In accomplishing Sustainable Development Goals (SDGs), the construction of roads played a substantial role in achieving economic equity. However, landslides due to problematic soil would hinder its construction process. Thus, it is essential to understand the mechanism of slope movement to reduce landslide problems. A landslide that happened during the construction of the Semarang-Bawen toll road was analyzed in this research. The landslide was known to fail between the interface of the tuff breccia overburden and the problematic clay shale soil. This research proposed the direct shear test to determine the interface shearing behavior. Before the test, the overburden was differentiated as various sand and weathered clay shale ratios. After the overburden soils were compacted, a multistage interface direct shear test was conducted with three different loadings. Water was added to the overburden layer to model the infiltration at the interface by increasing the water content. From the test, results such as the interface cohesion, friction angle, and average stress ratio were obtained. Overall, the interface shear strength decreased as the water content increased. The decreasing value was due to the wetting at the interface. Thus, it would moisten the interface and disrupt the structure of both the top and bottom layers of the sample. In conclusion, the interface direct shear test was able to describe the shear behavior at the clay shale interface. It also indicated that water had a considerable role in triggering interface landslides for two different soil layers. © 2024 Penerbit UTM Press.