Journal of the Civil Engineering Forum.
September 2026, 12.
:293-304 DOI 10.
22146/jcef.
Available Online at https://jurnal.
id/v3/jcef/issue/archive Effect of Relative Density Variation on The Effectiveness of Fly Ash in Stabilization of Sandy Soils for Liquefaction Mitigation Yunan Yakuta Wangsawitana1 .
Muhammad Fauzan1 .
Heriansyah Putra1,* .
Erizal1 .
Minson Simatupang2 1 Department of Civil and Environmental Engineering.
IPB University.
Bogor.
INDONESIA 2 Departement of Civil Engineering.
Halu Oleo University.
Kendari.
INDONESIA *Corresponding author: heriansyahptr@apps.
SUBMITTED 28 November 2025 REVISED 02 February 2026 ACCEPTED 23 February 2026 ABSTRACT Indonesia is highly vulnerable to earthquakes that can trigger soil liquefaction, particularly in coastal regions where saturated sandy soils with low relative densities are prevalent.
This study investigated the influence of varying Class C fly ash contents .
%, 10%, and 15%), relative densities .
%, 70%, and 90%), and curing periods .
and 14 day.
on the unconfined compressive strength of fully saturated sandy soils as a liquefaction mitigation approach.
The methodology included physical soil characterization, sample preparation with specific fly ash and relative density variations, and unconfined compressive strength testing after the curing period.
The results demonstrated that increasing the fly ash content, relative density, and curing time significantly enhanced the strength of the samples.
The highest strength occurred at 15% fly ash and 90% relative density, with 426.
22 kPa .
57 kPa .
The most notable improvement occurred with an increase in fly ash content from 5% to 10% and relative density from 50% to 70%.
Fly ash was more effective at enhancing the strength of sandy soil by promoting interparticle cementation, whereas excessive compaction tended to disrupt the soil structure and generate fine particles that reduced cohesion, as evidenced by the UCS test results and SEM observations.
The most significant strength gain between 7 and 14 days was observed at 50% relative density and 5% fly ash, with a twofold increase.
The combination of 10% fly ash and 70% relative density was optimal for field implementation, as it yielded strength values exceeding the liquefaction resistance thresholds, while maintaining practical compaction levels and material efficiency.
A comparative cost assessment considering materials, labor, equipment, and construction time is recommended to evaluate the practical feasibility of fly ash stabilization relative to alternative ground improvement methods for Indonesian projects.
KEYWORDS Fly Ash.
relative density.
saturated sand.
UCS.
A The Author.
This article is distributed under a Creative Commons Attribution-ShareAlike 4.
0 International license.
1 INTRODUCTION
Indonesia is situated at the convergence of four active global tectonic plates: the Indo-Australian.
Eurasian.
Philippine, and Pacific plates.
This geotectonic setting renders the country highly susceptible to seismic activity (Ministry of Public Works and Public Housing (PUPR), 2.
One of the significant consequences of earthquakes is the loss of soil strength owing to the intensity and duration of ground shaking, potentially leading to liquefaction.
This risk is especially relevant in coastal regions like Indonesia, where saturated sandy soils commonly exhibit low relative densities and high void ratios (Sholihah et al.
, 2.
Furthermore, tidal fluctuations can influence the bearing capacity of sandy coastal soils, increasing their vulnerability to liquefaction under saturated conditions (Adinda et al.
, 2.
Tidal cycles can elevate the groundwater level and reduce the initial effective stress in sandy soils, thereby increasing their susceptibility to excess pore-water pressure.
Nevertheless, the development of excess pore water pressure during seismic events is primarily governed by earthquake-induced cyclic loading, while tidal fluctuations mainly act as a preconditioning factor that brings the soil closer to a critical stress state rather than serving as the principal cause of liquefaction (Dong et al.
, 2.
Liquefaction frequently occurs when the shear strength of soil is significantly reduced owing to continuous shaking, particularly during earthquakes (Anidhea and Suliyanah, 2.
According to the Indonesian Agency for Meteorology.
Climatology, and Geophysics (BMKG), the magnitude 7.
4 tectonic earthquake that struck 26 km north of Donggala Regency.
Central Sulawesi, on September 28, 2018, triggered a tsunami along the western coast of Sulawesi Island with waves reaching nearly 6 m in height.
This event was followed by widespread liquefaction, which caused the soil to lose its load-bearing capacity, leading to ground failure, tilting, subsidence, and structural collapse.
Liquefaction typically affects saturated sandy soils with Journal of the Civil Engineering Forum low relative densities, resulting in a saturation ratio of 100% (Sr = 100%) (Laia, 2.
In fully saturated soils, all void spaces are filled with water.
under loading, this leads to increased pore water pressure.
An elevated pore pressure reduces the effective stress, which in turn decreases the shear strength of the soil.
This risk is further exacerbated in loose sandy soils, which often exhibit relative densities below 50% (Legrans, 2.
even higher densities (>70%), sands remain compact and retain shear strength under excess pore pressure, with a relative density (Dr ) of 90% sands demonstrating significantly greater resistance to liquefaction owing to stronger interparticle contact and internal friction (Bensoula et al.
, 2022.
Pratiwi, 2.
Various soil stabilization methods have been developed to mitigate liquefaction, including the use of fly ash, a coal combustion byproduct.
Fly ash possesses pozzolanic properties, reacting with water and calcium hydroxide to form strong calcium silicate hydrates (CSH), enhancing soil strength, stiffness, and reducing permeability.
Two main types of fly ash.
Class C and Class F, are typically used in soil stabilisation.
Class F fly ash contains low CaO (<10%) and is purely pozzolanic, requiring lime activation to induce cementitious behaviour (Nurdiyanto et al.
, 2.
Aydilek and Arora .
demonstrated that sandy soils stabilised with 40% Class F fly ash and lime exhibited increased unconfined compressive strength (UCS) from 300 kPa to 400 kPa over curing time.
However, this also results in decreased plasticity.
Cristelo et al.
found that Class F fly ash activated with sodium silicate and sodium hydroxide produced lower UCS values than cement-based mixtures, requiring prolonged curing times .
Ae365 day.
to achieve significant strength gains .
0Ae43500 kPa at 40% fly as.
Class C fly ash contains a higher CaO content .
Ae20%) and possesses pozzolanic and cementitious properties, enabling self-cementation (Rahmayanti et al.
, 2.
Simatupang et al.
investigated the mechanical behaviour of Pohara sand stabilised with 5Ae30% Class C fly ash under relative density of 50% and curing periods of 7Ae56 days across various saturation levels.
Their findings indicated increased UCS and shear strength with increasing fly ash content and decreasing fly ash The highest UCS and shear strength values under Sr = 100% were obtained with 30% fly ash:
95 kPa and 326.
56 kPa, respectively, attributed to the binding effect of fly ash particles forming larger, more cohesive agglomerates.
Subsequent work by Simatupang et al.
further confirmed the tensile strength enhancement in sandy soils stabilized with 5Ae20% Class C fly ash at varying saturation levels.
The highest splitting tensile strength .
was recorded for 20% fly ash under 30% saturation after 28 days of curing.
The strength gain was attributed to the pozzolanic activity, cementation, and Vol.
12 No.
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hardening, which were more effective at lower saturation.
These findings confirm the superior performance of Class C over Class F fly ash.
Yazici and Unsever .
reported that stabilizing fine sand (Dr 80%) with 3% cement and 5Ae25% Class C fly ash over 7Ae28 days resulted in the maximum friction angle increase at 5Ae10% fly ash .
53%, from 47.
51A).
Similarly.
Febrian et al.
found that 10Ae15% Class C fly ash with 7-day curing enhanced the shear strength by 22.
9% .
53 to 11.
71 kP.
, whereas increasing the dosage to 20% produced only marginal improvement.
Turan et al.
also observed that 10Ae15% fly ash with 7-day curing yielded the highest UCS increase of 30.
56% .
6 kPa 4 kP.
SEM observations by Dissanayake et al.
indicated that fly ash contents above 16% resulted in an abundance of unreacted particles, thereby limiting effective cementitious bonding.
Although a higher fly ash content generally increases strength, the rate of improvement diminishes beyond the optimum dosage owing to reduced reaction efficiency and possible matrix disruption.
Therefore, 5%, 10%, and 15% Class C fly ashes were selected as the effective stabilization ranges in this study.
In addition to its technical effectiveness, fly ash stabilization offers economic advantages over conventional liquefaction mitigation methods, such as stone columns and deep soil mixing.
As an industrial byproduct, fly ash is generally more cost-effective because of its lower material costs and reduced construction complexity, while simultaneously contributing to waste utilization and environmental sustainability (Gupta et al.
, 2024.
Karthikeyan and George, 2.
While previous studies have confirmed the beneficial effects of Class C fly ash in improving the strength of sandy soils, most have been restricted to partially saturated conditions and have not simultaneously examined the influence of relative density and fly ash dosage under full saturation.
Under Sr = 100%, sandy soils exhibit very low undrained shear strength and are highly susceptible to flow failure (Zeybek and Madabhushi.
Achieving fully saturated conditions in laboratory settings poses challenges due to the potential presence of occluded air within the pore spaces (Sakaguchi et al.
, 2.
To mitigate this issue, a controlled addition of water based on the calculated pore volume (Vv ), along with sealed curing and stringent control of dry unit weight, was employed to ensure consistent relative density throughout specimen preparation and This study aims to evaluate the effectiveness of Type C fly ash in stabilizing fully saturated sandy soils (Sr = 100%) under three relative densities .
%, 70%, and 90%) using fly ash contents of 5Ae15%, with strength behaviour assessed through unconfined compressive strength (UCS) testing.
The findings are expected to Vol.
12 No.
3 (September 2.
provide new insights into the interaction between saturation, fly ash dosage, and relative density, offering a scientific basis for the application of fly ash as a practical liquefaction mitigation strategy.
2 METHODS
1 Material The soil sample used in this study was classified as Bangka sand.
The specific gravity (Gs ) of the sample 53, which fell within the standard range of 2.
4Ae 7, as specified by ASTM C 128-07a, indicating that the material was consistent with typical sand properties (American Society for Testing and Materials, 2.
The minimum void ratio .
min ) reflects the densest packing condition of sand .
41Ae1.
, whereas the maximum void ratio .
max ) represents its loosest state .
6Ae .
(Briaud, 2.
Laboratory testing yielded a minimum void ratio .
min ) of 0.
46 and a maximum void ratio .
max ) of 0.
The particle size distribution curve (Figure .
produced characteristic diameters of D10 = 0.
24 mm.
D30 = 0.
mm, and D60 = 1.
06 mm.
The calculated uniformity and gradation coefficients were Cu = 4.
42 and Cc = 01, respectively, classifying the soil as poorly graded sand (SP) under the USCS (ASTM D2487-.
(American Society for Testing and Materials, 2.
Poorly graded sand is highly susceptible to liquefaction under increased pore-water pressure.
Plotting the grading curve on the Tsuchida liquefaction potential chart confirmed its location within the liquefaction susceptibility zone (Koester and Tsuchida, 1.
According to Koester and Tsuchida .
, soil is considered to have liquefaction potential if D10 falls within the range of 0.
01 mm < D10 < 1.
00 mm, and high liquefaction potential if 0.
04 mm < D10 < 0.
30 mm.
The D10 value obtained in this study was 0.
24 mm, indicating a high liquefaction potential.
This relatively low D10 value is associated with an increased presence of fine Figure 1.
Liquefaction potential curve based on soil grain Journal of the Civil Engineering Forum particles, which reduces the coefficient of permeability of the soil and promotes the buildup of excess pore water pressure under dynamic loading conditions (Lestari, 2 Experiment Design Three parameters were varied in this study: relative density, fly ash content, and curing time.
The relative densities .
%, 70%, and 90%) represented loose, medium, and dense sand conditions, respectively (Frempong and Shukla, 2.
The target relative densities (Dr ) were achieved by controlling the dry unit weight .
) of each specimen.
The maximum and minimum void ratios .
max and emin ) were determined, and the corresponding target dry unit weights .
) were calculated using the standard relative density equation.
The required mass of dry sand was then placed into the mold in layers and compacted to the predetermined height to ensure that the target dry unit weight was Fly ash contents .
%, 10%, and 15%) were selected based on previous studies showing significant strength improvement within this range (Febrian et al.
, 2024.
Turan et al.
, 2.
The fly ash dosage was controlled by determining the specific gravity and void ratios of each mixture to calculate its actual void ratio.
From the mold volume (Vmould ) and the volume of solids (Vs ), the dry weight of solids (Ws ) and pore volume (Vv ) were calculated.
The dry weight of solids (Ws ) represented the combined proportion of sand and fly ash, with 5%, 10%, and 15% fly ash measured relative to the dry weight of sand and homogenized thoroughly before compaction.
Curing durations of 7 and 14 days were selected because the peak early age strength of Class C fly ash-stabilized soils typically occurs within this period (Misra, 1.
Turan et al.
reported that Class C fly ash exhibits greater strength enhancement than Class F fly ash due to its higher CaO content, which promotes both hydraulic and pozzolanic reactions.
The elevated calcium content promotes rapid formation of cementitious products, particularly calcium silicate hydrate (CAeSAeH) and calcium aluminate hydrate (CAeAAeH) (Dissanayake et al.
, 2017.
Yazici and Unsever, 2.
UCS testing was selected because it effectively evaluates the bonding and cementation effects of calciumrich fly ash in sandy soils (Kolias et al.
, 2.
Previous research has indicated that a UCS value of approximately 98 kPa may represent an indicative lower-bound strength associated with reduced liquefaction susceptibility in sand (Zen et al.
, 1.
Therefore, the UCS values obtained at 7 and 14 days were compared with this reference level to assess the effectiveness of strength development over the curing time.
Journal of the Civil Engineering Forum Vol.
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Table 1.
Variations of test samples Test
Samples A01
A02
A03
B01
B02
B03
C01
C02
C03
Concentration Dr (%) Fly ash (%) Treatment Sr = 100% Curing 7 and 14 days Full saturation (Sr = 100%) was achieved by filling the calculated pore volume (Vv ) with water.
The verification and preservation of Sr = 100% were ensured by lining each mold with plastic and storing it in a sealed container or desiccator during curing to maintain 100% humidity and to prevent moisture loss.
Three specimens were prepared for each parameter combination, yielding 27 samples per curing period, for a total of 54 samples.
Each specimen was tested, and the average UCS value was used for the analysis, whereas the standard deviation assessed the data variability.
For microstructural investigation, selected specimens were oven-dried before SEM analysis to eliminate free moisture and preserve the internal microstructure for accurate imaging.
The detailed combinations of parameters and corresponding sample quantities are listed in Table 3 Specimen Preparation The sandAefly ash mixture samples were prepared using cylindrical PVC moulds with a height of 10 cm and a diameter of 5 cm, following the standard procedure for unconfined compressive strength (UCS) testing of sandy soils.
Before sample placement, the moulds were positioned vertically and lined with plastic wrap.
This lining facilitated the demolding process after compaction and preserved the cylindrical shape of the specimen, thereby preventing surface damage during the The test specimens, consisting of sand mixed with varying percentages of fly ash under different relative density (Dr ) conditions, were compacted into moulds using the layer compaction method.
The specimens were compacted into five equal layers to achieve target relative densities (Dr ) of 50%, 70%, and 90%, with the compaction effort adjusted according to the procedure reported by Simatupang et al.
Each layer was subjected to 15, 25, and 40 uniformly distributed blows to attain loose (Dr 50%), medium dense (Dr 70%), and dense (Dr 90%) conditions, respectively.
Compaction was performed manually using a laboratory rubber mallet, which was raised approximately 5 cm above the soil surface prior to each blow.
Figure 2.
Schematic of test sample preparation.
During this process, the interaction between sand and Class C fly ash was critical to the integrity of the The high CaO content triggered hydration, forming CAeSAeH and CAeAAeH compounds that bonded, coated, and bridged particles, filled voids, and enhanced interparticle cementation, allowing the mixture to maintain its cylindrical shape after demoulding (Kolias et al.
, 2005.
Siddique, 2004.
Transportation Research Board, 1.
The samples were subsequently cured for 7 and 14 days after compaction.
UCS testing was conducted to evaluate the influence of relative density on the mechanical behavior of fly ash-stabilized A schematic illustration of the sample preparation procedure using cylindrical moulds is presented in Figure 2.
4 Test Method The unconfined compressive strength (UCS) test was conducted using SNI 3638:2012 on the Method of Unconfined Compression Test for Cohesive Soils (National Standardization Agency of Indonesia, 2.
Although the Unconfined Compressive Strength (UCS) test does not directly capture cyclic behavior, increases in UCS enhance undrained shear strength and stiffness, thereby indirectly improving cyclic resistance and indicating a reduced susceptibility to liquefaction (Park et al.
, 2.
After compaction and curing for 7 or 14 days in a sealed container, the specimens were carefully demoulded from the PVC moulds by gradually pulling the plastic lining upward in the axial direction to avoid end damage.
The height and diameter of each specimen were measured using a vernier caliper with an accuracy of A0.
1 mm.
The test was performed by placing the cylindrical specimen on the lower platen of the compression testing The upper platen was then adjusted to contact the top surface of the specimen.
Before loading, the deformation indicator was set to zero to ensure accurate displacement measurements.
The load was applied vertically from below, activating both the load and displacement gauges, thereby allowing displace- Vol.
12 No.
3 (September 2.
ment data to be acquired throughout the test.
Loading was continued until the initial signs of failure were observed, such as the appearance of cracks or a significant drop in the peak load.
This procedure aimed to capture the maximum axial load that the specimen could withstand without complete structural failure.
A schematic of the UCS testing setup is shown in Figure 3.
Figure 3.
Schematic of uniaxial compressive strength testing with UCS apparatus.
Journal of the Civil Engineering Forum Table 2.
Properties of test sample variations FA (%) 55, respectively.
Theoretical specific gravities, calculated using a weighted average of sand and fly ash contents, were 2.
55, 2.
56, and 2.
58, respectively.
This testing followed SNI 1964:2008 on the Determination of Soil Specific Gravity (National Standardization Agency of Indonesia, 2.
, with kerosene used as the medium instead of water because fly ash can react with water, as also reported by Annisa and Widayanti .
The specific gravities obtained from the tests were higher than the theoretical values obtained from the literature.
This discrepancy is attributed to the use of kerosene in the sandAefly ash mixtures, which has a lower density and different adhesion properties than water (Andariana et al.
, 2.
The minimum .
min ) and maximum .
max ) void ratios were determined for each mixture and used to calculate the actual void ratio at relative densities (Dr ) of 50% .
50 ), 70% .
70 ), and 90% .
90 ).
The results are summarized in Table 2.
Scanning Electron Microscopy (SEM) was employed to characterize the microstructural morphology by examining the interaction of a high-energy electron beam with the sample surface to produce high-resolution images (Temiz, 2.
This technique enables the visualization of hydration products, such as calcium-silicatehydrate (CAeSAeH) and calcium-aluminate-hydrate (CAe AAeH) phases generated from pozzolanic reactions between fly ash and cementitious constituents, as well as textural features, including sand particles, fly ash particles, pore networks, and microcracks in the matrix (ChyliEski, 2.
SEM observations were conducted at a magnification of 1000y and an accelerating voltage of 15 kV to ensure an appropriate resolution for microstructural assessment.
As the fly ash content increased, the actual void ratio at each relative density (Dr ) level also tended to increase.
This is due to the characteristics of fly ash, which contains larger air voids than typical soils.
According to Upadhyay et al.
, fly ash exhibits air voids ranging from 5% to 15%, whereas soils generally have approximately 1% to 5% under maximum density conditions.
Furthermore, some fly ash particles are hollow or internally porous, thereby contributing additional voids to the overall soilAefly ash system (Yadav et al.
, 2.
3 RESULTS
2 Effect of Fly Ash Concentration 1 Material Characteristics The unconfined compressive strength (UCS) test results for each variation in fly ash content and relative density are illustrated in Figure 4, with detailed numerical values presented in Table 3.
The characterization of the sandAefly ash mixtures was conducted following standard procedures used for natural soils.
The specific gravity of the fly ash, the specific gravity of sandAefly ash mixtures, and the void ratio of each mix were determined.
These parameters are essential for determining the appropriate proportions of sand, fly ash, and water during sample preparation.
The fly ash used in this study was classified as Class C, with a measured specific gravity (Gs ) of 2.
This value falls within the generally reported range of 15Ae2.
80 (Mehta, 2.
, while broader ranges of 1.
6Ae 1 have also been reported (Ulum, 2.
The specific gravities of the sandAefly ash mixtures with 5%, 10%, and 15% fly ash were measured as 3.
02, 3.
At 7-day curing, the highest UCS values were recorded at Dr 90% for 5%, 10%, and 15% fly ash contents, with corresponding values of 65.
53 kPa, 206.
67 kPa, and 22 kPa, respectively.
Similarly, at 14-day curing, the maximum UCS values were also observed at Dr 90% for 5%, 10%, and 15% fly ash, with values of 69.
27 kPa, 97 kPa, and 438.
57 kPa, respectively.
This increasing trend was also observed for Dr 70% and Dr 50%, although the latter consistently exhibited the lowest UCS values.
Notably, adding fly ash from 5% to 10% resulted in a more substantial improvement in the UCS than the increase from 10% to 15% across all relative Journal of the Civil Engineering Forum Vol.
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Figure 4.
Unconfined compressive strength (UCS) in relation to varying fly ash content.
Figure 5.
Unconfined compressive strength (UCS) in relation to varying relative density.
Table 3.
UCS values of sand samples with varying fly ash content and relative density The most notable gain from 50% to 70% was observed at 10% fly ash, with UCS values increasing by 73 times at 7-day curing .
23 kPa to 189.
37 times at 14-day curing .
07 kPa 58 kP.
, surpassing the improvement observed at other fly ash levels.
Fly Ash (%) Dr (%) UCS .
UCS .
density levels.
The most pronounced strength gain due to this 5Ae10% increment occurred at Dr 70%, where UCS values increased by approximately 4.
4 times at 7day curing .
36 kPa to 189.
25 kP.
5 times at 14-day curing .
82 kPa to 227.
58 kP.
, compared to other Dr .
Moreover, as shown in Figure 5, the curing duration .
14 day.
significantly affected the UCS improvement.
At Dr 50%, the highest UCS gain from 7 to 14 days was observed at 5% fly ash, with a twofold increase, followed by 10% and 15% fly ash with increases of 1.
39 and 1.
29 times, respectively.
At Dr 70%, the most significant gain occurred at 10% fly ash .
20 time.
, followed by 5% .
17 time.
and 15% .
For Dr 90%, the highest increase was also recorded at 10% fly ash .
49 time.
, followed by 5% .
06 time.
and 15% .
03 time.
4 DISCUSSION
3 Effect of Relative Density 1 Material Characteristics The unconfined compressive strength (UCS) values increased with increasing relative density across all combinations of fly ash content and curing duration.
The UCS values for each relative density level are shown in Figure 5.
The specific gravity of the fly ash used in this study was relatively high .
A higher specific gravity is generally associated with a more complex physical texture, which is attributed to variations in coal sources and combustion processes (Gunawan and Nono, 2.
The specific gravity of the sandAefly ash mixtures exhibited an increasing trend with increasing fly ash content.
In practice, the measured specific gravity values were consistently higher than those obtained from theoretical calculations.
As shown in Table 3, an increase in the Dr corresponded to a decrease in the void ratio.
This reduction in porosity reflects an increase in soil compactness, which reduces void spaces within the soil matrix (Pangaribuan et al.
, 2.
Moreover, the actual void ratio at each Dr tended to increase with higher fly ash content.
This can be attributed to the physical characteristics of fly ash, which typically contains more internal air voids than natural soil.
A previous study At 7-day curing, the highest strength was obtained with 15% fly ash content at Dr of 50%, 70%, and 90%, with corresponding UCS values of 190.
23 kPa, 377.
03 kPa, 22 kPa, respectively.
A similar trend was observed for the 14-day curing period, with UCS values 83 kPa, 415.
41 kPa, and 438.
57 kPa for Dr 50%, 70%, and 90%, respectively.
Although lower strength values were recorded at 10% and 5% fly ash, the same increasing trend with the Dr was maintained.
As presented in Figure 5, an increase in relative density from 50% to 70% led to a more substantial improvement in UCS than an increase from 70% to 90% across all fly ash Vol.
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by Upadhyay et al.
suggested that fly ash particles may exhibit internal voids ranging from 5Ae15%, whereas soils generally have air voids from 1Ae5% under maximum compaction conditions.
2 Effect of Fly Ash Concentration The strength improvement of sandy soil in this study is attributed to the characteristics of Class C fly ash, which contains a high percentage of calcium oxide CaO along with reactive silicate and aluminate compounds, such as SiO2 .
Al2 O3 , and Fe2 O3 .
These compounds react with water to form calcium hydroxide (Ca(OH)2 ), which dissociates in water, releasing OH- ions.
This creates an alkaline environment that facilitates the dissolution of SiO2 and Al2 O3 , leading to the formation of calcium silicate hydrate (CAeSAeH) and calcium aluminate hydrate (CAeAAeH) compounds (Kolias et al.
, 2005.
Transportation Research Board, 1.
These hydration products fill soil pores, improve particle gradation, reduce plasticity, and effectively bind soil particles (Siddique, 2.
These bonding agents strengthen the interparticle contact between sand grains, enhancing soil cohesion and internal friction (Pandian, 2.
In addition, fly ash contributes to partial water absorption within the mixture, coating and binding the soil As a result, even under fully saturated conditions (Sr = 100%), the mixture does not experience excess free water that would otherwise weaken the soil instead, it undergoes densification, enhancing its stability.
The increase in fly ash content from 5% to 10% promoted more effective pozzolanic reactions, particularly the formation of calcium silicate hydrate (CAeSAe H), which enhanced soil cohesion and shear strength, with peak efficiency observed at 10% fly ash (Golewski.
This result suggests that the optimum improvement in unconfined compressive strength (UCS) was achieved at 10% fly ash content.
Beyond this optimum content .
% fly as.
, the excess fly ash particles no longer act effectively as pore fillers but behave as inert particles.
Dissanayake et al.
stated that surplus fly ash can become passive and fail to participate in pozzolanic reactions.
Additionally, excessive fly ash increases the total pore volume, disrupting the formation of adequate particle bonding (Osok et al.
, 2.
These findings indicate that once the optimum fly ash content is exceeded, additional fly ash no longer contributes actively to pozzolanic product formation, particularly the generation of calcium silicate hydrate (CAe SAeH).
The addition of lime or cement to fly ash may activate these otherwise passive particles by providing additional Ca2 and alkalinity, thereby potentially promoting pozzolanic reactions and improving the mechanical strength of the soil (Di Sante et al.
, 2.
Journal of the Civil Engineering Forum 3 Effect of Relative Density Increasing the relative density of sandy soil results in a tighter particle arrangement and stronger interlocking among grains Igwe .
According to Mahvash et al.
, a denser particle distribution enhances the contact between fly ash and sand particles, allowing more effective reactions and the formation of calcium silicate hydrate (CAeSAeH), thereby improving the interparticle interaction and pozzolanic bonding that reinforces the soil structure.
A higher density or lower porosity also increases the shear resistance between particles, enabling the soil to withstand greater loads.
This is attributed to reduced voids and enhanced particle contact, which strengthen the soil matrix (Mawlood et al.
Ruiz-Chyvez et al.
, 2022.
Santana et al.
, 2.
The more significant increase in UCS between low (Dr 50%) and medium (Dr 70%) densities compared to the increase from medium (Dr 70%) to high (Dr 90%) densities indicated that porosity reduction was more prominent at lower density ranges.
At a Dr of 70%, sand particles formed a relatively stable structure with limited mobility and enhanced interlocking (Zhang et al.
, 2.
Consequently, increasing compaction to Dr 90% resulted in a shorter densification process and a faster stress peak, limiting the strength improvement due to the density increase.
Therefore, the UCS gain from Dr 70% to 90% was less pronounced than that from Dr 50% to 70%.
These findings indicate that the most significant rate of strength improvement occurred at a medium relative density, particularly at a Dr of 70%.
This finding aligns with that of Herwandi et al.
who reported a more substantial decrease in the sand permeability coefficient at low to medium relative densities.
Interestingly, sand stabilized with 15% fly ash at a Dr of 50% produced a higher UCS than sand with 10% fly ash at a Dr of 70% or 5% fly ash at a Dr of 90%, indicating that increasing fly ash content had a more pronounced effect on strength development than increasing relative density alone.
The addition of fly ash is more effective in enhancing the strength of cohesionless sandy soils because fly ash contributes to the chemical bonding among particles.
In contrast, increasing the relative density through compaction .
, tampin.
may damage the existing soil structure, resulting in strength gains that are not proportional to those achieved by fly ash addition.
Structural damage from compaction can produce fine particles that fill intergranular voids and behave like silt or clay, reducing cohesion, decreasing permeability, and creating potentially weak zones within the soil mass (Bagheri et al.
, 2.
Scanning Electron Microscopy (SEM) is essential for examining the microstructure of fly ash-stabilized sand, including pore distribution, particle contacts, and reaction products.
This analysis elucidates the mechanisms by which fly ash contributes more effectively to strength devel- Journal of the Civil Engineering Forum opment than compaction alone.
Vol.
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ash in a loosely compacted matrix may disrupt these bonds and limit early-age strength development, as reported by Noaman et al.
At Dr 70%, the highest UCS gain over the 7Ae14 days curing period occurred at 10% fly ash .
20 time.
, followed by 5% .
17 time.
and 15% .
10 time.
At Dr 90%, the highest UCS increase was also recorded at 10% fly ash .
49 time.
, followed by 5% .
06 time.
and 15% .
03 time.
These findings confirm that the relative density and curing duration significantly influence the strength performance of fly ash-stabilized sand, with 10% fly ash content emerging as the most effective level for strength optimization.
Figure 6.
SEM analysis of microstructural characteristics of fly ash-stabilized sand SEM observations at 1000y magnification of the specimen containing 15% fly ash at a Dr of 50% indicate that strength development is primarily governed by chemical bonding rather than mechanical densification.
A continuous cementitious matrix formed by the reacted fly ash and pozzolanic products coats the sand grains and bridges the adjacent particles, while discrete spherical particles remain as unreacted fly ash microspheres (Vennapusa and Eluru, 2.
The presence of this chemically bonded network at a relatively low relative density demonstrates that the cohesion enhancement is not solely dependent on densification (Mohsen et al.
, 2.
Localised pores and microcracks further suggest that excessive compaction may disrupt the cement matrix and reduce effective bonding (Baloochi.
Strength improvement during curing occurred because fly ash absorbed water and initiated pozzolanic reactions.
These reactions produced cementitious compounds, such as calcium silicate hydrate (CAeSAeH) and calcium aluminate hydrate (CAeAAeH), which filled the voids and strengthened the interparticle bonds.
Over time, this cementation process enhanced cohesion and soil density, significantly increasing the UCS (Marsindi et al.
, 2.
At Dr 50%, the UCS increase from 7 to 14 days of curing was most significant at 5% fly ash content, with a two-fold increase, followed by 10% and 15% fly ash contents with 1.
39 and 1.
29 times increases, respectively.
This suggests that 5% fly ash by sand weight was the most efficient dosage at low densities over the curing period.
A fly ash content exceeding 5% tended to reduce the bonding efficiency, as its finer particles and larger air voids contributed more to porosity in a loosely compacted matrix (Upadhyay et al.
, 2.
This trend suggests that, in low-density soils, curing allows pozzolanic reactions to develop and strengthen interparticle bonding sufficiently.
In contrast, excessive fly Increasing the relative density decreases the void ratio, thereby increasing the number of sand particles per unit volume (Darwis, 2.
This condition necessitates a higher fly ash content to form sufficient bonding between particles at a higher Dr .
Additionally, at a high Dr (Ou 70%), more effective densification helps minimize air voids caused by fine fly ash particles, resulting in a denser microstructure (Turan et al.
, 2.
Overall, 15% fly ash yielded the lowest UCS increase during the 7Ae14 day curing period at all Dr .
This suggests that excessive fly ash content leads to passive particles that do not significantly contribute to pozzolanic compound formation.
The selection of the optimal relative density and curing duration .
Ae14 day.
for fly ashAestabilized sand should consider strength, constructability, material efficiency, and cost-effectiveness.
The highest relative UCS gain during curing was observed at Dr 50% and 5% fly ash, with a two-fold increase.
However, the resulting UCS values were relatively low, from 18.
26 kPa .
to 53 kPa .
Zen et al.
noted that a UCS value exceeding 98 kPa is considered a critical threshold for liquefaction resistance in sandy soils.
Based on the test results.
Dr 50% sand required 15% fly ash to reach UCS values between 190.
23Ae244.
83 kPa, exceeding the liquefaction safety threshold.
Meanwhile.
Dr 70% and 90% sand only required a minimum of 10% fly ash to achieve UCS values between 189.
26Ae438.
kPa.
Nevertheless, achieving higher Dr s requires more intensive compaction efforts directly related to the equipment type and associated costs (Arumningsih.
From a practical standpoint.
Dr 50% with 15% fly ash may be viable in the field owing to the minimal compaction effort and UCS values above the liquefaction However, in actual field conditions, the relative density is likely to increase naturally owing to construction activities such as worker foot traffic, heavy equipment movement, or fill placement, rendering the initial Dr 50% condition less representative (Keramatikerman et al.
, 2.
From a fly ash utilization efficiency perspective, a Dr of 70% with 10% fly ash Vol.
12 No.
3 (September 2.
is considered optimal, as it yields UCS values above the liquefaction resistance threshold while using less stabilizing material and requiring compaction levels that are still practical in field conditions.
Therefore, the most realistic and efficient combination is a Dr of 70% with 10% fly ash.
A cost analysis, including material, labour, and compaction equipment, is essential to determine the most economically viable and technically effective stabilisation strategy.
Moreover, based on Government Regulation (PP) No.
22/2021 on the Implementation of Environmental Protection and Management, fly ash is no longer classified as hazardous waste .
ocally known as Bahan Berbahaya dan Beracun or B.
However, its application still requires careful management owing to its potential to accumulate heavy metals in the environment (Government of the Republic of Indonesia, 2.
5 CONCLUSION
This study investigated the influence of Class C fly ash content, relative density (Dr ), and curing time on the unconfined compressive strength (UCS) of sandy soil as part of efforts to improve its resistance to liquefaction.
The hypothesis that both fly ash addition and compaction enhance soil strength was confirmed, with fly ash contributing more effectively than density increase alone, as evidenced by the UCS results and the SEM observations.
The UCS increased with higher fly ash content and relative density, reaching maximum values of 22 kPa .
57 kPa .
at 15% fly ash and Dr 90%.
The most significant improvement was observed when the fly ash content increased from 5% to 10% at a Dr of 70%.
The most optimal increase in UCS occurred when Dr increased from 50% to 70%, particularly at a fly ash content of 10%.
The optimal condition, balancing strength, material efficiency, and constructability, was achieved at 10% fly ash and Dr 70%, surpassing the liquefaction resistance threshold.
From a practical standpoint, this condition represents a level of field compaction effort that is more readily achievable than the high degree of densification required by conventional liquefaction mitigation methods.
Compared with techniques such as rapid impact compaction or stone columns, fly ashAebased stabilization provides a simpler alternative and offers potential cost advantages over cement- or lime-based stabilization.
These findings extend the current understanding of fly ash utilization for sandy soil stabilization, particularly under varying density conditions.
However, the study was limited to laboratory-scale UCS testing under controlled moisture and curing conditions.
therefore, field-scale validation is recommended for future research to confirm the long-term performance of the fly ash-stabilized sand.
A comparative cost assessment considering materials, labor, equipment, and construc- Journal of the Civil Engineering Forum tion time is recommended to evaluate the practical feasibility of fly ash stabilization relative to alternative ground improvement methods for Indonesian projects.
DISCLAIMER
The authors declare no conflict of interest.
REFERENCES