## 1 Introduction  
In recent years, the ecological restoration of slopes has become a research hotspot in resource conservation and soil and water conservation. Among various restoration techniques, hydroseeding has been widely applied in the rehabilitation of slopes along highways, railways, and mining areas due to its high construction efficiency and broad material applicability [1]. The core of hydroseeding lies in the establishment of a suitable substrate that facilitates seed germination under poor or even harsh environmental conditions while ensuring long-term slope stability. However, under the influence of rainfall erosion, wet-dry cycles, and wind erosion, conventional spray substrates often suffer from structural looseness, low erosion resistance, and severe shrinkage cracking, which significantly restrict the survival and long-term growth stability of vegetation on slopes [2].  
To enhance the stability of spray substrates, researchers have proposed the concept of fiber reinforcement [3, 4, 5]. Compared to early synthetic fibers, such as polypropylene, natural plant fibers offer greater environmental benefits and renewability, while their degradation products can improve soil organic matter content [6, 7]. Coir fiber, in particular, has attracted significant attention due to its high tensile strength, elastic modulus, and hygroscopicity, enabling it to form an effective three-dimensional network structure within the soil [8; 9]. This structure enhances interparticle friction and interlocking forces, significantly increasing soil cohesion and erosion resistance. Additionally, the anchoring effect of fibers mitigates the expansion and connectivity of shrinkage cracks, thereby delaying soil deterioration caused by moisture loss [10; 11]. However, studies have indicated that an insufficient fiber content fails to establish an effective reinforcement network, while excessive or overly long fibers tend to agglomerate, resulting in localized weak zones in the soil [12; 13]. Moreover, different soil textures, such as sandy soil, silty soil, and clayey soil, exhibit varied responses to fiber reinforcement. Sandy soil, due to its larger porosity, benefits from a more pronounced network effect, whereas silty and clayey soils rely more on fiber reinforcement to restrict crack expansion and improve the overall mechanical performance. Therefore, when determining the optimal fiber content and length, multiple factors, such as soil texture, moisture content, and fiber characteristics, must be considered to achieve a balance between soil stability and improved physical structure [14; 15].  
Based on the abovementioned considerations, this study employs coil fiber as a reinforcement material and examines its effects on the stability and structural improvement of spray substrates in sandy and silty soils. A series of triaxial shear tests, physical property measurements, and cracking tests were conducted to evaluate the influence of fiber length and content on soil performance. Principal component analysis (PCA) and Response Surface Methodology (RSM) were applied to identify the optimal fiber parameter combinations. The findings aim to provide a scientific basis for the rational design of fiber-enhanced spray substrates and contribute to the development of more stable and effective slope restoration techniques.