The Application of Fine Sand in Subgrades: A Review

Abstract

The subgrade serves as the foundation of road construction, typically involving a significant amount of earthwork during its establishment. However, in coastal and desert areas, soil sources are often scarce. Local soil extraction significantly damages cultivated land, impacting the local ecological environment. Transporting soil over long distances inevitably raises construction costs. Fortunately, these regions often feature abundant fine sand distribution, presenting an opportunity to utilize it as subgrade filler in coastal regions. This review comprehensively introduces the properties of fine sand as a raw material, its engineering applications, and the associated construction technologies. It emphatically discusses the road use characteristics and treatment technology of fine sand filler and puts forward a prospect combining the characteristics and development trends of fine sand so as to provide a new perspective and basic material for the application of fine sand in the subgrade. To foster the adoption of fine sand in subgrade construction, it is recommended to advance research on the evaluation and treatment of fine sand foundations, analyze its suitability and structural behavior as a filler, and refine construction methodologies and quality control measures specific to fine sand subgrades.

1. Introduction

The utilization of high-quality soil as a subgrade filler has long been a conventional practice in road construction [1]. However, in environmentally sensitive areas such as riverside, coastal, and desert regions, acquiring high-quality soil for subgrade filling presents challenges, often leading to the depletion of arable land [2]. In response, the adoption of on-site river sand, sea sand, or aeolian sand has emerged as a primary alternative, aligning with principles of environmentally friendly and sustainable engineering [3].

Fine sand filler, distinct from traditional high-quality subgrade fillers, presents unique characteristics in terms of silt content, compaction behavior, California Bearing Ratio (CBR) strength, and gradation features [4]. Unlike conventional soil-based fillers, fine sand often exhibits a singular particle structure with irregular gradation, posing challenges to conventional compaction practices. Lu et al. [5] examined the application of cohesive soil wrapping on fine sand subgrade slopes, identifying optimal compaction methods and quality evaluation indicators for achieving high compaction quality in subgrade construction. Consequently, rigorous experimentation is imperative to determine optimal gradation standards tailored to fine sand specifications [6].

The compaction curve of fine sand often shows characteristics of “double peaks” or “multiple peaks”, lacking a definitive optimal compaction moisture content. Therefore, selecting appropriate compaction control standards necessitates a careful consideration of site-specific conditions and available roller machinery. Feng et al. [7] investigated the water stability and improvement measures for sandy lean clay used in highway subgrades, focusing on its basic properties, water effects, and compaction quality control. These findings revealed that while the CBR strength of fine sand can potentially fulfill subgrade design requirements, elevated silt content compromises its strength, underscoring the importance of defining maximum permissible silt content through urgent research endeavors [8]. This field of research has garnered considerable attention, with Shubber and Saad’s study [9] evaluates and compares the effectiveness of various mechanical and chemical stabilizing mechanisms in enhancing the strength capacity of different subgrade soils for pavement construction, using CBR tests and pavement thickness reduction as the metrics. Furthermore, Elkafoury and Azzam’s investigation [10] explored the stabilization of Used Cooking Oil (UCO)-contaminated fine sand soil using xanthan gum, evaluating its effectiveness in enhancing soil’s CBR and reducing pavement thickness requirements, while also examining the durability under wet–dry cycles.

This comprehensive review aims to summarize the current research progress on fine sand’s performance as a subgrade filler. It encompasses the definition and classification of fine sand, a detailed discussion of its physical and mechanical properties, and an exploration of its dynamic characteristics. This review further explores various treatment methodologies for fine sand subgrade, including cement and fiber treatments as well as innovative materials. Ultimately, the application of fine sand in subgrade engineering is thoroughly investigated, with a specific focus on windblown sand subgrades, sand-filled subgrades, and sand blowing subgrades.

This review seeks to contribute to the ongoing discourse on sustainable and environmentally responsible road construction practices, highlighting the potential of fine sand as a viable alternative to traditional soil-based subgrade fillers. By understanding fine sand’s properties and treatment methods, engineers can develop effective strategies for its use, ensuring road stability and durability while minimizing environmental impact.

2. Materials

2.1. Definition and Classification of Fine Sand

Fine sand filler is typically regarded as a material with undesirable engineering properties in the traditional sense, and it has the potential for vibration liquefaction, making its practical application relatively rare.

From the perspective of the physical and mechanical properties of sand materials, sand demonstrates non-plasticity, high permeability, easy looseness, and difficulty in compaction, requiring methods such as vibration or water saturation for compaction. It has low cohesive strength, poor gradation, and a single-particle structure. It remains loose when its moisture content is low and becomes plastic when its moisture content is high. Under vertical stress, it is not prone to forming a cohesive mass and is susceptible to significant lateral displacement. Therefore, it is essential to prioritize the stability and protection of subgrade slopes to prevent instability and loss of sand particles. It is necessary to conduct systematic research on fine sand subgrades.

2.2. Physical and Mechanical Properties of Fine Sand Subgrades

The engineering application of sand-filled subgrades has predominantly focused on gravel, river sand, and aeolian sand, while fewer studies focus on the practical application of fine sand in existing subgrades. A notable instance is the utilization of sand dunes in the subgrade of Interstate 1–94 in northern Indiana [6]. The soil in this case is classified as poorly graded sand (SP) in the Unified Soil Classification System and as A-3 in the AASHTO classification, indicating poorly graded gravel or sand, or sandy gravel with little or no fines. Kim [11] et al. conducted non-destructive testing of the existing sand soil subgrade using a falling weight deflectometer combined with core drilling tests to determine the particle size distribution of the sand soil and the thickness of the asphalt pavement structure. Additionally, the research analyzed the nonlinear characteristics of the sand soil subgrade’s resilient modulus and its relationship with the particle size distribution of the sand soil. The obtained sand soil particle size distribution curve is shown in Figure 1.

Sand from the Osório region in southern Brazil is classified as non-plastic homogeneous fine sand, characterized by spherical particle shapes (Figure 2) with a specific gravity of 2.63. Mineralogical analysis indicates that the predominant mineral in the sand grains is quartz. The average effective diameter D50 of the fine sand is 0.16 mm, with a uniformity coefficient of 1.9 and a curvature coefficient of 1.2. The minimum void ratio and maximum void ratio are 0.6 and 0.9, respectively [12].

Guo et al. [13] conducted sieve analysis experiments on six representative sand samples from Shanghai, China. The particle sizes of the sand samples were mostly in the range of 0.075~0.3 mm, with the coefficient of uniformity for each sand sample generally being less than 5. Although some differences were observed, fine sand samples exhibited a relatively uniform particle size distribution, indicating poorly graded sand.

2.3. Dynamic Characteristics of Fine Sand

Dynamic resilient modulus and permanent deformation are two main indicators for evaluating the dynamic performance of fine sand fillers, and the dynamic characteristics of fine sand under traffic loads are the primary factors affecting the performance of subgrade structures over their service life. Currently, there are several laboratory test methods for evaluating dynamic performance including dynamic modulus tests, static creep tests, and repeated load triaxial tests.

Laboratory tests were conducted on windblown sand (SP, A-3), which is used in the subgrade of Interstate 1–94 in northern Indiana, to simulate the effects of impact and vibratory compaction [6]. The assessment employed an MTS-810 soil testing system (Figure 2). A comprehensive test for granular soil encompasses six preparatory stages and twenty-seven distinct testing phases, each involving various combinations of confining stress and repeated deviator stress magnitudes. To administer the repeated loading, a haversine stress pulse was employed, characterized by a 0.1 s load duration within a 1 s cycle. During the resilient modulus assessment, it was noted that at a confining stress level of 6.9 kPa (equivalent to 1 psi), the cumulative deformation consistently surpassed the displacement gauge’s measurement capacity, which was set at 4 mm.

The resilient modulus of a granular material is generally approximated by Equation (1):

$$M_R=k_1{\theta }^{k_2}$$

(1)

where M_R is the resilient modulus, θ is the first invariant of stress (σ₁ + σ₂ + σ₃), and k₁ and k₂ are experimentally determined constants. This equation, supported by repeated load triaxial test data, summarizes the key resilient factors of granular materials: stress state, saturation, initial density, and gradation. Parameters k₁ and k₂ are plotted against relative compaction in Figure 3. It is observed that parameter k₁ increases almost linearly as relative compaction increases, while parameter k₂ is almost constant. Therefore, when the relative compaction degree is between 95% and 103%, the resilient modulus demonstrates a linear increase with dry density, and a regression equation for the resilient modulus was established using Equation (2) as follows:

$$M_R=\left(-\mathrm{20,163}+232.886{\mathrm{R}}_{\mathrm{C}}\right){\theta }^{0.595}$$

(2)

where R_C is relative compaction degree in %, the ratio of dry density is obtained from 5 min of vibration per layer to the dry density, and θ is the first invariant of stress in psi.

The results revealed that at a lower moisture content, the compaction curves for both impact and vibratory compaction were comparable. However, at higher moisture levels, vibratory compaction was found to produce a higher dry density. When comparing specimens compacted under the same moisture content and dry density, it was observed that those compacted using the vibratory method exhibited significantly less permanent deformation than those compacted by impact. Furthermore, the elastic modulus of the vibratory compacted specimens was approximately 40% higher, indicating that vibratory compaction is more effective in enhancing the resilient modulus response of sand dune sands. It is noteworthy that the compaction moisture content had a minimal impact on the resilient properties of the sand.

Kim [11] proposed a simple method for estimating the in situ resilient modulus of sandy subgrade soils while obtaining the particle size distribution of core samples and non-destructive testing at various locations within the subgrade soil. Non-destructive testing was conducted using a falling weight deflectometer at different load levels, and the subgrade resilient modulus was calculated based on the data collected from various pavement areas. When higher load levels are applied to the pavement, stress hardening effects result in higher subgrade moduli response for sandy subgrade soils. However, fine-grained or plastic subgrade soils may not exhibit this trend, as the modulus response is expected to decrease with increasing vertical deviatoric stress, as described by Equation (3) as follows:

$$E_{\mathrm{subgrade}}=19729+322{\mathrm{g}}_{\mathrm{a}}-1.31{\mathrm{g}}_{\mathrm{m}}-5022{\mathrm{H}}_{\mathrm{UAB}/\mathrm{HMA}}+0.183{\mathrm{W}}_{\mathrm{Load}}-157{\mathrm{g}}_{\mathrm{a}}\cdot {\mathrm{g}}_{\mathrm{n}}$$

(3)

where E_subgrade is the subgrade resilient modulus in psi, g_a, g_m, and g_n are parameters for fitting the gradation curve, H_UAB/HMA is the thickness ratio of the crushed stone base and asphalt concrete pavement, and W_Load is the load magnitude in lb.

Akili [14] conducted repeated load triaxial tests on uniformly compacted dry sand from the Arabian Gulf region, examining the relationship between permanent deformation of dry desert sand, stress conditions, and placement density. The authors also monitored the elastic strain occurring simultaneously with plastic strain and the elastic modulus. Furthermore, the constitutive relationships were established for utilization in conjunction with the “elastic layering” method to estimate the contribution of sand subgrades to permanent deformation in flexible pavement systems. The experiments showed that permanent axial strain is predominantly influenced by the magnitude of deviatoric stress, with elastic strain occurring simultaneously with permanent strain increasing with stress. A correlation between permanent strain and the number of load cycles was identified when the stress level was below 65% of the ultimate specimen strength, as described by Equation (4) as follows:

$${\epsilon }^p=A{N}^b$$

(4)

where ${\epsilon }^p$ is the permanent axial strain and N is the number of repeated loading cycles.

3. Treatment Technology for Sand-Filled Subgrades

3.1. Mechanical Analysis of Cement Treatment Sand

The treatment methods for sand materials mainly include cement treatment and fiber treatment [15,16,17,18,19]. In recent years, many researchers have made significant contributions to the understanding of cement treatment behavior [20,21,22,23,24]. Amhadi and Assaf’s research [25] proposes an innovative approach to improve the strength of desert sand for pavement design by adding manufactured sand, ordinary Portland cement, and fly ash as a binder, demonstrating optimal results with a specific mix ratio of CFA, NDS, OPC, and FA in terms of strength, compaction, and bearing capacity.

Tariq [26] conducted a comprehensive investigation into the strength and durability of cement-treated sand, with a particular emphasis on its response to stress. The study involved experimenting with various mixtures of cement, sand, and limestone powder, which were shaped into different forms to examine the material’s mechanical behavior under compression and tension.

Previous studies by Consoli [27], Schnaid [28], and Hamidi [29] have also delved into the investigation on cement-treated sands. It is widely acknowledged that the type of cement used is an important parameter in examining the stiffness of cement-treated soils. Additionally, it was observed that the rate at which tangent stiffness increases at the cementitious yield point varies depending on the amount of cement utilized. With an increase in cement content, the bonding and ultimate yield surfaces move upward. The tangent stiffness of cement-treated soil is consistently exceeding that of untreated soil, and this difference is more pronounced at lower confining stresses. However, it diminishes with increasing confining pressure. This trend is observed under both drained and undrained conditions, where the difference between drained and undrained tangent stiffness decreases with increasing confining pressure, while the tangent stiffness increases with increasing cement content.

The study examined strength characteristics were studied by artificially adding Portland cement to sandy soil. The mechanical behavior of the soil was analyzed through unconfined compression tests and drained triaxial compression tests, employing local strain measurements and scanning electron microscopy (SEM) testing. A series of SEM images of the samples indicated that specimens without cement addition or untreated specimens exhibited a more open and porous matrix, whereas those with cement addition displayed a more closed and dense structure.

3.2. Mechanisms and Effects of Fiber Treatment for Fine Sand Subgrade

In contrast to cement-treated sand, fiber treatment not only increases the peak shear strength of the sand but also exhibits characteristics of toughness. Fibers often undergo significant plastic tensile deformation before fracturing, even under isotropic compression [30]. From a strength perspective, different types of fibers have varying effects on subgrade soils. Palm fibers have higher friction coefficients and resilient moduli compared to geotextile fibers [31]. Figure 4 presents a mechanism to explain the fiber treatment process, where fibers undergo both stretching and fracturing under tensile stress, even when the total stress applied is isotropic [32]. The initial fiber shape is represented here as a straight line, and isotropic compression leads to relative movement between particles, generating tensile stress within the fibers. Additionally, there may be an additional mechanism of fiber fracture during testing, involving the squeezing and crushing of sand grains, resulting in the cutting of the fibers trapped between them. However, this is evidently not the primary mechanism, as otherwise the fibers would not elongate.

Maher [33] conducted laboratory triaxial compression tests to determine the static stress–strain response of discretely distributed randomly oriented fiber-reinforced sands and observed the influence of different fiber properties and soil properties on the soil behavior. Furthermore, a model was developed based on statistical theories of composite material strength to predict the contribution of fibers to strength under static loading. The results indicate that randomly distributed fibers significantly enhance the ultimate strength and stiffness of the sand soil, with strength and stiffness being functions of sand particle size and fiber properties. The sand–fiber composite materials display linear or bilinear flexural failure envelopes, with failure occurring at critical confining pressures. The magnitude of critical confining pressure decreases with increasing sand gradation, particle angularity, and fiber aspect ratio, and increases with increasing fiber modulus while being insensitive to changes in sand particle size and fiber content.

Tingle [34] proposed an investigation to validate the outcomes of extensive laboratory experiments assessing the impact of various variables on the performance of fiber-stabilized sand. Laboratory unconfined compression tests on sand specimens reinforced with randomly oriented discrete fibers, leading to the design of field test sections for validating their application in low-volume roads. The primary conclusion is that fiber-stabilized sands present a viable alternative to traditional materials for constructing temporary or low-volume roads.

For reinforced sand, conducting direct shear tests on dry sands reinforced with different fiber types yields the following observations: the shear strength of sand is directly proportional to the concentration or area ratio of fibers. The maximum influence is observed when the initial fiber orientation is at 60° relative to the shear plane. Similar results are obtained for reinforced sands tested under both loose and dense states [35,36]. Park [37] further utilized finite element analysis to investigate the effects of short fibers (60 mm) on the vertical and horizontal soil pressure, displacement, and settlement of sand-fine soil reinforced walls, comparing the results with measurements from full-scale tests. The findings demonstrate that using short-fiber-reinforced soil enhances the stability of the wall, reducing soil pressure and displacement. This effect is more pronounced when short-fiber soil is combined with geogrids.

Latha [38] conducted triaxial compression tests to examine the influence of different reinforcement configurations (Figure 5) on the strength of geosynthetic reinforced sand. The observations reveal that, among the planar and honeycomb reinforcement configurations, the honeycomb reinforcement is more effective in enhancing strength, considering the low tensile strength of joints. The stress–strain curves of geocells remain almost flat after reaching the peak value under all confining pressures, unlike other configurations where strength loss is observed after the peak. Compared to geotextiles, polyester geocells exhibit high efficiency in increasing sand strength due to the formation of indentations. However, the use of discrete fiber forms of steel reinforcement in the study performs worse than planar or honeycomb forms, which may be attributed to the reduction in overall confinement effect caused by small-sized fiber elements.

Ahmad [39] conducted tests on Oil Palm Empty Fruit Bunch (OPEFB) fibers coated with a polyacrylonitrile–butadiene–styrene thermoplastic material to determine the influence of coating on the reinforcement effect. The coated fibers increased the frictional contact between the fibers and the soil by increasing the surface area. Compared to unreinforced fine sand, under undrained loading conditions, the friction angle and cohesion of reinforced fine sand containing 0.5% coated fibers (30 mm long) increased by approximately 25% and 35%, respectively.

Diambra [40] performed conventional triaxial compression and tensile tests on polypropylene short-fiber-reinforced sand. The contribution of fibers to strength is significant during compression but limited during tension. The stiffness matrix of the sand soil is based on the Mohr–Coulomb model, and the constitutive model of the fiber-reinforced sand composite material was calibrated based on the results of drained triaxial compression and tensile tests.

3.3. Innovative Meterials for Sand Subgrade Treatment and Engineering Cases

Furthermore, scholars have proposed the utilization of materials such as shredded tires, sludge, and fly ash for the sand subgrade treatment [41,42,43,44]. This approach not only enhances the stability and performance of sand subgrade but also promotes waste utilization, aligning with the prospects of sustainable development.

Yoon [45] conducted inspections and monitoring on an experimental embankment constructed using a mixture of shredded tires and sand, with a weight ratio of 23/77 (volume ratio of 50/50). The results indicate that within one year of open traffic, the maximum settlement is only 12 mm. The maximum relative lateral displacement with respect to the embankment base was observed to be approximately 2 mm. No signs of internal heating were found, indicating no risk of spontaneous combustion. It is recommended to promote the use of mixtures of shredded tires and soil in embankment construction.

The practical engineering cases of fine sand subgrade improvement are shown in

Table 1. Studies on the improvement of fine sand subgrades.

Proposer	Treatment Methods	Main Results/Conclusions
Li Fen [46]	Cement-modified fine sand	Cement-modified fine sand as filler on subgrade surface, with optimal cement-mixing ratio
Li Shenghua [47]	P.P.T polymeric road-building agent	Agents improve soil cohesion, enhancing basic properties
Jia Zhenzhong [48]	Cement and lime-improved aeolian sand	Optimal properties and mix ratio for cement–lime-stabilized aeolian sand obtained
Zhang Guohui [49]	Cement-modified fine sand	Silty fine sand met 200 km/h railway subgrade filler specs
Shen Lei [50]	Cement improvement, quicklime improvement, and sandstone improvement	Combined chemical and physical improvement for suitable plan
Fan Yun [51]	50% fine sand mixed with 50% round gravel	Solution meets K30 foundation coefficient, subgrade body porosity

. Although fine sand in coastal areas is not traditionally considered an ideal subgrade material, the rapid development of high-grade highways and the demands of engineering construction necessitate adherence to the principles of using local materials and environmental protection. Consequently, coastal areas inevitably face the questions of whether fine sand can serve as a subgrade material and how to utilize it.

Traditional methods for detecting subgrade issues mainly rely on excavation and probing, which are costly, time-consuming, information-intensive, subjective, and can easily cause damage to the surveyed route. The traditional detection methods for fine sand subgrades may result in an overall instability of the subgrade due to sand leakage. Therefore, the development of a non-destructive detection and identification technology for defective subgrades is crucial for ensuring the long-term performance of fine sand subgrades during the operation and maintenance stages.

4. Application of Fine Sand Subgrade Engineering

4.1. Application of Windblown Sand Subgrades

There are relatively few examples of using aeolian sand for road construction abroad, and most of them were studied between the 1960s and the 1980s. Studies during this period primarily focused on material characteristics and road performance, as shown in

Table 2. Literature review on aeolian sand [6].

Year	Author (Application)	Research Topics
1963	Haynes and Yoder [52]	(1)
1967	Kallas and Riley [53]	(1)(2)
1967	Monismith [54]	(3)
1967	Seed [55]	(1)
1971	Hicks and Monismith [56]	(1)(2)(3)(4)(5)(6)(7)(8)(9)
1973	Kalcheff and Hicks [57]	(3)(4)(5)
1974	Brown [58]	(3)
1974	Allen and Thompson [59]	(3)(4)(6)(7)
1977	Knutson and Thompson [60]	(7)
1981	Rada and Witzcak [61]	(1)(2)(7)(8)
1985	Khedr [62]	(3)
1989	Nataatmadja and Parkin [63]	(9)

Notes: (1) the influence of stress sequence and stress repetition on elastic behavior; (2) the influence of stress sequence on plastic deformation; (3) the influence of stress pulse duration on the resilient modulus; (4) the influence of confining pressure on the resilient modulus; (5) the relationship between experimentally measured constants k₁ and k₂; (6) a review of resilient modulus characterization models for granular materials; (7) the influence of density and saturation on k₁ and k₂; (8) the impact of saturation on resilient modulus response; (9) the influence of gradation on the resilient modulus.

.

Desert areas in the western region of China are densely distributed, with aeolian sand being the most abundant and economically viable material in these regions. It is primarily used in the construction of desert roads. Domestic aeolian sand subgrade construction is mainly located in the Taklamakan Desert area of Xinjiang and the Mu Us Desert area in the energy-rich region of northern Shaanxi, dating back to the early 1990s. Construction in the eastern coastal regions commenced with the establishment of the Liaoxi section of the Beijing–Shenyang Expressway in 1999. Deserts in China are mainly distributed in the western provinces and in regions such as Xinjiang, Gansu, Inner Mongolia, Ningxia, and Qinghai [64,65]. The natural dry density and moisture content of aeolian sand in various desert regions are shown in

Table 3. National dry density and moisture content of aeolian sand.

Location	Particle Size (mm)	Average Loose Density (g·cm−3)	Average Maximum Dry Density (g·cm−3)	Natural Moisture Content (%)
Southeastern Ulan Buh Desert	0.074–0.50	1.56	1.70	0–2
Hinterland of the Mu Us Desert	0.074–0.50	1.20	1.60	<3.8
Yulin Transit Line of the Mu Us Desert	0.074–0.6	1.56	1.69	1.4–3.3
Hinterland of the Taklamakan Desert	0.05–0.50	1.385	1.69	0.5–1.5
Eastern Edge of the Taklamakan Desert	0.05–1.25	1.715	2.02	0.5–1.5
Southern Edge of the Gurbantünggüt Desert	0.05–1.25	1.69	1.93	0.5–1.5

[66,67]. Sieve analysis of aeolian sand from various desert regions indicates uniform sand particles and fine mechanical composition, representing a typical case of poor gradation and non-cohesiveness [68,69,70]. Compaction performance presents a challenge during construction processes [71].

Chen Zhongda [72] concluded from experiments that significant differences exist in the compaction process between aeolian sand and cohesive or sandy soils. A relatively high degree of compaction can be achieved with a moisture content of around 1%. Based on this, dry compaction can be applied to sand bases under vibratory compaction, which holds significant significance for regions with arid and low rainfall conditions, as well as for economic benefits. Factors such as impact energy, vibration frequency, ballast weight, and compaction speed are all important factors affecting compaction effectiveness. Yuan Yuqing [73] suggested using heavy rollers with high frequency (45–50 Hz) and small amplitude (0.4–1.0 mm), with compaction speed controlled between 3 and 6 km/h, achieving a compaction degree of up to 97.7%.

A summary of typical aeolian sand fill subgrades is presented in

Table 4. Overview of typical sand-filled subgrades with aeolian sand.

Project Name	Material Characteristics	Slope Protection	Sealing Structure
Shenshan Expressway [74]	5% natural moisture, 3–5% clay content	Edge width > 1.6 m, 30 cm loose laying thickness, 1:1.75 slope ratio	Aeolian sand
Shenyang to Zhangwu Expressway [75]	Low powder and clay content, lacks stickiness, 1–4% natural moisture	Wrapped clay with a thickness of 2.0 m	80 cm gravel or gravel soil
Liaoning Central Ring Expressway [76]	Fewer silt particles, lacks stability, forms slurry when wet, loose when dry	Fill subgrade sides with cohesive soil, center with windblown sand, roll simultaneously	High-strength materials such as weathered sandstone and stone slag, with a cross slope of 2% for road arches
Yushen Expressway [77]	Minimal cohesive and plastic properties due to high sand and low silt content	Cohesive soil edging, outer slope 1:1.5, inner slope 1:1, 2.0 m thick	Gravel soil
Liaoning Central Ring Expressway [78]	Contains few silt particles, optimal moisture 10–15%	Fill subgrade sides with cohesive soil for edge wrapping	Mountain soil, weathered rock, crushed stone soil, and stone slag, with a thickness of 80 cm
National Highway 218 in Xinjiang [79]	Fine sand, without clay particles	Wrap clay for windblown sand and fine-grained soil, slope 1:2 to 1:3. Fill subgrade directly, slope ≤ 1:3	-

. Compared to coastal fine sand subgrades, the engineering environments for these two types of subgrades differ significantly. Aeolian sand subgrades are located in arid, low-rainfall, and windy desert areas, where the predominant subgrade issues are wind erosion and sand burial. In contrast, coastal fine sand subgrades are mostly found in southern, water-rich regions with high humidity and rainfall, where the predominant subgrade issues include uneven settlement, slope instability, and erosion damage. Additionally, there are significant differences in compaction mechanisms between the two types of subgrades due to different methods of moisture content control. Aeolian sand subgrades can be compacted under nearly dry conditions. Therefore, the design and construction experience of coastal fine sand subgrades cannot be easily applied to aeolian sand subgrades.

4.2. Application of Sand-Filled Subgrade

The uniqueness of sand-filled subgrades lies in the fact that the core filling material is sand, rather than the traditional notion of fine-grained clay. Due to the unique characteristics of the filling material, designers and construction personnel must carefully consider the impacts on the structural system of sand-filled subgrades.

The engineering practices of typical sand-filled subgrades in China are summarized in

Table 5. Overview of typical sand-filled subgrades.

Project Name	Sand Source	Material Characteristics	Slope Protection	Sealing Structure
Suiyuezhong Expressway [80]	Alluvial sand on the Han River beach	Fine sand	Subgrade enveloped in 6% lime soil wrapping, 25 cm thick layers with fine sand filled between	Gravel
Shantou Jinhong first-class highway [81]	Beach fine sand	Fine sand, fineness modulus of 1.21~1.25, silt content <15%	Clay applied on both sides of sand-filled embankment for edge soil fill, simultaneous with sand filling	Lithosol
Chongming Island Connection Project of Shanghai Suiming Cross River Channel [82]	Fine sand at the mouth of the Yangtze River	Non-adhesive, water content is 11.22%	Subgrade covered with 1.0 m clay, encased in geotextile, surrounded by sandbags	Cement-stabilized soil, cement-stabilized fine sand, lime-stabilized soil
Jiangxi Lewen Expressway [83]	Gan River and Laofu River Sand	Fine sand, medium sand, silt content 1~6%	Subgrade: 1.0 m clay layer, geotextile wrapping. Embankment slope: sandbags, beveled concrete precast blocks for stabilization	Cement-stabilized soil, cement-stabilized fine sand, lime-stabilized soil
Mazhu Expressway [84]	Gravel containing river sand	Non-viscosity, small compressibility, large void ratio, loose structure	Subgrade: 1.5 m 3% lime soil wrapping, transition to stepped configuration at river sand junction	3% lime soil

. Compared to embankments and stockpiles formed by sand accumulation, sand-filled subgrades exhibit significant differences in aspects such as filling height, types of lateral confinement, drainage design, construction techniques, and quality inspection. For example, while coastal embankments serve as water barriers where slope stability must be carefully verified, such considerations are less relevant in the design of sand-filled subgrades.

4.3. Application of Sand-Blowing Subgrades

Hydraulic filling, also known as hydraulic blowing, is a method of soil excavation that utilizes hydraulic machinery to agitate and displace sediment. A certain concentration of slurry is pumped through pre-laid pipelines into the surrounding area enclosed by embankments, allowing it to gradually dewater and consolidate.

Hydraulic filling is suitable for areas where rivers intersect and large vessels can freely navigate. The main construction process is illustrated in Figure 6. Reinforcement of the hydraulic fill foundation generally employs methods such as vibratory stone column installation, dynamic compaction, cemented soil mixing, vibro-compaction without backfill, and high vacuum preloading.

5. Discussions

This review summarizes the research progress on fine sand’s performance as a raw material, involving its definition, classification, and its physical and mechanical properties, as well as its dynamic characteristics. Furthermore, various treatment methods for fine sand subgrades are analyzed, including cement treatment, fiber treatment, and other innovative materials. Additionally, the application of fine sand in subgrade engineering, specifically focusing on windblown sand subgrades, sand-filled subgrades, and sand-blowing subgrades, is discussed, underscoring its relevance and effectiveness in practical engineering applications.

6. Conclusions

Ensuring the long-term performance of the subgrade is essential for extending the lifespan of pavement structures. The assurance of subgrade performance spans various stages, including design, construction, and ongoing maintenance. Upon reviewing existing research, several academic and engineering challenges that urgently need resolution in the application of fine sand subgrades can be presented.

• Subgrade evaluation and treatment techniques.

The subgrade must possess sufficient bearing capacity, resistance to deformation, and stability to provide a solid foundation for road construction. For fine sand subgrades with specific characteristics, particular attention must be paid to their risk of dynamic instability under seismic or traffic loading. Therefore, scientifically evaluating fine sand subgrades and developing rational treatment techniques are the primary challenges that need to be addressed in the initial construction of fine sand subgrades.

2.

Dynamic characteristics of fine sand filler.

Due to the unique geographical environment of fine sand, significant differences exist in natural moisture content, capillary rise height, and other aspects. Currently, there is no unified classification method for fine sand fillers, further complicating the widespread utilization of fine sand. Therefore, accurately evaluating the suitability of fine sand as a subgrade filler has become one of the key challenges that urgently needs to be addressed.

3.

Suitable fine sand subgrade structural system and structural behavior.

Fine sand is a loose material with no cohesive force, posing challenges for compaction and vulnerable to erosion by water flow. Therefore, traditional subgrade structural systems are insufficient to ensure the long-term performance of fine sand subgrades. Hence, identifying a subgrade structural system suitable for fine sand fillers and accurately analyzing its behavior are critical challenges that must be addressed in the design process of fine sand subgrades.

4.

Construction and quality control technology of fine sand subgrades.

Analyzing compaction mechanisms in fine sand subgrades and developing construction techniques and quality control methods that align with sand core structures are challenging aspects of fine sand construction technology, necessitating thorough research.

5.

Diagnosis and treatment technology for fine sand subgrade issues.

Traditional diagnostic methods for subgrade issues primarily involve excavation and probing, which are costly and time-consuming, yield excess data, lack precision, and can potentially damage the surveyed route. Traditional diagnostic methods may lead to an overall instability of fine sand subgrade due to sand leakage. Therefore, the development of non-destructive detection and identification techniques for defective subgrades is crucial during the operational maintenance phase to ensure the long-term performance of fine sand subgrades.