Dilatancy and pore collapse in rocks play an important role in petroleum and mining engineering problems. Experiments were conducted on rock samples to study the dilatant behavior. An elasto-plastic constitutive model is proposed to describe the shear-volume coupling accurately. Parameter evaluation is carried out using the laboratory test data and a complex optimization procedure. Model predictions are found to be in good agreement with the experimental data. Further extension of this model to predict pore collapse is currently in progress.
In many petroleum and mining engineering problems, an accurate prediction of volume change behavior, especially dilatancy and pore collapse in rocks, plays an important role. Dilatancy can be described as an increase in volume of the material during shearing, and is often accompanied by hardening. When rocks undergo dilatant deformation and the total volume of pore fluids is fixed, pore pressures decrease and effective confining stresses increase. As a result, the rock becomes stronger. However, this increase in strength is only temporary since the pore fluids present in adjacent unstressed rocks start to flow into this dilatant region. In addition, mechanical instability can occur depending on the manner in which newly created pore space varies with shearing (Brace, et al. 1966, Frank 1965). As a result, the ability to be able to predict the onset of dilatancy, its expected magnitude and the corresponding stress levels acquire great significance when dealing with rocks. The theory of plasticity is often employed to describe the strength-deformation behavior of rocks (Desai and Salami 1987, Dragon and Mroz 1979, Faruque and Chang 1986). In the context of incremental plasticity, failure criteria such as the Mohr- Coulomb and the Drucker-Prager are often called upon. Non-associative plasticity and critical state concepts have also been used for rock behavior characterization (Ishikawa, et al. 1982). A majority of earlier plasticity-based models focused on pre- diction of stress-strain response and did not adequately address the volume change behavior (Desai and Siriwardane 1984). This inadequacy is particularly evident in terms of prediction of the onset of dilatancy and post-dilatant behavior. The degree of dilatancy usually decreases with an increase in confining pressure. At very high confining pressures, rocks generally experience only compressive volume change during shearing up to failure. In general, the magnitudes of initial porosity and the confining pressures at which transition from dilatant to compressive mode takes place vary from rock to rock. For a given rock, specific values of these variables must be evaluated experimentally.
Based on the coupled shear-volumetric response of rocks as observed experimentally, two distinct states, termed as characteristic states, can be identified. The first one relates to the state of failure when a rock experiences progressive shear deformation at constant volume (i.e., volumetric strain rate is zero). The second one is identified with the onset of dilatancy at which the rate of volumetric strain momentarily vanishes as the rock passes from the compressive mode of deformation to the dilatant mode of deformation.
