The frictional properties of staple fiber needle-punched geotextile at the soil interface are key to its stability in projects such as slope protection and foundation reinforcement. Their performance is influenced by the interaction of multiple factors, including fiber properties, processing technology, soil conditions, and environmental conditions.
Fiber properties fundamentally determine the micromechanisms of interfacial friction. The raw materials used for staple fiber needle-punched geotextile must balance flexibility and surface roughness. Natural fibers (such as hemp) have a rough surface and are highly hygroscopic, which allows for a strong mechanical bond with the soil, but they are susceptible to degradation due to prolonged moisture exposure. Synthetic fibers (such as polyester and polypropylene) are treated with surface modification techniques (such as plasma treatment) to increase their roughness, thereby improving the coefficient of friction while maintaining durability. The matching of fiber diameter and length is also crucial. Thin fibers increase contact area, but if they are insufficiently long, they are easily pulled out by soil shear forces. Coarse fibers, while offering high pullout strength, may buckle if they are too long due to insufficient bending stiffness, thereby reducing friction efficiency. Furthermore, fiber orientation significantly influences friction properties. The three-dimensional structure created by the needle punching process allows for multi-directional fiber alignment throughout the thickness, enhancing the depth of interlocking with the soil and increasing the friction coefficient by approximately 20%-30% compared to unidirectionally aligned fibers.
The processing technique directly influences interfacial friction performance by controlling the fiber entanglement structure. Needling density is a key parameter. Low-density needling (e.g., 50-70 needles/cm²) creates fewer fiber entanglement points, resulting in larger surface pores on the geotextile. While this facilitates drainage, it also reduces friction. Medium-density needling (80-100 needles/cm²) ensures drainage while increasing fiber entanglement points to create a uniform capillary pore structure, allowing soil particles to embed within the fiber interstices and significantly enhancing mechanical interlocking. High-density needling (>120 needles/cm²) may lead to excessive fiber breakage, resulting in a loose structure and, in turn, a lower friction coefficient. The needling depth must be matched to the fiber length. Typically, a needling depth of 7-12mm combined with a medium-density process creates a stable fiber-soil interface, preventing friction loss caused by fiber warping or insufficient soil penetration. Furthermore, multi-step needling (e.g., pre-needling followed by main needling) can optimize fiber orientation step by step, resulting in a three-dimensional distribution of fibers across the thickness, further enhancing interfacial shear strength.
The impact of soil conditions on frictional properties is scenario-dependent. Soil particle size distribution is the primary factor. A high proportion of fine particles (e.g., clay and silt) tends to fill the pores between fibers, forming a "soil-fiber-soil" composite friction layer and enhancing cohesion. On the other hand, a high proportion of coarse particles (e.g., gravel) reduces the contact area between the fiber and the soil, and friction relies primarily on mechanical engagement between the particles. In this case, increasing fiber density or surface roughness can compensate. Soil moisture content affects frictional behavior by changing pore water pressure. In clay, increasing moisture content weakens the bonds between soil particles, reducing the friction coefficient between the fiber and the soil. In sandy soil, due to its good drainage, changes in moisture content have a minimal impact on the friction coefficient. However, extreme dryness or saturation can lead to lubrication of the fiber-soil interface. Soil density is also crucial. Higher compaction increases the contact area between the geotextile and the soil, increasing the fiber's embedding depth and correspondingly increasing the friction coefficient.
Environmental conditions indirectly influence frictional properties by altering the physical state of the interface. Rising temperatures accelerate fiber aging, reducing surface roughness and decreasing the friction coefficient with increasing temperature. Low temperatures can cause water in the soil to freeze, forming ice crystal-fiber complexes. This can increase friction in the short term, but long-term freeze-thaw cycles can lead to fiber breakage and reduced durability. Ultraviolet radiation is the primary cause of polymer aging. Untreated staple fiber needle-punched geotextiles experience a decrease in surface fiber strength and, consequently, a reduction in friction coefficient after UV exposure. Using fibers containing carbon black or antioxidants can significantly slow this process. Furthermore, chemical corrosion (such as acid rain and saline-alkali soils) can damage the fiber surface structure, leading to an irreversible decrease in friction coefficient. Therefore, corrosion-resistant fiber materials must be selected based on the project environment.
