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 x 40 cm nominal contact are a) blocks of gabbro were sheared in a direct shear apparatus at room t emperature, 5 MPa normal stress, and slip velocities from 0.1 to 10 mu m/s. The apparatus was servocontrolled using a displacement feedback measurement made directly between the gabbro blocks. Two surface rough nesses were studied (rough, produced by sandblasting, and smooth, prod uced by lapping with #60 grit) and accumulated displacements reached 6 0 mm. Measurements of surface topography were used to characterize rou ghness and asperity dimensions. Step changes in loading velocity were used to interrogate friction constitutive properties. Both rough and s mooth surfaces showed appreciable displacement hardening. The coeffici ent of friction mu for rough surfaces was about 0.45 for initial slip and 0.7 after sliding 50 mm. Smooth surfaces exhibited higher mu and a greater tendency for unstable slip. The velocity dependence of fricti on a-b and the characteristic friction distance D-c show systematic va riations with accumulated displacement. For rough surfaces a-b started oat positive and became negative after about 50 mm displacement and D -c increased from 1 to 4 mu m over the same interval. For smooth surfa ces, a-b began negative and decreased slightly with displacement and D -c was about 2 mu m, independent of displacement. For displacements <3 0 mm, rough surfaces exhibit a second state variable with characterist ic distance about. 20 pm. The decrease in a-b with displacement is ass ociated with disappearance of the second state variable. Our data indi cate that D-c is controlled by surface roughness in a complex way, inc luding but not limited to the effect of roughness on contact junction dimensions for bare rock surfaces. The data show that simple descripti ons of roughness, such as rms and peak-to-trough, are not sufficient t o infer D-c. Our observations are consistent with a model in which D-c scales with gouge thickness.