## 2 Materials and Experimental Process
### Materials
#### 2.1.3 Interfacial Structure Model  
To represent the structural member for laboratory testing, solid material blocks were fabricated via 3D printing using a polylactide (PLA) filament. The PLA filament is often  
Figure 1: Particle size distribution of jumunjin sand.  
Figure 2: Shear rate dependent viscosity reduction of xanthan gum hydrogels at different xanthan gum to water (\(M_{xy}/M_{w}\)) ratios in mass.  
suggested as a geosynthetic bioplastic material that has the advantage of rapid manufacturing and sustainability. [27; 28]. The interfacial structure model is composed of a single structure without asperity and three other structures having single triangular asperities with tilt angles of 10\({}^{\circ}\), 20\({}^{\circ}\), and 30\({}^{\circ}\) (Figure 3a). The interfacial structure was designed with dimensions of 60 mm diameter and 12.5 mm height. A 20 mm-wide asperity was carved along the middle axis of the disc, and the tilt angles of the asperity varied from 0\({}^{\circ}\) (no asperity) to 30\({}^{\circ}\), as shown in Figure 3a [29]. The surface roughness of measuring length of 5000 \(\upmu\)m for 3D-printed solid structure was evaluated by surface profiler equipment (Dektak-8, Veeco) for 100 s. [PERSON] and [PERSON] (1986) introduced the normalized relative roughness (\(R_{n}\)), which expresses the ratio between the maximum peak-to-valley distance of asperities along the solid surface and the average particle size (\(D_{50}\)) of interacting particulate material as [30]:  
\[R_{n}=\frac{R_{max}\left(L=D_{50}\right)}{D_{50}}. \tag{1}\]  
The \(R_{n}\) between the interfacial structure model (Figure 3b) and jumunjin sand used in this study has been assessed as 0.12. Meanwhile, literatures report the normalized \(R_{n}\) values of materials used in actual geotechnical practices as listed in Table 2[31; 32; 33; 34] and an interface can be defined as smooth (\(R_{n}\) < 0.02), rough (\(R_{n}\) > 0.2), and intermediate (\(0.02\leq R_{n}\leq 0.2\)) [35]. Generally, \(R_{n}\) exceeds 0.05 for a typical pile surface in practice, and \(R_{n}\) used in this study (\(R_{n}\) = 0.12) is found competitively to represent common structural members (e.g., medium concrete, \(R_{n}\) = 0.12; rough steel, \(R_{n}\) = 0.17) used in geotechnical engineering practices [36]. As the \(R_{n}\) along the block surfaces is regarded to be constant (\(R_{n}\) = 0.12), a single asperity was added at the middle of the block surface to investigate the asperity effect on the geometric dilation and shear resistance which is an important consideration along solid-solid anomalous interfaces [37].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Interface Condition (Type)** & \(R_{n}\) **in mm (\(D_{50}\))** & **References** \\ \hline \(R_{n}\) in this study & 0.12 (\(D_{50}\) = 0.47) & \\ \hline Very rough steel & 0.43 (\(D_{50}\) = 0.65) & \\ Rough steel & 0.17 (\(D_{50}\) = 0.65) & \\ Intermediate steel & 0.08 (\(D_{50}\) = 0.65) & \\ Smooth steel & 0.03 (\(D_{50}\) = 0.65) & \\ \hline Brass & 0.03 (\(D_{50}\) = 0.65) & \\ \#120 Sand paper & 0.26 (\(D_{50}\) = 0.65) & [32] \\ \#50 Sand paper & 1.14 (\(D_{50}\) = 0.65) & \\ \hline Smooth concrete & 0.002 (\(D_{50}\) = 0.5) & \\ Medium concrete & 0.12 (\(D_{50}\) = 0.5) & [33] \\ Rough concrete & 0.2 (\(D_{50}\) = 0.5) & \\ \hline Smooth woven geotextile & 0.03 (\(D_{50}\) = 0.65) & \\ Moderately rough woven geotextile & 0.19 (\(D_{50}\) = 0.65) & [34] \\ Rougher woven geotextile & 0.30 (\(D_{50}\) = 0.65) & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Normalized relative roughness (\(R_{n}\)) used in this study.