# Investigation of the Mechanical Behavior of Polypropylene Fiber-Reinforced Red Clay
## 1 Introduction  
Rapid socioeconomic development, particularly accelerating industrialization and urbanization, is accompanied by the rapid emergence of massive urban buildings, high-speed railways, and freeways. Such development inevitably leads to the challenge of foundation engineering on unfavorable soils. Reinforcement techniques for soil are popular in geotechnical engineering. Geosynthetics (for instance, geogrids, geotextiles, geomembranes, and geocells) are widely utilized as reinforcement materials and have shown significant improvement effects in subgrade bed reinforcements, foundation base treatments, and applications to special-soil subgrades [1, 2, 3, 4, 5]. However, geosynthetics have vulnerabilities. In particular, when subjected to an external load, the reinforcement-filler interface may easily develop failure surfaces, thereby compromising the overall performance of the reinforced structure [6]. To effectively overcome this vulnerability of geosynthetics, researchers have attempted to use fibers to reinforce soils [7, 8, 9, 10]. Fibers are mixed into soils at a certain ratio to create a composite material. The fiber is randomly and uniformly distributed in the soil to connect the fiber filaments and soil particles effectively. The fiber serves to confine the soil deformation in all directions, inhibit the development of failure surfaces, and thus enhance the shear strength through the fiber's tensile strength and its friction and interlocking effect on the soil particles. Studies have shown that fiber could reinforce the shear strength, tensile strength, and liquefaction resistance of soils, but the improvement effect is influenced by a multitude of factors, for instance, fiber type, content, and length as well as cell pressure and particle gradation [11, 12, 13, 14, 15]. Many kinds of fibers are used in the method of fiber reinforcement to improve soil properties, including artificial fibers and natural fibers. Commonly used artificial fibers are PVA, polypropylene, nylon, and carbon fiber. [PERSON] et al. [16] demonstrated that the toughness and strength of fiber-reinforced clay increased due to the addition of longer fibers, and the performance of PVA fiber is better than that of inert fiber. [PERSON] and [PERSON] [17] concluded that freeze-thaw cycles decreased the UCS and ultrasonic pulse velocity of fiber-reinforced clay and that the polypropylene fiber was more resistant to freeze-thaw action than basalt fiber. [PERSON] et al. [18] indicated that polypropylene fiber acts as a reinforcing material and limits the development of tension cracks through the bridge effect on fiber-reinforced clay soil. [PERSON] et al. [19] concluded that the UCS and axial strain of clay were improved through the addition of nylon fiber, and the failure modes changed from brittle to ductile. [PERSON] et al. [7] pointed out that the UCS and brittle failure of clay were significantly improved with the addition of carbon fiber, and the reinforcement mechanism was a single carbon fiber (one-dimensional effect) and a fiber network (three-dimensional effect), respectively. Common natural fibers include basalt, hair, and coir fibers. [PERSON] et al. [20] concluded that the basalt fiber could obviously improve the unconfined compressive strength (UCS) of clay soil, and the optimum values of FC and \(FL\) were 0.25% and 12 mm, respectively. [PERSON] and [PERSON] [21] investigated the shrinkage cracking properties of fiber-reinforced clay soil and concluded that the geometric and morphic behaviors of crack shapes were obviously changed by fiber action, and the polypropylene fiber was more effective in preventing cracking than the human hair and coir fibers. [PERSON] et al. [22] observed that the shear strength and durability of marine clay were improved by nanomodified coir fibers, and the tensile strength and friction between the interfaces were the primary mechanisms to control the reinforcing effect.