Scale effect in shaft friction from the direct shear interface tests

The thickness of the shear band mobilised on the pile shaft subjected to loading is practically the same for a small model and a large-diameter prototype. At the same normal stress applied to the shaft of the model and of the prototype pile, the normal stiffness of the interface and so the shear conditions are different for both cases. Lateral friction measured for the model and the prototype will differ. This scale effect in shaft friction related to the ratio of the diameter of the model to a mean grain size is studied with the direct shear interface tests, where the pile diameter is modelled with a constant normal stiffness applied to the shear box. The scale effect whose value is lower than unity is obtained for the dilative soil behaviour within the interface. The value of the scale effect smaller than unity is deduced from the direct shear interface tests with the soil contractancy.