## 1 Introduction  
Rainfall-induced shallow landslides are common natural hazards in the Republic of Korea [1; 2; 3; 4]. They often occur during heavy rainfall events, especially during the summer season (June to October). Many shallow landslides that occurred in weathered granitic and greaisic soils in mountainous region source areas have a typical soil thickness of 1-2 m. Landslide materials encountered in weathered soil deposits typically consist of sandy and gravelly grain sizes. Owing to its grain size distribution, many slope failures result in a catastrophic landslide. When a landslide occurs, it is mostly accompanied by large-scale collapse and high spreads due to the disintegration of agglomerates. In particular, they belong to debris avalanches and flows by landslide classification [5; 6]. To prevent these kinds of landslide risks, deterministic and probabilistic analyses of landslide occurrences are often carried out. The mechanical properties of landslide materials are of importance for both analyses to determine the slope stability. To examine the shear characteristics of geomaterials, numerous laboratory experiments have been used, e.g., direct shear tests, triaxial compression tests, simple shear tests, and ring shear tests [7; 8]. Ring shear testing is by far a very common method used to determine the residual shear resistance of landslide materials. Residual shear stress is defined as the minimum shear resistance of landslide materials to resist slope deformation. Compared with the others, the greatest advantage of this apparatus is that it is not limited by the shear displacement. A typical ring shear apparatus was used for fine-grained sediments [9; 10; 11], but in recent years, it can be applicable for coarse-grained sediments by using a relatively large-sized ring shear box [12; 13; 14; 15]. The latter may cover sand- and gravel-sized particles. In addition, a ring shear apparatus was also used to examine the mechanical characteristics of landslide materials with respect to high mobilization, e.g., long runout distance and large spreads [16; 17; 18; 19]. In the actual test program, the shear strength of soils depends on the variation in normal stress and saturation [20; 21]. Saturation (or water content) may affect the shear stress development during shearing. Lower water content may cause higher shear stress for carbonaceous silty mudstone and lime mudstone [21]; dry conditions may cause higher shear stress for silty clay soil from the landslide source area [20]. Saturated cases cause higher shear stress for weathered serpentinite soil samples [22]. Interestingly, the shear thickness has almost no difference under drained and undrained conditions for a relatively high shear speed of 100 cm/s, but it is affected by imposed shear speed. Earthquake-induced landslides can also be demonstrated by the undrained cyclic loading applied: the strength parameters, e.g., the friction angles, determined from the relation between shear stress in response to pore water pressure and displacement, can be investigated. It is well known as a phenomenon called sliding-surface liquefaction, which occurs when a landslide has rapid motion due to pore water pressure development related to the creation of particle crushing in the shear zone [23]. Compared with the typical phenomenon of liquefaction, the pore water pressure in sliding-surface liquefaction can be generated after slope failure and resulting from significant particle crushing along the shear surface at a high shear speed. This phenomenon implies that it has the potential for high landslide mobilization. It should also be noted that understanding the transitional mechanism from sliding to flow is very important to mitigate landslide hazards. However, it is still complicated to examine the development of shear stress against strain at the onset of mass movements. In this context, the shear resistance of landslide materials at the initiation stage of landslide occurrence should be determined to examine reliable slope stability.