A model of a fault in the Earth is a sand-filled sawcut in a granite cylinder subjected to a triaxial test. The sawcut is inclined at an angle a to the cylinder axis, and the sand filling is intended to represent gouge. The triaxial test subjects the granite cylinder to a constant confining pressure and increasing axial stress to maintain a constant rate of shortening of the cylinder. The required axial stress increases at a decreasing rate to a maximum, beyond which a roughly constant axial stress is sufficient to maintain the constant rate of shortening. Such triaxial tests were run for sawcuts inclined at angles a of 20°, 25°, 30°, 35°, 40°, 45°, and 50° to the cylinder axis, and the apparent coefficient of friction ma (ratio of the shear stress to the normal stress, both stresses resolved onto the sawcut) at failure determined. Subject to the assumption that the observed failure involves slip on Coulomb shears (orientation unspecified), the orientation of the principal compression axis within the gouge can be calculated as a function of ma on the sawcut for a given value of the coefficient of internal friction mi. The rotation of the principal stress axes within the gouge in a triaxial test can then be followed as the shear strain across the gouge layer increases. For mi 0.8, and appropriate value for highly sheared sand, the observed values ma imply that the principal axis of compression within the gouge rotates so as to approach being parallel to the cylinder axis for all sawcut angles (20°< a < 50°). In the limiting state (principal compression axis parallel to cylinder axis) the stress state in the gouge layer would be the same as in the granite cylinder, and the failure criterion would be independent of the sawcut angle.
AGU Index Terms: 5104 Fracture and flow; 8160 Rheology-general; 8140
Keywords/Free Terms: Coulomb plasticity, apparent friction, fault models.
JGR-Solid Earth 96JB02094