# Effect of the Surface Roughness on the Shear Strength of Granular Materials in Ring Shear Tests
## 1 Introduction  
The shear strength of granular materials is a very important parameter in engineering practice when examining the frictional and viscous characteristics of geomaterials [1; 2]. Granular materials have complicated shear deformation characteristics involving solid- and liquid-like behaviors [3; 4]. A previous study found that granular materials have much more complex elastoplastic and viscous behaviors than materials with finer particles [5]. For this reason, surface roughness has often been taken into account in examining soil-structure interactions in geotechnical engineering practices for piles, foundations, retaining walls, tunnels, embankment, and earth reinforcement [6; 7; 8; 9; 10], because the soil-structure interaction is significantly influenced by the material's properties, shape, roughness, and loading conditions (i.e., monotonic and cyclic loading). Numerous studies have focused on the frictional and viscous behaviors of loose and dense sands as representative granular matter [6; 11]. According to [PERSON] and [PERSON] [8], for a sandy size particle (0.075-2 mm), elastic perfect-plastic behavior is dominant for a smooth interface, while strain localization is dominant for a rough interface. The importance of surface roughness in various grain sizes has also been evaluated and presented by numerous researchers, because inter-particle friction can be affected by the Young's modulus, particle shape, and surface roughness of the materials [11; 12; 13]. The reason for this is that natural materials have a wide spectrum of surface features in nature; it has generally been observed that inter-particle friction increases with surface roughness. According to [PERSON] and [PERSON] [11], the friction angle depends on both sand density and interface roughness. In order to investigate the effect of roughnesson the frictional behavior of particle-particle and soil-structure interfaces, different types of tests have been used under monotonic and cyclic loading: The simple shear test [14; 15], the direct shear test [8; 9], and the ring shear test [16; 17]. The effect of surface roughness on the mechanical behavior of granular materials is also crucial for natural disaster prediction and prevention, such as landslides and debris flows. In landslide areas, the mechanical properties that may lead to rock avalanches with rapid, massive, and dynamic movements of fragmented rocks are evaluated by the surface roughness determined by in situ and laboratory tests [9; 18]. In geology, joints and weathering are related to fragmentation [19]. The shear strength, permeability, and compressibility of granular materials are also greatly affected by the fragmentation phenomenon when they are subjected to changes in stress. An empirical method to estimate the travel distance of fragmented rocks in the Wenchuan earthquake area was presented by [PERSON] et al. [1]. High mobile mass movements can be caused by the soft base effect [20], which is related to the increase of fine-grained sediment during a landslide motion. In general, the surface roughness of granular materials may result in particle breakage, also called grain crushing. In the shear stress (or force) and strain relationship, three stages of particle breakage under compression can be identified: (i) Local rearrangements and sliding, (ii) fragmentation by abrasion, and (iii) fragmentation by fracture [21]; it is assumed that grain fragmentation can be characterized by the progressive propagation of a transversal crack (including compression, abrasion, and fracture) inside the grain. In general, the breakage potential of a soil particle increases with its size [22]. Using a large-sized ring shear apparatus, particle breakage can be measured by the grain size distribution curve before and after shearing [23; 24]. However, the mechanical behavior due to particle breakage in different drained and shear velocity conditions is not fully understood.