## 1 Introduction  
The shear characteristics of the pile-soil interface are influenced by many factors. Many studies have reported that the normal stress, relative density of sandy soil, particle size, structural material, and surface roughness are the main factors affecting the shear characteristics of the pile-soil interface [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. Among these factors, the structural material, which has received much attention from researchers, is an important factor affecting the shear properties of the pile-soil interface. [PERSON] [17] studied soil-structure interfacial shear tests of sandy soil with concrete, steel, and wood, and found that the interfacial friction angle was largest for sand with concrete, second largest for sand with wood and smallest for sand with steel. They concluded that the material difference could be attributed to the different surface roughness values. [PERSON] and [PERSON] [18] and [PERSON] and [PERSON] [19] also conducted soil-structure interfacial shear test studies on sand with concrete, wood, and steel, and verified the conclusions obtained by [PERSON] [17]. Additionally, they claimed that, compared with concrete and wood, the surface of steel was smoother, and thus, the sand-steel interfacial friction angle was the smallest. In fact, different structural materials have significantly different surface hardness values. [PERSON] et al. [20] argued that interfacial shear failure of softer materials was dominated by sliding failure of sandy soil particles along the structural surface when the normal stress was low. With increasing normal stress, interfacial shear sliding failure gradually changed into the plowing of soil particles into the structural surface; that is, there was a significant plowing effect. Hence, the surface hardness of the structure had a significant effect on the interfacial shear characteristics. [PERSON] and [PERSON] [21] investigated shear mechanisms at interfaces between particles and relatively smooth materials using contact mechanics and basic friction theory, and revealed that for softer materials, as the normal stress increased, the contact area between the soil particles and structure surface increased. Soil particles quickly moved into the structure surface during the interfacial shearing process, which increased interfacial shear stress for softer materials. Thus, surface hardness has a significant effect on interfacial shear properties. Furthermore, [PERSON] and [PERSON] [22] and [PERSON] et al. [23] proposed that surface hardness had a significant effect on interfacial shear properties. Obviously, there is a limitation to attributing the change in the peak interfacial friction coefficient caused by the differences in structural material to only the difference in surface roughness. The difference in structural surface hardness is also an important factor affecting the peak interfacial friction coefficient, but previous studies did not clarify the relationship between structural surface hardness and the peak interfacial friction coefficient. Therefore, to clarify the relationship between the peak interfacial friction coefficient and surface hardness, it is necessary to conduct tests on the shear characteristics of the pile-soil interfaces with different surface hardness values. There have been numerous studies on the influence of particle size on the shear characteristics of the pile-soil interface. [PERSON] et al. [24] argued that the increase in particle size essentially increased the relative roughness of the soil-structure interface, which increased the interfacial shear strength. [PERSON] and [PERSON] [25] suggested that the interlocking effect between particles and the plowing effect between particles and structural surface were the main mechanisms leading to an increase of the peak interfacial friction coefficient. [PERSON] and [PERSON] [26] and [PERSON] and [PERSON] [27] reported that the interfacial friction angle or interfacial shear strength increased with increasing particle size. However, [PERSON] et al.