# The Application of Fine Sand in Subgrades: A Review
## 3 Treatment Technology for Sand-Filled Subgrades
### 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. [PERSON] [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. [PERSON] [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  
Figure 4: Fracture mechanism of fiber under isotropic loading. 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\({}^{\circ}\) relative to the shear plane. Similar results are obtained for reinforced sands tested under both loose and dense states [35; 36]. [PERSON] [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. [PERSON] [38] conducted triaxial compression tests to examine the influence of different reinforcement configurations (Figure 5) on the strength of geosynthetic reinforced sand.