, and organic [81,100,110,119].

Stabilization with fibers
Reinforcing soil with fiber is not a new concept; it has been used for more than 5000 years [50,122].Fiber-reinforced soil behaves as a composite material in which the fiber is of relatively high tensile strength.Thus, it would be helpful to predict its workability and durability [60,98].Mainly natural fibers such as coconut (coir) fiber, sisal, palm fiber, jute, flax, barley straw, bamboo powder, and cane fiber, have recently been identified as promising natural fibers for soil reinforcement [50,60,73,98].The natural fiber is readily biodegradable, non-toxic, and enhances greater tensile strength for soil reinforcement [24,35,106].
There are also many types of synthetic fiber, such as polypropylene, polyester, polyethylene, nylon, steel, and glass fiber, that are used for soil stabilization because of their availability and easy engineering application [54,97,116].Hence, soil reinforced by these materials will have greater strength, a lower water content, and soil compressibility [60,73].On the other hand, wheat straw fiber, rice husk, coffee husk, and corn husk fiber have been used in combination with lime or cement to stabilize clay soil [34,55,82,122].More importantly, fiber-reinforced soil has exhibited greater toughness and ductility and a smaller loss of post-peak strength [50,116].

Enzymatic induced calcium precipitation/EICP methods
The rapid increase in concern about replacing cement-intensive soil stabilization techniques with biologically based techniques has recently gained research attention in many parts of the world.EICP (Enzymatic induced calcium precipitation) method is primarily used to improve the soil property in the form of using calcium carbonate precipitation with the urease enzyme purification process [17,95].Some research investigated that EICP is effective in some soils such as sands and it improves efficiency without introducing any cementing materials [7,47].EICP utilizes agricultural-derived enzyme products instead of using microbially produced urease enzymes.Hence, several studies show that the EICP method may be more sustainable, efficient, environmentally friendly, and less costly.The basic engineering properties of the soil such as shear strength, UCS, and permeability have been significantly improved [7,17,62].However, the method may lack reliable improvement results since it depends on the different plant species that possess the urease activity [125].Enzymatic soil stabilization methods mainly employed the urease enzyme as a catalyst, and it is expensive material on the other hand.

Stabilizers from agricultural wastes
Using agricultural residue for soil stabilization is a timely alternative way to utilize disposed solid wastes.Nowadays, agricultural waste materials are abundantly available [5,32,108].However, generating a large quantity of agricultural waste is creating a serious problem in the handling and disposal system.The disposal of agricultural waste has a potentially negative environmental impact, causes air and water pollution, and ultimately affects the local ecosystems [15,18,108].To balance this, it is believed that the practice of the sustainable use of agricultural waste in soil stabilization is beneficial for effective waste management and ensuring a green environment [12].
The agricultural waste additives should be carefully examined and assessed for potential environmental impact assessment and must be proven harmless [5,54,72].Moreover, because of its availability, agricultural and industrial wastes are considered for soil admixtures, and focus is being placed on improving the engineering properties of soil [12,34,44,89].Figure 3 describes some of the preparation processes of local agricultural waste additives.Making powder from some agricultural waste could be difficult.Bamboo stem and Ensete fiber might be among the difficult materials to crush due to their strong nature and fiber content.Other agricultural waste additives such as wheat straw, sawdust, and sugarcane bagasse, are slightly easier to crush than bamboo.Although, the powders can be obtained from the different parts of plants, mainly the leaf, stem, straw, and stalk.
Numerous researchers including [15,108] have extensively studied the mechanisms to improve the unsuitable properties of expansive soil by using coffee husk ash, cornhusk fiber, coir fiber, wheat straw, and other types of agricultural waste/biomass.The results confirmed an improvement in CBR, UCS, and shrinkage, as well as a reduction in cracks, were obtained [5,78,89,127].Hence, these materials could be used to improve the strength of the road base/subgrade for low-volume gravel roads [5,18,20].In Table 2, the various types of agricultural waste utilized for soil stabilization are illustrated in detail along with their respective references.The effectiveness and results described in Table 2 for agricultural wastes used for soil stabilization purposes vary from one another.However, in many cases, the results have met at least the minimum requirement of soil stabilization when compared with an untreated soil sample.Moreover, Table 2 presents the merits, and demerits of plant-based agricultural waste used for soil stabilization in the form of powder or ash, or as a fiber.Since there is a limitation in the detailed and broad study of the advantages and disadvantages of plant-based agricultural wastes, some of the rarely studied research works on various agricultural waste additives are presented in Table 2.

Production and processing of agricultural waste
Agriculture is the world's largest waste-generating activity [8,13,108], at the same time it plays a key role in contributing to the growth of Gross Domestic Product (GDP) in developing countries [44,65] and generates income for low-income people.Many agricultural wastes generate after harvesting the crops such as wheat, rice, barley, sugarcane, forest biomass, and other non-food source plant resources or from industrial sources [5,12,49].For instance, rice produces by-products or residue like rice husk, straw; wheatpods, straw; maize-cob, husk, stalk, and stover; sugarcane-bagasse, leaves; soybean-husk and stalk, and others [8,55].In some parts of the world, the straw or cobs and waste leaves burnt in the open air and consequently cause air pollution, and disposal of this huge amount of waste could be a problem for landfilling.
To curb this worldwide problem, locally available plant-based wastes can be used for various purposes such as soil modification purpose [5,78,100,108,127].Several researchers mainly studied those agricultural wastes that have been used for biofuel   production to generate energy and some raw materials for power production [2,44,65,124].On the other hand, the greatest advantage of utilizing agricultural residues is to enhance economic benefit as well as use for engineering purposes [120].In some countries, it could be considered a leftover, but others use it for several important activities such as fertilizers or compost, bioelectricity (biofuel, ethanol), cellulose, and lignin production [53,121,124].

Geotechnical characteristics
The concept of sustainability in soil stabilization is very crucial in terms of natural resource aspects and alternative approaches for sustainability.Hence, agricultural waste additives produced from waste materials could be promising alternative material sources for stabilizing expansive soil and promoting sustainable resource utilization mechanisms [12,89,127].The conventional way of expansive soil stabilization significantly improves the strength and lowers the volume-change behavior in terms of the series of cationic exchange and pozzolanic reactions between the additives and the soil particles [16,37,107,123].In this regard, geotechnical researchers have started to investigate methods for treating expansive soil with agricultural waste additives [12,96].Therefore, since expansive soil is weak when in contact with water, agricultural waste additives ascertain the interaction of water with the soil particles in terms of decreasing water absorption and increasing workability [82,89,104].The primary purpose of using agricultural waste additives could be to alter the insitu soil characteristics with locally available materials [32,43].Therefore, the application of agricultural waste additives is now being viewed as enormously promising for in-situ soil modification in terms of being easy to use, local availability, eco-friendliness, and innovative technique [15,51,54,89,100].Moreover, an experimental study carried out on agricultural waste additives showed an improvement in the geotechnical properties, such as the bearing capacity, compressive strength, and shear strength, as well as a reduction in the permeability and compressibility of clayey soil [26,27,123].Road subgrade soil treated by agricultural waste additives, such as rice husk, sugarcane bagasse, coir or palm fiber, coffee husk, wheat straw, enset, and bamboo powder, shows significant improvement and meets the required standards of stabilization [12,31,32,127].
Table 3 presents the changes in engineering properties of the soil treated by eight different agricultural waste additives.Simultaneously, it is important to notice that the size, composition, and parts of the agricultural waste additives used for soil stabilization in the Table 3 vary from one material to another.Hence, Table 3   basic engineering properties of the soil improved by agricultural plant-based materials.Although, in Fig. 4, the fundamental improvement to the soil is described graphically.The CBR value in Fig. 4 is an important engineering parameter and is used to determine the strength of the soil.Thus, most agricultural waste additives demonstrate higher CBR values than untreated expansive soil.Accordingly, the UCS values in Fig. 4; Table 3 show higher values that could be gained in early curing periods of different curing durations.
The results depended on the agricultural waste additives' size and fiber length.Similarly, the improved property of the treated soil was observed in terms of PI, FSR, and OMC/ MDD values.

Durability characteristics
The concern of using agricultural waste additives for soil stabilization has not been properly explored yet.In the process of soil stabilization with agricultural wastes, the durability in terms of long-term should be considered.The waste generated from local agricultural waste has been used by local people for simple construction purposes, such as making mud walls and floors of houses and other non-standard work activities [24,35].Some materials could be highly durable and resistant to moisture, sun, and other    3) environmental factors, although some may become worn out before providing proper service and lose their performing abilities [35].
It is important, therefore, to guarantee not only the improvement but also the durability to promote their utilization of agricultural waste additives.When agricultural waste additives are mixed with other forms of conventional soil stabilization materials, their durability and performance are increased [128].Mainly, the durability and performance can be tested in freeze-thaw cycles that result in cracking, spalling, and heaving to the stabilized soils.The study examined the properties of clay soil samples reinforced and unreinforced with different types of fiber and observed an improvement in terms of strength [70].However, a significant decrease in strength was observed in samples treated with agricultural waste additives after 15 freeze-thaw cycles [70,116].
Soil stabilization with natural fibers, agricultural waste additives, and other local materials requires special attention for its longevity and performance [123].Furthermore, the factors that can affect the decomposition of agricultural waste after soil stabilization are, [29,129]; -soil aeration: organic matter existing in the soil can stimulate decomposition due to the soil aeration process.-temperature: decomposition of plant materials occurs based on seasonal changes and microorganisms that are active under moist warm conditions.-soil pH reaction: the soil is very sensitive to different pH levels and, as a result, the decomposition can be influenced by the acidity or basicity of the soil.-effect of moisture: soil holds a significant amount of water and contributes to decomposition.-microbial activities: addition of plant-based organic matter into the soil increases the rate of decomposition and multiplies the degradation.
The freeze-thaw or wet-dry cycle of agricultural waste additives should be considered as important as other engineering parameters.The decomposition problem and longevity issues are the main concern for agricultural waste additives [42,92].Wetting-drying conditions in different cycles expose the treated sample to the extent that the resistance to severe conditions.The sugarcane bagasse, wheat straw, and rice husk-treated soil specimens were prepared and compacted with the optimum moisture content to evaluate different wetting-cycling conditions [10,22,70,129].The rice husk-treated soil observed weight loss, a decrease in CBR, (7% to 3.4%), and an increse in swll, (5.5-7.1%) in 4 wetting-drying cycles, and a similar trend was observed for other agricultural waste additives treated soils [25,63,129].

Microstructural characteristics
The microstructural changes in the stabilized soil help to understand the mineralogical phases, structural crystallinity, and patterns of peaks.Using XRD/ X-ray diffraction and SEM/ EDS methods could reveal the formation of soil matrix in terms of mineralogical composition and morphology [56,74,101].Although, the structural shifts can be seen in stabilized expansive soils treated with agricultural waste additives.Microstructural examination of SEM analysis shows that the presence of several void spaces and interlayer bonding between soil particles and additives [6,85,118,131,132].Silica and alumina are the major chemical composition in expansive soils.Montmorillonite, quartz, kaolinite, and illite minerals are the dominant minerals that could be revealed by XRD analysis [15,28,71,86].Therefore, various reactions such as cation exchange, the appearance of aggregation/flocculation, or decrement/increment of peaks could take place in expansive soils due to the presence of clay minerals and stabilizing agents.
According to a study by [3,15,64,118], coffee husk, ensete fiber, rice husk, and other agricultural waste additives used in various amounts contributed to changes in the morphological and microstructural process.Figure 5 (a, b,) illustrates XRD analysis and SEM images used for the expansive soils treated by several agricultural waste additives.

Pros and cons of expansive soil stabilization techniques
Geotechnical engineers often apply soil stabilization, paying particular attention to the geo-environmental implications and utilization of waste materials.The aim is usually to effectively reuse the waste material, whilst improving the engineering properties of the soil and ensuring that harmful compounds are not released in quantities that may induce an adverse environmental effect [54,58].However, there are pros and cons to each soilstabilization technique.The positive and negative impacts are inevitable, but the levels of the impacts can be maintained [37,46,127].As many recent studies have revealed, soil-stabilization techniques should be free of intense resource requirements and use local resources as good candidate input materials [50,51].Moreover, Table 1, shows that all the techniques adopted for expansive soil stabilization have benefits and drawbacks since they comprise different improvement elements in terms of pozzolanic reaction with soil particles and reverse reaction during and after mixing with soil additives.A summary of the pros and cons of different soil-stabilization techniques is briefly described in Table 1.

Conclusion
Road infrastructure is an essential part of society to develop and access vital services.However, the roads mainly in the developing world failed and collapsed due to several reasons such as design problems, subgrade material failure, and other reasons as described in this review.
This study has revealed many untouched possibilities of using agricultural waste for soil stabilization purposes.A substantial amount of waste is being generated globally and polluting the environment and its ecosystem causing health problems for humans.Indeed, on the other hand, it could be a useful resource in terms of engineering perspectives, resource utilization aspects, and economic benefits.Moreover, this review article explored the enormous importance of agricultural plant-based waste additives for soil stabilization and concluded with the following points: i) In this review, the testing techniques and test results for agricultural waste additives were well organized, matriculated, and accurately addressed from different researchers' perspectives.ii) It is believed that agricultural waste additives are economically viable, locally available, bio-degradable, and eco-friendly.The sustainability practice was thoroughly assessed and discussed in response to utilizing the waste material for soil stabilization.iii) It examined the significant improvement in CBR, UCS, PI, shear strength, and other important engineering properties in the treated soil, however, the results varied from one additive to another due to the soil type and employed testing mechanisms.iv) The wet-dry cycle and freeze-thaw cycles are critical indicators of the performance and durability of agricultural waste additives.When the number of cycles increases, weight loss, decrease in strength, and increase in swelling could be observed.v) This study further confirmed that agricultural waste additives could be a better replacement for the expensive and resource-intensive soil-stabilization techniques with less technical skills and as well as cheaper local materials.vi) The expansive soil stabilized with agricultural waste additives has shown significant improvements in terms of strength development, compressibility, and reduction in cracking.However, recommendations would be given for the careful selection of plant resources, and microstructural and mineralogical formations in soils in future studies.Natural fibers are prone to decay and decomposition hence, the article would suggest further study on durability of fiber additives in terms of applicability and current construction practice.

Fig. 3
Fig. 3 Process of material preparation for plant/agricultural waste additives (own study)

70 *
The results are mainly obtained from ash byproducts.Fibers of different sizes and lengths have been used.

Fig. 4
Fig. 4 Engineering property improvement of soil stabilized by agricultural waste additives.(adapted from Table3)

Fig. 5
Fig. 5 microstructural analysis for expansive soil stabilized with agricultural waste additives, (a) XRD for coffee husk treated expansive soil and XRD for expansive black cotton soil; (b) SEM image analysis for the eight agricultural waste additive treated soils

Table 1
Advantages and disadvantages of various expansive soil stabilization techniques.(adapted from various sources)

Table 2
Practices of the utilization of agricultural waste for soil stabilization purposes (own study adapted from other sources)

Table 3
Improvement in engineering properties of treated expansive soils (own preparation adapted from various sources)