# Effect of the Surface Roughness on the Shear Strength of Granular Materials in Ring Shear Tests
## 2 Materials and Methods
### Methods  
#### 2.2.1 Ring Shear Test  
A ring shear apparatus is commonly used to examine the residual shear strength of soils based on a shear stress-shear strain relationship. It is a well-known special instrument with an artificial slip surface that allows for measurements of the shear stress when forces act on a sample [25; 26; 27]. One of the main advantages of the ring shear test is the unlimited shear deformation developed in a soil sample. There is a constant contact area during the test, so extra stress concentration with large shear deformation does not occur [8]. To investigate the undrained shear strength of soils, a large-sized ring shear apparatus was developed by various researchers [25; 26; 27; 28; 17; 29]. This ring shear device is easy to assemble and has simple drainage and shear velocity control systems (Figure 1). The device incorporates one of the largest ring shear boxes, with a maximum outer diameter of 250 mm. It is applicable for both fine- and coarse-grained sediments. The samples were contained in the ring shear box, which is also called the ring shear cell [30]. Vertical loading can be applied to the samples through three piston chambers (Figure 1a,b). However, normal stress-dependent frictional strength and shear zone formation were beyond the scope of this study.  
After mounting, the drained or undrained condition is selected using the open/close valve in the upper plate. The shear velocity can be varied from 0.01 to 100 mm/s. In this study, three shear velocities were selected: 0.1, 0.5, and 1 mm/s, with a particular emphasis placed on surface roughness. These velocities were selected because the grain crushing of gravel is much higher (excessive) when the shear velocity is higher than 1 mm/s in ring shear tests [30]. However, the effect of surface roughness on the shear stress in the shear zone is not clear, because the inside of the ring shear box cannot be observed during the test. Normal stress was not applied during the tests regardless of the drainage condition; that is, the upper ring plate just contacts the upper part of the gravel material being tested without applying any load because the frictional behavior is strongly influenced by applied normal stress [8; 11], which was not the focus of this study. The grain size and surface roughness also have significant influence on the shear resistance [14]. To minimize the normal stress effect on the granular materials, a constant normal stress is applied during a test (i.e., 0 kPa). However, the gravel may resist the dilative force during shearing when the normal stress is applied to maintain a zero vertical loading value. Thus, the materials tested are continuously subjected a contrac  
Figure 1: Ring shear apparatus and material: (**a**) Ring shear apparatus; (**b**) ring shear box; (**c**) gravel; and (**d**) schematic view of ring shearing.  
be negligible within the relatively low shear velocities imposed (e.g., 0.1-1 mm/s). During the tests, the shear stresses (i.e., the torques) were measured by two arms on the sides of the ring shear box. The variations in vertical displacement and normal stress were also measured. The testing procedures were identical to those detailed by [PERSON] et al. [25; 26].