# Experimental Study on Pullout Behavior of Coir Geotextiles Based on Transparent Soil
## 3 Results and Discussion  
### Influence of Normal Stress  
Figure 7 shows the relationship curve between the pullout force and the pullout displacement under different normal stresses. As it can be seen from Figure 7, with the increase in normal stress, the pullout force in the whole process increased significantly. The mechanical response of the pullout test was basically similar, that is, with the increase in pullout displacement, the pullout force first increased rapidly, then slowly increased to the peak value, and then gradually decreased with the fluctuation. A similar pattern appears in Figures 11 and 13 below. When the normal stress increased from 10 kPa to 40 kPa, the corresponding peak value of the pullout force increased from 1.8 kN to 2.2 kN and 2.4 kN, respectively. At the beginning of the pullout test, the greater the normal stress, the greater the slope of the pullout curve, because, when other things were equal, the greater the normal stress, the more soil entered the coir geotextile mesh. As the interlocking effect between the coir geotextile and the soil increased, the resistance to be overcome in the pullout test increased. In order to evaluate the reinforcement effect of the coir geotextile, the shear strength and the friction coefficient of the reinforced soil interface were analyzed. The relevant calculation formulas of shear stress, normal stress, and interfacial friction coefficient of the reinforced soil are shown in Equations (2)-(4), respectively [34]. \[\tau=\frac{T_{d}}{2 LB} \tag{2}\]  
\[\sigma_{v}=\frac{P}{A} \tag{3}\]  
\[f_{gs}=\frac{\tau_{max}}{\sigma_{v}} \tag{4}\]  
where \(T_{d}\) and \(P\) are the horizontal tension and the normal load (kN), respectively, \(L\) and \(B\) are the length and width of the reinforcement, respectively, \(\tau\) and \(\sigma_{v}\) are the shear stress and the normal stress (kPa), respectively, \(A\) is the horizontal area of the pullout box (m\({}^{2}\)), and \(f_{gs}\) is the interfacial friction coefficient of the reinforced soil. Figure 8 shows the relationship between the shear strength of the coir geotextile and the normal stress. The shear strength parameters of the coir geotextile interface can be obtained after linear fitting, in which the cohesion was 5.68 kPa and the internal friction angle was 3.43\({}^{\circ}\). Figure 9 shows the relationship between the interfacial friction coefficient of the reinforced soil and normal stress. The interfacial friction coefficient of the reinforced soil is an important index of the comprehensive strength of the interfacial friction effect, which is the ratio of the interfacial shear stress to the normal stress. In general, the greater the interface friction coefficient, the better the reinforcement effect. Since the strain at each point of the reinforced material is not the same, the stress at the interface of the reinforced  
Figure 8: The relationship between the shear strength of the coir geotextile and the normal stress. Figure 7: The relationship curve between the pullout force and the pullout displacement under different normal stresses. soil is not evenly distributed. Only when the reinforced material is pulled out, the friction resistance at the upper and lower interface can be considered to be evenly distributed. As it can be seen from Figure 9, the interface friction coefficient was a quantity that changed with the change in normal stress, rather than a constant. The tests conducted by [PERSON] [35] and [PERSON] et al. [36] show a similar situation. [PERSON] [35] conducted a pullout test on a geogrid with a tensile strength of 40 kN/m in transparent soil. It can be seen that, under a normal stress of 10 kPa, the interfacial friction coefficients of the coir geotextile and the geogrid were close to each other, and the gap between them began to appear as the normal stress increased.