Scaling of rock friction constitutive parameters: The effects of surface roughness and cumulative offset on friction of gabbro

We describe experiments in which large (14×40 cm nominal contact area) blocks of gabbro were sheared in a direct shear apparatus at room temperature, 5 MPa normal stress, and slip velocities from 0.1 to 10 μm/s. The apparatus was servocontrolled using a displacement feedback measurement made directly between the gabbro blocks. Two surface roughnesses were studied (rough, produced by sandblasting, and smooth, produced by lapping with #60 grit) and accumulated displacements reached 60 mm. Measurements of surface topography were used to characterize roughness and asperity dimensions. Step changes in loading velocity were used to interrogate friction constitutive properties. Both rough and smooth surfaces showed appreciable displacement hardening. The coefficient of friction μ for rough surfaces was about 0.45 for initial slip and 0.7 after sliding 50 mm. Smooth surfaces exhibited higher μ and a greater tendency for unstable slip. The velocity dependence of friction a-b and the characteristic friction distance D_c show systematic variations with accumulated displacement. For rough surfaces a-b started out positive and became negative after about 50 mm displacement and D_c increased from 1 to 4 μm over the same interval. For smooth surfaces, a-b began negative and decreased slightly with displacement and D_c was about 2 μm, independent of displacement. For displacements <30 mm, rough surfaces exhibit a second state variable with characteristic distance about 20 μm. The decrease in a-b with displacement is associated with disappearance of the second state variable. Our data indicate that D_c is controlled by surface roughness in a complex way, including but not limited to the effect of roughness on contact junction dimensions for bare rock surfaces. The data show that simple descriptions of roughness, such as rms and peak-to-trough, are not sufficient to infer D_c. Our observations are consistent with a model in which D_c scales with gouge thickness.