Rock creep, the continuing movement of rock with time, can be a significant engineering problem whenever large loads must be sustained for long durations. As is true in almost all rock engineering phenomena, the creep of rock masses in situ will be governed primarily by the behavior of the discontinuities -- the bedding planes, faults, and in particular, joints. However, nearly all previous research on rock creep has been aimed at determining the form of the creep law for small, unjointed laboratory specimens. While this essential first step has defined the mechanisms and variables for the intact rock case -- which will also be some of the mechanisms and variables for the jointed rock case -- it is not sufficient for the development of a comprehensive quantitative creep model for rock masses.
In this paper, we will briefly review some of the previous research on rock creep and then present some preliminary test data from our own ongoing study of the creep of a jointed rock-like material.
Time-dependent deformations of rock can be broken into two categories: (1) creep (often called "squeezing" in tunneling), a term which in its most general usage refers to any time-dependent rock behavior but which in practice usually connotes time-varying, primarily shear deformations, and (2) consolidation and/ or swelling, which are more restrictive terms referring to purely volumetric time-dependent deformations. These two types of behavior involve fundamentally different mechanisms in the rock. Consolidation/swelling is usually associated with the flow of water out of or into the rock pores. Creep, on the other hand, is primarily the product of time-dependent microfracturing of the rock; if the rock is dilatant, this microfracturing will produce both shear and volumetric strains (Evans and Wood, 1937; Matsuhima, 1960; Wawersik, 1974).
Time-dependent microfracturing is the dominant mechanism in rock creep, but it is not the only one. Robertson (1964) summarizes some of the other, secondary mechanisms: (1) twin and translation gliding in individual mineral crystals, (2) recrystallization, especially at high temperatures, (3) dislocations at grain boundaries, and (4) viscoelasticity of the matrix material in aggregated rocks (e.g., sandstone, shale). The microfracturing process, though, is the mechanism that has been most thoroughly investigated. In one recent study by Kranz (1979), for example, the actual crack growth patterns in granite were observed using a scanning electron microscope. For creep tests under high constant stresses (87% of the unconfined compressive strength)," ... the average length of the cracks increases in time, as does the total stress-induced cracked space, at a rate which parallels the strain measured on the rock surface." (Kranz, 1979). The relationship between microfracturing and rock creep has become so well established that one new constitutive model for rock creep incorporates crack density as an explicit variable (Dragon and Mroz, 1979).
Rock creep is usually expressed as a creep function relating strain or strain rate to time. In its most general form, this creep function can be expressed as (Jaeger and Cook, 1976):