# Effects of Polypropylene Fiber on the Liquefaction Resistance of Saturated Sand in Ring Shear Tests
## 1 Introduction  
Soil liquefaction may cause loss of lives and property during earthquakes. Liquefaction may result in ground failure, infrastructure damage, landslides and other hazards [1; 2; 3; 4; 5; 6; 7]. In recent decades, liquefaction resistance of cohesionless soils of slopes, dams, embankments and foundation materials has attracted more attention [8; 9; 10; 11; 12]. Much effort has been made in an attempt to prevent sand liquefaction, including draining, densification and soil reinforcement. Fiber reinforcement has been considered as an effective approach to enhance the anti-liquefaction property of sand [13; 14; 15; 16; 17]. [PERSON] et al. studied the liquefaction of a sand-fiber mixture with static situation using tri-axial undrained tests and found that fiber inclusion reduces the occurrence of liquefaction [18]. [PERSON] et al. carried out a battery of ring shear tests on saturated specimens with an undrained situation, and the results show that for dense sand the reinforcement changes its undrained behavior [19]. In addition to those mentioned, many previous studies have advocated that fiber reinforcement sand can effectively prevent liquefaction of sand. However, neither of them proposed a simple and effective method to evaluate the effect of fiber on the improvement of the anti-liquefaction property of sand.  
Some methods have been considered to determine the parameter to evaluate the liquefaction resistance of soils. The shear stress and cycle number were firstly proposed by [PERSON] and [PERSON] as a criterion for evaluation [20]. Subsequently, the shear strain was used to define the liquefaction resistance. However, it was difficult to determine an effective uniform shear stress or shear strain in tests. Meanwhile, an energy approach was proposed by [PERSON] and [PERSON] as a useful method to assess soil liquefaction resistance [21]. This energy method has been used by many researchers [22; 23; 24; 25; 26]; it presents a liquefaction representation based on the dissipated energy, increased pore pressure and initial effective stress. [PERSON] and [PERSON] built a relation between energy release and pore pressure generation [23]. [PERSON] et al. proposed an energy attenuation equation to assess the soil liquefaction potential [24]. [PERSON] et al. studied the evaluation of soil liquefaction by energy principles [25]. [PERSON] and [PERSON] introduced that the parameters of shear strain and cycle number could be replaced by shear energy [26]. These researches indicated that the energy approach is a simple method to assess soil liquefaction resistance.  
In traditional sand liquefaction research, the direct shear test and triaxis test are mostly used. However, the specimen volume and strain range of the conventional direct shear apparatus and the triaxis shear apparatus are small. The results of these two tests have some limitations, which limit the evaluation of sand liquefaction. Therefore, we used a late-model shear device performing the associated tests in the present research. The used ring shear device has a big strain range and can keep the area of the shear surface constant in comparison with others [27; 28; 29], which are widely used in geologic hazard assessment [30; 31].  
In this study, we use an energy approach to assess the effects of polypropylene fiber on the liquefaction resistance of saturated sand in undrained ring shear tests, and the effects of sand density, fiber content and shearing method on dissipated energy were examined. The variation of dissipated energy with shear stress and pore pressure generation was analyzed. Finally, the interaction mechanism between sand and fiber is discussed.