## Abstract  
The shear characteristics of the interfaces between soil and concrete structures are essential for the safety of the structures. In this study, a large-scale direct shear test apparatus was developed to measure the mechanical parameters of soil-concrete interfaces under conditions with different soil types, soil moisture contents, and interfacial filling materials. The results showed that the shear stress of the soil-concrete interface increased initially and then became stable with the increase in the shear displacement. The shear displacement of the sandy soil when the shear stress became stable was smaller than that of the claye soil. The silty sand-concrete interface had a smaller friction angle than the interface with the medium-coarse sand. Moreover, with the increase in the soil moisture content, the friction angle of the claye soil-concrete interface decreased rapidly, whereas the cohesion first increased and then decreased, and the peak cohesion was near the plastic limit of the soil. Under the same moisture content, the friction angle and cohesion of the clay-concrete interface was reduced by filling the interface with a thin layer of sandy soil, while filling the silty sand-concrete interface with a thin layer of silt reduced the friction angle and increased the interfacial cohesion. Nonetheless, the filling had little impact on the overall shear strength of the interface.  
soil-concrete interface; shear characteristics; large-scale direct shear test; shear strength 122  
pile foundation using a three-dimensional laser scanner and proposed a morphological evaluation method [8]. [PERSON] et al. (2021) studied the shear softening characteristics and failure load of a soft soil-concrete interface under a constant normal load. Then, a hyperbolic interface model was established based on a direct shear test [1]. [PERSON] et al. (2020) obtained hysteresis and backbone curves under different vertical pressures and shear-displacement amplitudes [9]. [PERSON] et al. (2020) and [PERSON] et al. (2022) proposed that the thermal cycle is also a potential factor affecting the soil-structure interface response [10; 11]. [PERSON] et al. (2020) evaluated the mechanical properties of concrete piles under thermal loads, and the results showed that there was an increase in the pile-soil frictional resistance after each thermal cycle [12]. [PERSON] et al. (2022) studied the behavior of a soil-structure interface under a constant normal stiffness using the discrete element method and proposed an algorithm to achieve a constant normal stiffness [13]. [PERSON] et al. (2019) argued that the interface friction angle could change during pile construction, and an on-site test was the best method to obtain the shear parameters of the soil-structure interfaces [14]. In summary, the above studies yielded many important conclusions; yet, there is still a lack of in-depth research on the influence of various factors, such as the soil type, soil moisture content, and interface filling material, on the shear strength of the soil-structure interface.  
In this study, large-scale direct shear tests were carried out on different types of soil materials, including silty sand, medium-coarse sand, clay, and sandstone, in order to study the influence of the soil moisture content, normal stress, and interface filling material on the shear parameters of the soil-concrete interface.