Design of safe, stable mine excavations requires consideration of time-dependent phenomena, a rather neglected area outside salt and potash mines. In this regard, viscoplasticity is the simplest extension of the standard elastic model, limited by rock mass strength, into the time domain. The main consequence ofviscoplasticity is time- dependent displacement, above the elastic limit, that grows with ever increasing strain, although at a diminishing rate. Rounded stair-step displacement versus time curves are similar to those often observed in the field. Such a process
Rock falls, pillar spaIls and floor heave in underground mine openings are very often time-dependent (e.g, Merrill 1952, Obert and Duvall 1967, Pariseau et al. 1984). Time-dependent closure is especially noticeable in salt and potash formations. Slope motion in surface mines is also quite often time-dependent (Zavodni and Broadbent 1980). In some instances the motion develops into catastrophic failure. However, despite much observational evidence for time-dependent behavior of rock masses, surprisingly little has been done to quantify the effects and to improve forecasting of potential instabilities and unsafe conditions. In response, a critical review of viscoplasticity was recently undertaken (Pariseau, 1998). This contribution is a condensed version of that report.
Viscoplastic effects are synonymous with strain-rate effects. Such effects range over orders of magnitude in time from fractions of a second in blast wave dynamics to hundreds of years in creep of geologic media, especially salt. The latter are often described as fluid-like viscoelastic media that have no well-defined elastic limit. Interestingly enough, the original viscoplastic model, proposed by Bingham (1922) and subsequently elaborated upon in continuum mechanics, was motivated by fluid mechanics observations. However, concern here is with extension of what may be called the standard model in rock mechanics, a material limited by strength or possibly stability criteria. Guidance for the validity and limitations of the model is found in laboratory test data for intact rock and mine measurements for rock masses. The objective is to achieve greater realism in engineering design by accounting for time-dependent rock mass behavior and eventually anticipating the development of fast, catastrophic failures. Here, a brief outline of theory is presented first; several test problems are then examined for the consequences of viscoplasticity.
