# The Application of Fine Sand in Subgrades: A Review
## 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. [PERSON] 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. [PERSON] 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 designrequirements, 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 [PERSON] and [PERSON]'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, [PERSON] and [PERSON]'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.