The mechanism of faulting in clay experiments

Abstract

Abstract In a series of experiments a nearly homogeneous strain-rate field was maintained in a body of soft clay paste, deformed continuously and slowly. The strain rate is of the general type, with all three principal strain rates different. In both elongation and compression four families of faults develop in the clay, none of which contain a principal axis of strain, and all of which form small angles with the plane containing the absolutely smallest and the intermediate principal axes of the strain. The array of all four fault-plane families has orthorhombic symmetry, the three mirror planes coinciding with the principal planes of strain. Measurements of the stress show nearly homogeneous stress fields with principal axes of stress parallel to those of the strain, and with the principal stresses proportional to the principal strain rates at any given time. The stresses grow with increasing total strain, at first rapidly, then more slowly, without a corresponding increase in strain rate i.e., the clay strain-hardens. The attitudes of the fault planes, and the directions of slip on them, do not agree with the Coulomb theory of fracture. However, the orientation of the fault planes can be explained if five assumptions about the mechanism of clay deformation are made: 1. (1) The clay is deformed exclusively by slip along fault planes. 2. (2) Moving faults interfere with the mobility of other faults. 3. (3) The magnitude of this interference is independent of the rate of slip, but dependent on the orientation of the mutually interfering faults and of their slip directions. 4. (4) Friction and viscous drag on the fault planes also offer resistance to slip, the former independently of the rate of slip and the latter rate-dependent. 5. (5) The deformation which occurs is one that maximizes the rate of work for the given stress. If these assumptions are accepted, a number of related phenomena also find an explanation. They are: fault patterns in clay experiments with plane-strain rates, or with different ductilities of clay due to different water contents, and the appearance of additional fault systems in late stages of the experiments with general strains. The properties of clay which lead to faulting not explicable by the Coulomb theory can be expected to exist also in a variety of geologic bodies. Therefore the clay experiments serve as model experiments for geologic bodies and the hypothesis of the interference of slip planes can be applied to them.