# Study on Stress-Strain Relationship of Coir Fiber-Reinforced Red Clay Based on Duncan-Chang Model
## 3 Experimental Results and Analyses  
### Stress-Strain Relationship  
By sorting out the triaxial test results for plain soil and coir fiber-reinforced soil samples, the stress-strain curves of each sample could be obtained by taking the axial strain \(\varepsilon_{1}\) as the abscissa and the principal stress difference \(\sigma_{1}\)-\(\sigma_{3}\) as the ordinate, and the stress-strain curves of each sample could be analyzed, as shown in Figure 5.  
As can be seen from Figure 5:  
1. Under different confining pressures and different fiber contents, the stress-strain relationship curves of plain soil and coir fiber-reinforced soil have similar trends, both of which are hyperbolic. With the increase in axial strain, the rate of increase in deviatoric stress decreases gradually, and the final deviatoric stress tends to a stable value. The stress-strain curve shows a typical strain-hardening type. 2. The stress-strain curve shows a typical strain-hardening type. 3. The stress-strain curve shows a typical strain-hardening type. 4. The stress-strain curve shows a typical strain-hardening type. 5. The stress-strain curve shows a typical strain-hardening type. 6. The stress-strain curve shows a typical strain-hardening type. 7. The stress-strain curve shows a typical strain-hardening type. 8. The stress-strain curve shows a typical strain-hardening type. 9. The stress-  
2. Compared with plain soil, the peak deviatoric stress of five coir fiber-reinforced soil samples with different levels of fiber content increased substantially, indicating that the addition of coir fiber can affect the strength and deformation resistance of soil. 3. Under low confining pressure (100 kPa, 200 kPa), the peak deviatoric stress of reinforced soil with different levels of coir fiber content shows little difference, indicating that the deviatoric stress is less affected by confining pressure under low confining pressure. Under high confining pressure (300 kPa, 400 kPa), the peak deviatoric stress of coir fiber-reinforced soil increases first and then decreases with the increase in fiber content, that is, there is an optimal coir fiber content. When the fiber content is 0.3%, the peak deviatoric stress reaches the maximum, and the reinforcement effect is the best. When the fiber content is too much, the fiber in the soil will be gathered together, such that the degree of occlusion and friction between the soil and the fiber becomes low, and it is easy to form penetrating cracks in the sample, which destroys the integrity of the soil due to the existence of potential failure surfaces. 4. When the axial strain of the soil is small (\(\varepsilon_{1}\) < 1%), the stress and strain curves of the plain soil and the coir fiber-reinforced soil are basically equivalent, indicating that the coir fiber cannot play a good role under a small strain. This is mainly because when the soil deformation is small, the external force is also small, the occlusal friction between the soil and the fiber is weak, the cohesion of the soil itself is not much enhanced, and the degree of fiber bending in the soil is not enough, and the spatial constraint effect on the soil is weak, so the reinforcement effect is not obvious. With the increase in axial strain \(\varepsilon_{1}\), the deviatoric stress of coir fiber-reinforced soil increases rapidly, and the greater the confining pressure, the greater the increase in the deviatoric stress, indicating that in the case of a certain small strain, coir fiber can play an early role in inhibiting the deformation of the soil, and the reinforcement effect of fiber soil is obviously reflected. When the axial strain reaches a certain value (\(\varepsilon_{1}\) > 4%), under the same fiber content, the greater the confining pressure, the greater the corresponding deviatoric stress. This is because there are more voids in the soil. The larger the confining pressure, the faster the void closure, the stronger the lateral restraint force, and the greater the deviatoric stress.