# Effects of Polypropylene Fiber on the Liquefaction Resistance of Saturated Sand in Ring Shear Tests
## 4 Discussion  
As introduced before, during the shearing process, the external shear energy was consumed by two parts. A part of energy was consumed first as sand internal energy, probably in the form of heat, kinetic, or surface energy of the grains. The rest was consumed by generating plastic deformation, including grains rearrangement, bonding distortion and grains crushing. It was expressed as the variation of shear stress and shear strain from the macro level, and therefore the required shear energy was dependent on the shear stress and shear strain of the sand, as shown in Equation (3). It was reported that if the sand grains could be crushed with the applied load, the rearrangement of sand grains or the grains crushing would increase the pore pressure and increase the potential of liquefaction [37]. For unreinforced specimens, an increase in sand density led to an improvement in the interfacial force between sand grains via increasing the effective contact area or the compactness of grains. Hence, more energy was required to keep shearing. This is why in all tests with unreinforced sand, the shear stress and shear displacement increased with the increment of sand density, resulting in larger total shear energy until liquefaction (\(W_{\text{total}}\)) of unreinforced specimens with higher density (Tables 2 and 3).  
The mixing of fibers in sand might provide a three-dimensional structure to interlock soil grains, resulting in an improvement in sand microstructure and restricting displacement. In addition, the lateral spreading of the soil caused by liquefaction can be limited or even prevented by mixing fibers [40]. The interfacial interactions between sand and fibers are greatly affected by dry sand density and fiber content. The increase in sand density not only effectively increased the contact area and compactness of grains, but also boosted the fiber surface roughness. The roughness of the fiber surface plays a vital role on fiber sliding resistance [39]. So, the shear stress of reinforced sand increased with the increase in both sand density and fiber content; thereby more energy was spent to overcome the resistance. This is the reason why \(W_{\text{total}}\) increased with increasing fiber content and sand density. Meanwhile, the fiber gets reoriented in the shear zone. Compacting the sand-fiber mixture during specimen preparation led to a preferred near-horizontal orientation for the fibers [41]. This fiber  
Figure 11: Some typical photographs of specimens after shearing.  
orientation is quick to expand its volume when in a greater density state, resulting in decreasing pore pressure and increasing shear stress; especially, the negative pore water pressure generated in specimens with higher density (Figure 5b). It was predicted that further shear deformation along the failure plane was caused by liquefaction or a steady value. If the specimens failed to induce grains rearrangement or overcome other resistance, it needed more energy to trigger liquefaction. Therefore, the shear energy was mainly dissipated after shear failure (Tables 2 and 3).  
The test results indicate that fiber reinforcement can be considered as an effective method to improve the anti-liquefaction ability of sand. This method will be used in soil reinforcement in the field, especially for foundation reinforcement. Firstly, the clean soil should be mixed with moisture content that is about 10% the proportion of dry weight of soil. After that, the proposed content of fibers should be mixed thoroughly with small increments by a stirrer to obtain a uniform mixture. Finally, the mixtures will be compacted with the designed densities layer by layer in the field. It is noted that although it will be difficult to add fibers to the sandy ground directly, we may add fibers to the sandy materials used for the construction of reclaimed lands.