The shear strength of the soil-rock mixture-bedrock interface is a key parameter for the stability of the slope and engineering design. In order to explore the influence of the block stone size on the shear mechanical behavior of the soil-rock mixture-bedrock interface

a series of large-scale laboratory shear tests of the soil-rock mixture-bedrock interface with different block stone sizes was carried out. The results show that the failure mode of the contact interface is not significantly affected by the size of the block stone and the normal pressure

and both are characterized by strain hardening. However

the normal pressure will weaken the impact of the block stone size effect. As the size of the block stone increases

the shear strength and the shear strength index (

c) first increased and then decreased. There are both a positive size effect and a negative size effect. The size effect has a limited influence on the angle of internal friction

which fluctuates around 29º. However

the size effect has a significant influence on the apparent cohesion c. The specimens with different block sizes show obvious shear contraction behavior under different normal pressures

and the normal pressure can enhance the shear contraction characteristics of the soil-rock mixture. The soil-rock mixture-bedrock interface is a weak potential slip surface. With the increase of the block size coefficient

the weak failure surface tends to transform into the soil-rock mixture.