## 2 Materials and Methods
### Indicators and Methods
#### 2.3.2 Triaxial Test  
Sample preparation was conducted according to the Standard for Soil Test Methods (GB/T 50123-2019) [20]. For each treatment, 3 kg of air-dried soil was weighed and placed in a mixing basin. Polyacrylamide was added and mixed evenly. The soil was then leveled, and coir fibers were incorporated before transferring the mixture into a sealed bag. A spray bottle was used to evenly distribute a predetermined amount of water.  
Based on the compaction test, the optimum moisture content corresponding to the maximum dry density was determined, and a moisture content of 12% was set for this  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**pH** & **EC** & **Total Phosphorus** & **Total Potassium** & **Organic Matter** \\
**(kg/m3)** & **(GPa)** & **(MPa)** & **(MPa)** & **(\%)** \\ \hline
7.74 & 8331 & 3.883 & 2.32 & 53.8 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Chemical properties and nutrients of organic fertilizers.  
experiment. According to the SL237-1999 Standard [21] for the soil test, the soil samples were moisturized for 36 h before testing. During sample preparation, compaction was applied uniformly, with each sample being compacted in three layers with 25 blows per layer. After preparation, the soil columns were weighed to ensure uniform mass across treatments.  
Test Equipment and Methods  
The test was conducted using a TSZ-6A strain-controlled triaxial apparatus (Nanjing Soil Instrument Co., Ltd., Nanjing, China). The triaxial apparatus applied confining pressure (where normal stresses in the x and y directions were equal) and axial pressure (in the z direction) to induce shear failure in the specimens. The cohesion and internal friction angle of the samples were determined based on the Mohr-Coulomb theory.  
The test was conducted following the SL237-1999 Standard for Soil Tests, using the unconsolidated undrained (UU) shear method. The concept of the root-containing (reinforcement) effect coefficient was introduced [14] to evaluate the impact of fiber reinforcement on soil strength, expressed by the following equation:  
\[\mathrm{R}\sigma=\frac{(\sigma 1-\sigma 3)\mathrm{fr}}{(\sigma 1-\sigma 3) \mathrm{fs}} \tag{2}\]  
In Equation (2):  
R\(\mathrm{\SIUnitSymbolMicro}\): Root-containing (reinforcement) effect coefficient.  
(\(\sigma_{1}\)-\(\sigma_{3}\))fr: Deviatoric stress at failure for fiber-reinforced or root-containing soil.  
(\(\sigma_{1}\)-\(\sigma_{3}\))fs: Deviatoric stress at failure for unreinforced soil.  
Interface fiber-quantity characteristics were analyzed by cutting the failed triaxial test specimens into two halves, exposing failure surfaces A and B. The number of fibers on each surface was counted separately. The total fiber count on surfaces A and B, the difference in fiber quantity between the two surfaces, and the interface dispersion coefficient were recorded. By analyzing the fiber quantity in quarter sections of each specimen, the fiber dispersion coefficient, \(\alpha\), in the soil was calculated using the following equation:  
\[\alpha=\mathrm{e}^{\mathrm{\SIUnitSymbolMicro}(\mathrm{x})} \tag{3}\]  
\[\mathrm{\SIUnitSymbolMicro}(\mathrm{x})=\frac{\sqrt{\sum(x_{i}-\mu)^{2}}}{n\mu} \tag{4}\]  
In Equations (3) and (4):  
A represents the fiber dispersion coefficient; \(\mu\) represents the average number of fibers in the sample; \(n\) represents the number of divided sections (sample count); and \(\mathrm{x}\) represents the number of fibers contained in the divided section (\(\mathrm{i}=1\), \(2\), \(3\), \(4\)).  
The dispersion coefficient reflects the distribution state of fibers within the soil. A dispersion coefficient closer to 1 indicates a more uniform fiber distribution.  
Microstructural observation: A scanning electron microscope (SEM), model S-3400N, was used for microstructural analysis. The failed triaxial test specimens were cut into small pieces using a scalpel, mounted onto sample holders, processed, and then placed in the SEM for observation at magnifications of 50\(\times\) and 100\(\times\).