## 2 Test Arrangement  
Extensive series of direct shear tests were performed on sand specimens reinforced with cotton fiber strips to investigate their mechanical characteristics. The sand tested was Fujian standard sand, whose mechanical and physical properties are presented in Table 1. The range of particle size was from 0.1 to 1 mm. The specimens were 30 mm in thickness and 60 mm in diameter. The fibers used in this study were commercially purchased woven strip cotton flexible fibers with a thickness <0.5 mm and width of 10 mm. The strip fibers had a high tensile strength (usually greater than 50 MPa) and negligible compressive strength and flexural stiffness. The length of fiber strips was selected as 30 mm, which was equal to the thickness of the sand specimens. To investigate the influence of initial orientation and content of fiber strips on the mechanical responses of sand specimens, they were selected as controlled variables in the direct shear tests. The initial orientation angel of fiber relative to the shear plane was the angle \(i\) shown in Figure 1. In this study, the initial orientation angles of 60\({}^{\circ}\), 90\({}^{\circ}\), and 120\({}^{\circ}\) were selected. Note that a new method to place the fiber strips was developed to maintain the flexible fiber strips at a predetermined orientation angle during the process of specimen preparation. As shown in Figure 2a, some plexiglass plates with orientated slots were manufactured, and some needles were inserted into the slots. The fiber strips were placed on the needles; thus, their orientation angles were determined by the slots. During the specimen preparation, plexiglass plates were first placed into the shear box (see Figure 2b). Then, the sand was introduced using a funnel and densified in layers using a steel sharp wire to rearrange the sand particles into a high packing order, along with small hammer blows to further increase the sand density. More attention was paid to the sand around the fibers to avoid the obvious influences of layered densification on the fibers. When the sand was filled close to the top of fiber strips, the plates and needles were slowly lifted. The fibers remained in the sand since the friction along the needle-fiber interface was negligible in comparison to that along the fiber-sand interface (see Figure 2c). In this method, the fibers in the sand specimens were distributed according to the angle of slots, i.e., the predetermined orientations. To prepare the reinforced specimens with initial fiber orientations of 90\({}^{\circ}\) and 60\({}^{\circ}\)/120\({}^{\circ}\), plexiglass plates with the slots at 90\({}^{\circ}\) and 60\({}^{\circ}\) relative to the horizontal direction were needed in this study. The average fiber content included in a reinforced sand specimen was defined as a percentage weight of fibers with respect to the dry unit weight of sand. In this study, unreinforced sand (fiber content = 0%) and reinforced sand with fiber contents of 0.04%, 0.2%, and 0.4% were selected for testing. Note \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Effective size, \(D_{\mathbf{10}}\) (mm) & 0.25 \\ Mean particle size, \(D_{\mathbf{50}}\) (mm) & 0.37 \\ Uniformity coefficient, \(C_{\mathbf{u}}\) & 1.92 \\ Coefficient of curvature, \(C_{\mathbf{c}}\) & 1.40 \\ Maximum dry density, \(\rho_{\mathbf{max}}\) (g/cm\({}^{3}\)) & 1.65 \\ Minimum dry density, \(\rho_{\mathbf{min}}\) (g/cm\({}^{3}\)) & 1.33 \\ Maximum void ratio, \(\epsilon_{\mathbf{max}}\) & 0.73 \\ Minimum void ratio, \(\epsilon_{\mathbf{min}}\) & 0.50 \\ Grain density, \(\rho_{\mathbf{8}}\) (kg/m\({}^{3}\)) & 2630 \\ Specific gravity, \(G_{\mathbf{s}}\) & 2.63 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical properties of Fujian standard sand. that, for each combination of controlled variables, three identical sand specimens were tested, and the average results were analyzed to ensure that the conclusions drawn in this study had statistical significance.