By modified DRS-I high pressure direct shear test apparatus

interface shear tests of gravel and different structural planes under medium and high pressures are carried out. With different structural planes having different peak-valley distances H

relative scale R is defined as the ratio of H to average particle size ( ). Effects of relative scale R on mechanical and deformation characteristics of interface shear process under medium and high pressures are studied. The test results show that the relative scale R affects the interface shear energy consumption and shear volume deformation directly; and influential rules are different between medium pressure and high pressure. Whether under medium or high pressure

there exist limited and critical R respectively described as and Before

the increase of R results in monotonous change of interface shear energy consumption and shear volume deformation. After

the increase of R will cause shear energy consumption and shear volume deformation meeting a critical state

and the corresponding relative scale is . For above rules closely relate to particle displacement and grain breakage

the limited and critical values of R give new thoughts to research the roughness which affected by relative scale of particles and structural planes under different pressures; and it also has some significance for describing the mechanics and deformation mechanism of interface under different pressures.