Effect of Lupine Hull Ash on Geotechnical Behavior of Weak Expansive Subgrade Soils

Abstract

Several road sections have failed due to volume changes of expansive soils during moisture variation. While there are various common stabilization techniques, the use of agricultural byproducts in stabilization works can minimize costs and environmental effects when compared to ordinary stabilization methods. This paper discusses an experimental approach used to study the influence of lupine hull ash on expansive soil as a subgrade material. The laboratory results show that the expansive soils belong to A-7-5 and CH classes according to AASHTO and USCS, respectively. The expansive soil was mixed with varying concentrations of lupine hull ash, namely, 3%, 6%, 9%, and 12%. The plasticity index and free swell index were reduced by 68.19% and 62.67%, respectively, when treated with a 12% lupine hull ash concentration. The compaction test revealed a decrement of MDD and increment in OMC values as the percentage of the stabilizer material increased. Additionally, the UCS and CBR values increased significantly as the concentration of the expansive soil increased. The mineralogical analysis showed that the untreated soil is composed of 46.2% silica, 28.7% montmorillonite, and 25.1% kaolinite minerals. The SEM and XRD analysis proved the microstructural and mineralogical changes when the expansive soil is mixed with the optimum lupine hull ash concentration. While the CBR value of natural soil is unsuitable for subgrade material, mixing it with a 9% concentration of lupine hull ash ensures that the expansive soil complies with the standard as subgrade material. The experimental findings of this study emphasize the suitability of lupine hull ash for treating expansive soil encountered in rural road construction works.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Expansive soils are abundant in many parts of the world, posing major risks and financial losses due to their swelling and shrinking behavior. Engineering problems due to expansive soils have been reported in many countries all over the world, costing millions of dollars due to severe damage to infrastructures. To prevent these problems, it is essential to stabilize the natural soils before erecting structures on them [1, 2]. Ethiopia is one of the countries where the occurrence and spatial distribution have been identified as noteworthy. Around 40% of the land in Ethiopia is covered by expansive soil. The Ethiopian Road Administration (ERA) office has prioritized the problem of expansive soil on road assets, and several stabilization techniques have been applied in the field. But the damage of expansive soils on road assets is still prevalent disrupting the intended purpose of road networks.

The magnitude of damage caused by expansive soil is enormous causing huge economic problems. Lightweight engineering structures, highways, runways, and subsurface utilities are examples of infrastructures heavily influenced by expansive soils [2, 3]. The content of montmorillonite mineral in the expansive soil strongly influences the swelling nature of the expansive soils [4, 5, 6].

The traditional additive cement and lime are not recommended nowadays due to their cost and associated environmental impact. Moreover, the conventional stabilizers caused negative influence to the environment by increasing the pH level of treated soil and its surrounding areas [7, 8]. A study by Zhang et al. [9] discussed the environmental impact of lime when compared with geogrid materials. A similar study by Ojuri et al. [10] also showed the geotechnical and environmental effects of lime–cement and mine tailing mixture for road construction purposes.

Recent trends in research works in the field of geotechnical engineering and construction materials focused on cheap, locally available materials, and environmentally sound materials such as bagasse ash, sugarcane straw ash, fly ash, rice husk ash, coconut husk ash, coffee husk, and eggshell ash [11, 12, 13, 14]. The influence of rice husk and lime mixture of on the properties of expansive soils has been discussed by previous studies [15, 16, 17]. A study by Adeyanju et al. [18] showed that 8% rice husk ash and 10% cement kiln dust improved the mechanical behavior of subgrade soil. Moreover, the application of industrial wastes for stabilization of expansive soils has been discussed previous studies [19, 20, 21, 22].

Nowadays, several researches focused on the potential of locally available materials from industrial and agricultural waste, to improve the properties of expansive soils and comply with geotechnical engineering design requirements. A study by Neguse et al. [13] showed that adding 10% of Enset ash improved index properties as well as the CBR value significantly. A study by Atahu et al. [23] discussed that 20% coffee husk ash improves the unsoaked CBR value by 28%, while the soaked CBR increases by 203%. The effect of eggshell ash and stone powder on the engineering behavior of expansive soils has been prevented by Teferra et al. [14]. The study highlighted that 6% eggshell ash and 15% stone powder improve the engineering behavior of the expansive soil. Previous studies discussed the influence of coconut pulp and groundnut pulp on the geotechnical behavior of expansive soil [24, 25]. Moreover, the admixtures increase CBR values gradually up to 6% and then decrease for both coconut waste pulp and groundnut waste pulp.

Even though several studies are showing the stabilization of expansive soil using various additives, plant products, and agricultural wastes, those studies are not still adequate [19, 25, 26]. A study by Hasan et al. [26] discussed the role of bagasse ash on the mechanical behavior of expansive soils. In addition, the effect of rice husk ash on the stabilization of subgrade soils has been highlighted [27]. Moreover, a large number of plant sources have been identified in Ethiopia as potential sources of construction materials and locally stabilizing agents in rural areas [12].

In this study, the influence of lupine hull ash (LHA) on the geotechnical behavior of the subgrade expansive soil has been discussed by employing mineralogical analysis techniques like XRD and scanning electron microscopy (SEM) besides index and shear strength tests. The presence of abundant lupine hulls in the study area made it suitable for stabilizing the expansive subgrade soils found in rural road networks. The X-ray fluorescence (XRF) analysis has been used to characterize LHA. Moreover, SEM and XRD analyses have been used to present the influence of LHA on the geotechnical behavior of weak expansive subgrade soils.

2. Materials and Methods

2.1. The Study Area

The study area, Debre Markos City, is located on the plateau of the northwestern highlands of Ethiopia. It is located at 10° N latitudes and 37° E longitudes covering a total area of 1,650 km². The elevation ranges from 1,750 to 2,450 m with an average of 2,360 m a.s.l. with gentle and some hill slopes. The plateau is surrounded by the deep Abay River gorge on the eastern and southern sides and western lowlands on the western side. The valley plains are swamps and water-logged areas during the rainy months. The morphological behavior also favors the formation of expansive soils. The study area has been selected by the authors to remediate the ongoing road failure and deformation by implementing locally available LHA as a stabilizing agent.

2.2. Materials Used and Their Properties

2.2.1. Expansive Soil

The disturbed expansive soil sample was recovered using pit excavation at a depth of 1.5 m adjacent to the roadway. The collected soil is black. The sample has been prepared as per ASTM-D421. Figure 1 shows the sample collection procedure adopted in this study. The mechanical properties of the expansive soil are reported in Table 1.

[figure(s) omitted; refer to PDF]

Table 1

Summary of geotechnical properties of expansive soils.

Geotechnical properties	Values		Standard methods
	Soil	ESA
Natural moisture content (%)	28.12	—	ASTM D2216
Optimum moisture content (%)	34.6	—	ASTM D698
Maximum dry density (g/cm3)	1.26	—	ASTM D698
Liquid limit (LL) (%)	108.3	—	ASTM D4318
Plastic limit (PL) (%)	42.66	—	ASTM D4318
Plasticity index (PI) (%)	65.64	—	ASTM D4318
Shrinkage limit (SL)	37.77	—	—
Linear shrinkage (%)	21.89	—	BS 1377-2
Free swell index (%)	152.5	—	ASTM D-720
Free swell ratio (%)	—	—	IS 2720-40
(%) finer of No. 200 sieve (%)	—	—	ASTM D6913
USCS soil classification	CH soil	—	ASTM D2487
AASHTO soil classification	A7-5	—	AASHTO M145
Specific gravity (Gs)	2.73	—	ASTM D854
UCS (kPa)	73.57	—	ASTM D2166
Compressibility index (Cc)	—	—	ASTM D2435
mv (Mpa−1)	—	—	ASTM D2435
Cv (m2/year)	—	—	ASTM D2435
Proctor compaction test	—	—	ASTM D 698-07
CBR and CBR swell	12.97	—	ASTM D 1883-99
Soaked CBR	0.89	—	—
Color	Black	—	—

2.2.2. Geotechnical Properties of Expansive Soil

Table 1 shows the geotechnical properties of the natural soil. As shown in Figure 2, the natural soil is composed of sand, silt, and clay particles. About 3.5% of sand passes a 4.75-mm sieve and retains on 75 μm and 45.12% of silt, and 51.38% of clay passes 2 μm. The amount of fine-sized particles less than 75 μm was 96.5%, and the remaining 3.5% was coarse materials.

[figure(s) omitted; refer to PDF]

The natural soil liquid limit (LL), plastic limit (PL), and shrinkage limit (SL) are 108.3% 42.66%, and 37.77%, respectively. Also, the plasticity index and linear shrinkage index are 65.64% and 21.89%, respectively. The specific gravity and natural moisture content of the soil are 2.73% and 28.12%, respectively. Also, the free swell index is 152.5%, which is greater than 100%. The standard proctor test gives the optimum moisture content (OMC) and the maximum dry density (MDD) which are 34.6% and 1.26 g/cm³, respectively.

The unconfined compressive strength (UCS) of the natural soil is 73.57 kPa peak strength at 2.4% strain. The CBR value is 0.89, and the CBR swell becomes 12.97%. According to the ERA 2013 technical specification manual, the natural soil sample does not fulfill the requirements as a subgrade material, and it is unsuitable for subgrade in road construction since it has very low load-bearing capacity and high swelling potential (ERA, 2013).

Moreover, the aforementioned laboratory results show that the soil sample used in this study can be categorized as highly expansive soil and it does not obey the standard set for subgrade material. This necessitates the need to employ stabilization intervention.

2.3. Lupine Hull Ash

2.3.1. Properties of LHA

The specific gravity of LHA is 1.09, which is lower as compared to the natural soil material. XRF analysis has been employed to identify the material internal compositions including major and minor oxides, loss of ignition, and water contents. Table 2 shows XRF results for the additive material.

Table 2

XRF result of lupine hull ash.

Oxides or constituents	Concentration (%)
SiO2	45.7
Al2O3	3.17
Fe2O3	1.48
CaO	11.6
MgO	2.36
Na2O	0.66
K2O	8.76
MnO	0.08
P2O5	2.56
TiO2	0.11
H2O	1.73
LOI	22.97

As mentioned in Table 2, the XRF or complete silicate analysis of LHA implies that the major abundant oxides are SiO₂, CaO, K₂O, and Al₂O₃. Also, the sum of SiO₂ + Al₂O₃ + Fe₂O₃ becomes 50.4% which is greater than 50%, the amount of water was 1.73%, and it was less than 3% that satisfy ASTM C-618-03 requirements to classify under pozzolana fly ash class C. Thus, the material has pozzolana content that can form calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH), and it can be used as a chemical stabilizer that can react with expansive soil by cation exchange, agglomeration, flocculation, and pozzolanic ways of chemical stabilizer. The LOI is 22.97%, which is a higher value as compared to the standard. This is attributed to the unburnt carbon or other volatile materials during the burning process. The LHA material had high pozzolanic content as compared to Enset ash and coffee husk ash because the LOI was low and the material was composed of many minerals [13, 14]. But, as compared to bagasse ash material, the LHA was low pozzolanic content due to unburned and volatile minerals.

2.4. Methodology

2.4.1. Sample Preparation

The preparation of LHA passes different and challenging procedures. Some of the lupine seeds were collected from a local market, and the hull part was separated from its seed using daily labor. Additional lupine hull was collected from local beverage houses. The overall sample was then air-dried. Then the air-dried lupine hull sample was subjected to an open-air burning process. Finally, all the open-air burned lupine hull materials were calcined with an additional burning process using a muffle furnace. The calcination temperature of 650 to 850°C was adopted to obtain ash. The pozzolanic content of the ash was checked by a complete silicate analysis (XRF) test in the Ethiopian Geological Survey laboratory. The procedure of preparation LHA is shown in Figure 3.

[figure(s) omitted; refer to PDF]

2.4.2. Laboratory Test Procedures

The geotechnical properties of the materials are determined considering ASTM, AASHTO, BS, and IS standards. The soil specimen passing sieve size below 4.75 mm has been used for particle size determination. The index properties, UCS, and CBR parameters were determined accordingly. The untreated soil is classified according to the USCS and AASHTO system. Table 2 shows a list of parameters and standards adopted in the analysis.

2.4.3. Microstructural Analysis

(1) SEM. The microstructural properties of the expansive soil and treated soil have been determined by using a SEM. The mechanical behavior and physical properties are predicted by the arrangement of elementary particles, bondage, and continuity as well as their sizes and shapes. The SEM analysis considers only the untreated natural expansive soil and treated expansive soil at optimum concentration (i.e., 9% LHA).

(2) XRD. The mineralogical properties were identified by using XRD analysis. The XRD analysis was employed to identify the presence of different minerals in natural soil and treated expansive soil at the optimum percentage of LHA. The representative samples were recovered from samples prepared for analysis of UCS and CBR tests. Then the samples were oven-dried and passed with a 75-μm sieve.

This test has been conducted in laboratory based on Bragg’s law of diffraction at constant wavelength lambda of 1.541874A° on Cu-Ka radiation. The values of 2θ versus relative intensity values were collected for both samples. The collected 2θ versus relative intensity values were passed through phase analysis software using a power diffraction manual of crystalline impact to identify major minerals of untreated soil and made a comparison on the extent of change when treated at optimum percentage of LHA [4].

(3) XRF. The XRF analysis has been employed to characterize LHA. It identifies the chemical composition of LHA recovered after burning in a muffle furnace. This test was performed in the Ethiopian Geological Survey Central Laboratory using 200 g of LHA that passes a 75-μm sieve. The identified major and minor oxides are shown in Table 2. The results have been interpreted based on ASTM C 618-08a standard.

2.4.4. Mix Design

The mix design of this experimental study is shown in Table 3. The LHA concentrations considered in this study are 3%, 6%, 9%, and 12%. The index tests, UCS, and CBR tests have been conducted for the blended soil and untreated soil samples. The optimum concentration has been identified by referring to experimental results. Then XRD and SEM analyses were conducted to explore the mineralogical and microstructural behavior of the expansive soil under optimum concentration of LHA.

Table 3

Mix design adopted in the experimental study.

Mix design	Expansive soil (ES) (%)	Lupine hull ash (LHA) (%)
ES + 0% LHA	100	0
ES + 3% LHA	97	3
ES + 6% LHA	94	6
ES + 9% LHA	91	9
ES + 12% LHA	88	12

3. Result and Discussion

3.1. Effect of LHA on Atterberg Limit Tests

The effect of LHA on the Atterberg limit of expansive soil performance is shown in Table 4.

[figure(s) omitted; refer to PDF]

Table 4

Effect of LHA on engineering properties.

Properties	Concentration of LHA (%)
	0%	3%	6%	9%	12%
Free swell index (%)	152.5	113.66	89.32	59.54	56.94
Plasticity index (%)	65.64	49.38	37.11	23.22	20.88
Plastic limit (PL) (%)	42.66	43.69	44.82	46.43	44.57
Liquid limit (LL) (%)	108.3	93.07	81.93	69.65	65.45
Shrinkage limit (SL) (%)	37.77	34.22	31.89	28.98	27.68
Linear shrinkage index (%)	21.89	18.18	14.54	11.75	11.21
Socked CBR (%)	0.89	4.26	7.9	9.8	8.51
CBR Swell (%)	12.97	7.46	4.36	1.94	3.25

The liquid limit of untreated expansive soil reduced by 14.06%, 24.35%, 35.69%, and 39.57% when using 3%, 6%, 9%, and 12% LHA concentration, respectively. This is due to the stabilizer material can react to expansive soil chemically quickly and reduce the amount of water to flow like a fluid, and the reduction of liquid limit is generally due to a decrease in the thickness of the double layer developed. A reduction in liquid limit generally indicates an increase in frictional resistance and a decrease in the cohesion of soil [11, 15, 27].

As mentioned in Table 4, the PI changes from 65.64%, 49.38%, 37.11%, 23.22%, and 20.88% when stabilizer concentration varies from 3%, 6%, 9%, and 12%, respectively. Figure 4 shows that the PI value has been significantly reduced when the concentration of the additive is 9%. Similarly, the shrinkage limit and linear shrinkage index decrease by 26.71% and 48.79%, respectively, by using 12% of LHA. The linear shrinkage index value does not change much when the LHA concentration varies from 9% to 12% (Figure 5). A reduction of PI generally indicates an increase in frictional resistance and a decrease in the cohesion of soil [13, 26].

[figure(s) omitted; refer to PDF]

3.2. Effect of LHA on Free Swell Tests

The free swell index of the untreated soil is 152.5%. The stabilizer reduces the free swell index to 113.66%, 89.32%, 59.54%, and 56.94% when the concentration of LHA varies from 3%, 6%, 9%, and 12% (Figure 6). Moreover, the stabilizer material significantly decreases the expansiveness behavior when 9% additive is considered, and it gets almost constant for further concentration increments (Figure 6). This shows that 9% is an optimum amount to minimize free swell index.

[figure(s) omitted; refer to PDF]

3.3. Effect of LHA on Standard Proctor Compaction Tests

The standard proctor compaction test was conducted for untreated expansive soil and treated expansive soil using different percentages of LHA. As shown in Figure 7, the compaction test was performed for untreated expansive soil and treated expansive soil by 3%, 6%, 9%, and 12% of LHA concentration.

[figure(s) omitted; refer to PDF]

As illustrated in Figure 7, the untreated expansive soil has 1.26 g/cm³ maximum dry density achieved at an optimum moisture content of 34.6%. The value of dry density decreases from 1.26 to 1.11 g/cm³ when increasing percentage of stabilizer from 0% to 12%, respectively. The result shows that the percentage of stabilizer is directly proportional to moisture content due to the stabilizer’s higher water absorption capacity but inversely proportional to dry density, due as compared to expansive soil, the stabilizer was low specific gravity. The increase in OMC is due to the pozzolanic reaction of silica and alumina in LHA and soil with calcium to form CSH and CAH, which are the cementing agents and due to structural changes in which the dispersed structure changes to a flocculated structure, and as a result, the mixture absorbs more water [15, 26].

3.4. Effect of LHA on UCS

As shown in Figure 8, the UCS is presented as various percentages of the stabilizer material. The peak strength of the natural expansive soil is 73.57 kPa at 3.67%. When the natural soil is blended with the LHA at 3%, 6%, 9% and 12%, the peak strength becomes 100.16, 124.15, 199.3, and 156.95 kPa, respectively.

[figure(s) omitted; refer to PDF]

The UCS value of the untreated soil has increased as the percentage of stabilizer increased to 9% percentage of LHA and decreased for further increments of LHA (Figure 8). Moreover, Figure 9 shows that the UCS value is maximum when the percentage of LHA is 9%. The strength gain is due to the ability of LHA in agglomeration, flocculation, and pozzolanic reaction with expansive soil and the formation of cementitious materials in the mixture [13, 27].

[figure(s) omitted; refer to PDF]

3.5. Influence of LHA on CBR Value

The strength of the subgrade soil depends on the CBR value. The untreated soil has a CBR value of 0.89%, which is weak to select as a subgrade material. When the soil is stabilized by LHA at different concentrations starting from 3%, 6%, 9%, and 12%, the CBR value is changed to 4.26%, 7.9%, 9.8%, and 8.51%, respectively (Figure 10). The CBR swell becomes 12.97% for untreated soil, and it is changed to 7.46%, 4.36%, 1.94%, and 3.25% when the concentration of stabilizer increases from 3%, 6%, 9%, and 12%, respectively. Figure 11 shows that the CBR value obtains optimum value at 9% LHA concentration.

[figure(s) omitted; refer to PDF]

As illustrated in Table 5, the untreated soil is categorized as S1, which is classified as poor material according to the ERA pavement design manual. The additive material to the natural soil makes the soil suitable for subgrade construction.

Table 5

Effect of LHA concentration on subgrade class.

Mix combination	Subgrade class
Untreated ES	Poor material for subgrade (S1)
Untreated ES + 3% LHA	Normal material for subgrade (S2)
Untreated ES + 6% LHA	Good material for subgrade (S4)
Untreated ES + 9% LHA	Good material for subgrade (S4)
Untreated ES + 12% LHA	Good material for subgrade (S4)

Figure 12 shows the 4 days socked CBR swell results. Similarly, the CBR swell reduces from 12.97% to 1.94% by adding 9% LHA; adding further concentration of the additive material negatively affects the CBR swell value.

[figure(s) omitted; refer to PDF]

3.6. Microstructural Property Analysis

The microstructural property analysis has been employed to explore the structure bondage at microlevel, and this was performed by SEM machine. Specifically, the SEM analysis has been worked out to compare the behavior of the untreated soil with the blended soil at optimum percentage, i.e., 9% of LHA.

As shown in Figures 13(a) and 13(b), the untreated expansive soil shows discontinuity, fissures, visible voids between particles each other, and which shows the presence of weak bond in the untreated soil. Similarly, Figures 13(c) and 13(d) show the SEM analysis results at 9% LHA. In the blended soil, there are few pores, continuous matrix, and rough texture between particles. The additive material creates strong bondage and cementation. Similar results were reported by previous study using Enset ash by Neguse et al. [13].

[figure(s) omitted; refer to PDF]

3.7. Mineralogical Analysis Results

XRD analysis using a Bruker CCD diffractometer of Cu-Ka radiation was performed on untreated expansive soil to identify the changes in mineralogical phases presented in the samples. The samples were scanned at an angle of 2θ; it ranges from 10° to 80° to provide enough XRD peaks to identify the most common soil minerals in the phase. The minerals are identified by comparing diffraction pattern with standard pattern data using diffract software provided by the manufacturer of the instrument. Match (Crystal Impact software) was used to analyze the data, and the detailed procedure of this test is supported by literature [4].

As shown in Figure 14(a), the major minerals found in the untreated expansive soil include silica, montmorillonite, and kaolinite with 46.2%, 28.7%, and 25.1%, respectively. Moreover, the peak intensity was recorded for silica, montmorillonite, and kaolinite at 2θ values of 27.09, 20.81, and 12.73, respectively (Figures 14(b), 14(c), 14(d), and 14(e)). When the expansive soil is treated by an optimum amount (9% of LHA), the XRD results are shown in Figure 14(e). The intensity of montmorillonite decreases due to the formation of CSH, CAH, and calcite. This shows the stabilizer can successfully react and bind with expansive soil minerals chemically by cation exchange, agglomeration, and flocculation and in pozzolanic ways [4, 13].

[figure(s) omitted; refer to PDF]

4. Conclusions

This study explores the influence of agricultural byproduct on the behavior of subgrade material using experimental approach. To this aim, LHA at concentration of 3%, 6%, 9%, and 12% have been considered. The following conclusions were drawn:

(i) The natural soil is fine-grained soil with 96.5% fine, and it is A-7-5 and CH soil based on AASHTO and USCS classifications, respectively.

(ii) Moreover, the free swell index is 152.5%, and the CBR and its swell become 0.89 and 12.97, respectively, which is highly expansive, low bearing capacity, and not recommended as a subgrade material.

(iii) The results of XRF analysis for LHA show the presence of oxides that can chemically stabilize expansive soil by cation exchange, flocculation, and agglomeration. Also, the sum of SiO₂ + Al₂O₃ + Fe₂O₃ obeys ASTM-C618-03 requirements to classify under pozzolana fly ash class C.

(iv) The plasticity index of the expansive soil decreased from 65.64% to 20.88%, and free swell index reduced from 152.5% of ES to 56.94%, while soil treated by 12% LHA. This is due to the bondage of LHA with expansive soil and consequently reduce double-layer of soil and avoid swell potential.

(v) The concentration of stabilizer material is directly proportional to OMC and inversely proportional to MDD due to the LHA can absorb more water and bind with soil particles and low specific gravity as compared to expansive soil in compaction test.

(vi) The performance of soil was improved when using LHA in UCS test. The stabilized material alters the values from 73.57 to 199.3 kPa and 156.95 kPa when mixed with 9% and 12% LHA, respectively.

(vii) Using LHA material plays important role in improving subgrade strength of road by increasing the CBR of ES. The socked CBR value of untreated ES 0.89% was a poor subgrade material. This value is changed to 9.8% and 8.51% when added with 9% and 12% LHA, respectively.

(viii) The XRD examination test at 9% LHA shows high intensity of kaolinite; decrement of montmorillonite; and the formation of CSH, CAH, and calcite due to cation exchange, agglomeration, flocculation, and pozzolanic ways with soil and form strong bond.

(ix) The SEM results witnessed the presence of strong bondage between particles when treated by 9% LHA at optimum amount.

(x) Based on the overall laboratory test results, untreated soil did not satisfy the requirements of the ERA 2013 Ethiopian Roads Administration standard specifications for subgrade material. However, after blending with 9% LHA, the mixed soil satisfies subgrade class S4 as good material of road construction.

5. Limitations and Future Works

Future studies can focus on the effect of LHA on the geotechnical behavior of the expansive soil considering the effect of curing time. The stabilization work can be further optimized by considering suitable additive in addition to LHA. The XRD analysis can be further enhanced by considering various concentrations of additives.

Disclosure

This study did not receive any funds but is a part of the employer at Addis Ababa Science and Technology University.

Acknowledgments

The authors are very thankful to Addis Ababa Science and Technology University, Civil Engineering Department, for allowing access to geotechnical laboratory.

Glossary

Abbreviations

AASHTO:American Association of State, Highway, and Transport Official

ASTM:American Society for Testing Material

BS:British standard

CSH:Calcium silicate hydrate compound

C_V:Coefficient of consolidation

C_c:Compressibility index

ES:Expansive soil

ERA:Ethiopian Road Administration

FSI:Free swell index

FSR:Free swell ratio

IS:Indian standards

JCPDS:Joint Committee on Powder Diffraction Standards

LL:Liquid limit

LOI:Loss of ignition

LHA:Lupine hull ash

m_v:Coefficient of volume compressibility

MDD:Maximum dry density

OMC:Optimum moisture content

PI:Plastic index

PL:Plastic limit

SEM:Scanning electron microscope

UCS:Unconfined compressive strength

USCS:Unified soil classification system

XRD:X-ray diffraction

XRF:X-ray fluorescence.

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