Creep (geology)

Creep involves small deformations under small stresses acting over long periods of time. The effect of time on rock properties is important in understanding geologic processes as well as deformation and failure. In general, creep results in a decrease in strength and an increase in ductile or plastic flow.

Environmental Factors

Creep is an important geologic process related to rock deformation. It involves small displacements that occur under the influence of small but steady stresses that act over long periods of time. Scientists and engineers involved in experimental rock and soil deformation and the assessment of creep commonly perform stress-strain tests. These experiments are designed to deform earth materials in the laboratory under controlled conditions.

The effect of environmental factors such as surrounding (confining) pressure, temperature, pore-fluid pressure, and strain rate (or time) have been documented through the years based on countless tests. In essence, these factors dictate whether rocks will fracture as brittle substances or whether considerable ductile flow and creep strain will occur prior to rupture. The effect of increasing confining pressure on dry rocks (containing no appreciable amounts of liquid pore fluid) at room temperature is to increase both the ultimate strength and the ductility. Rocks tested under constant confining pressures tend to weaken and become more ductile as temperature increases. An increase in confining pressure on rocks saturated with pore fluids generally results in a decrease in both ultimate strength and ductility. This result is because part of the load (or stress) is carried by the pore fluid and less by grain-to-grain contacts. Decreasing the strain rate (or increasing the time during which the stress is applied) lowers ultimate strength and increases ductility—which basically defines the influence of creep strain on rock properties.

Deformation Stages

The mechanism of creep may be expressed as follows: Rocks subjected to the steady action of small stresses first undergo elastic deformation. After a given period of time, the elastic limit is exceeded. (The elastic limit is the point of no return beyond which deformation is permanent or nonrecoverable.) Following elastic deformation, rocks undergo strain hardening, a phenomenon characterized by a continuous rise in stress with increasing strain because of dislocations moving within individual mineral grains, interfering with one another and causing a literal “traffic jam” at the interatomic level. This initial stage of deformation comprising elastic behavior and strain hardening is termed transient creep; following the transient creep stage, steady-state creep is achieved. During this stage, rocks deform by plastic or ductile flow under a constant strain rate. Deformation mechanisms are characterized by gliding flow (intracrystalline movements) and by recrystallization. Gliding flow may take the form of translation or twin gliding. In translation gliding, layers of atoms slide one interatomic distance or a multiple thereof relative to adjacent layers. The overall mineral grain changes shape, but the interatomic lattice (arrangement) remains unchanged. In twin gliding layers, atoms slide a fraction of an interatomic distance relative to adjacent layers, distorting the interatomic lattice. Recrystallization involves rearrangements of the deforming minerals at the molecular scale through solution and redeposition by local melting or by solid diffusion. A common type of recrystallization occurs by local melting at those grain contacts experiencing the greatest stress and by precipitation (or redeposition) along grain contacts subjected to low stress. Recrystallization can also occur through mixing and rearrangement of the atoms and molecules in mineral grains by “spreading” into each other, analogous to the mixing of gases and liquids through the process of diffusion. Beyond steady-state creep, the final stage, known as accelerated creep, is reached. During accelerated creep, strain rate increases rapidly, ending in rock failure by fracturing or faulting. Deformation mechanisms during this final stage are characterized by cataclasis and the formation of voids or pores. Cataclasis involves the mechanical crushing, granulation, fracturing, and rotation of mineral grains. It results in intergranular movements.

Creep strain is equal to the sum of all the stages of deformation, starting with elastic strain, followed by the transient stage, and culminating with steady-state and accelerated flows prior to failure by rupture. The rate at which creep strain occurs is very sensitive to temperature, with creep rates increasing rapidly as temperature rises. In fact, increasing temperature has been used as an alternative to experiments involving low strain rates or deformations over long periods of time. Increasing temperature or lowering strain rates affects rocks in a similar fashion by decreasing the ultimate strength and increasing the overall ductility.

Laboratory and Field Study

The study of creep is conducted in the laboratory in special experiments under controlled conditions. Environmental factors such as confining pressure, temperature, pore pressure, and strain rate are closely monitored and regulated. Among the methods that have been used to study creep are tension, bending, uniaxial compression, and triaxial compression. Pure tension has been utilized mainly to study creep in metals but has not been common in testing rocks. Bending is a simple method that has been used in creep studies of coal. By far, however, uniaxial and triaxial compression experiments have been utilized most often in testing creep behavior in rocks.

In uniaxial compression, rock samples are loaded with the stress directed vertically. The sample itself is generally unconfined laterally. The vertical or axial load is maintained at a constant level, and percent strain is plotted as a function of time.

In triaxial compression, a rock sample is loaded in a pressure chamber, and an all-around confining pressure is applied. The magnitude of the confining pressure can be significant, simulating pressure conditions expected several kilometers below the earth's surface. An axial or vertical load is then applied and maintained constant. The total stress along the vertical axis of the specimen is the sum of the axial load plus the confining pressure. The deforming stress therefore equates to the axial load. The latter is often referred to as differential stress or deviatoric stress because it is the stress that deviates from the all-around confining (or hydrostatic) pressure. The deviatoric stress is maintained at a constant level until failure occurs. Some pressure vessels are equipped with heating elements so as to increase the surrounding or ambient temperature; others have the additional capability of recording the increase in fluid pressure for samples saturated with pore fluids. Some of the recent designs have the capacity to subject samples to confining pressures of 20 kilobars (20,000 atmospheres), temperatures of 1,000 degrees Celsius, and strain rates as low as 10⁻¹⁰ per second.

Creep tests are not easy to run from a purely mechanical point of view. For example, the choice of magnitude of the deviatoric stress is a matter of difficulty and importance because each experiment may occupy an apparatus for a considerable time. (It is not unusual for creep tests to last for a period of one year.) In addition, if the stress is too low, little effect is produced; if it is too high, failure may occur too quickly. Temperature effects must be closely controlled because they can accelerate creep rates. Also, with many rocks, the absorption of water produces effects similar to creep, so humidity must be monitored and regulated.

Earth and soil creep that may eventually result in landsliding or damage to foundations and retaining structures can be studied in the field and laboratory. Evidence of creep strain along slopes may be detected by direct observation; bent or distorted tree trunks are common indicators. The rate of creep strain is recorded through installation and monitoring, or strain (displacement), gauges. The magnitude of pressures exerted on human-made structures resulting from creep flow can be predicted through laboratory experiments designed to record shear strength and shrink-swell (potential volume change) of soils and argillaceous (clay-rich) rocks. Specialized laboratory experiments simulating pressure-temperature conditions expected in the earth's mantle have been designed to study the effects of creep as a mechanism for releasing stored strain energy resulting in earthquakes.

Petrographic Study

Mechanisms common to creep (such as translation and twin gliding, recrystallization, and cataclastic flow) are routinely documented by studying thin sections of deformed rock specimens using the petrographic (polarizing) microscope and the universal stage. The petrographic microscope differs from a conventional model in that it is equipped with two polarizing elements and other accessories. When both polarizers (or nicols) are engaged, a ray of light transversing a mineral grain is generally refracted into two rays that vibrate in planes at right angles to each other. Analysis of the refraction of these rays makes it possible to identify the types of crystals and rocks that are involved and the nature of the creep process. In contrast, when the lower polarizer is the only one engaged, the light impinging on the mineral grain is plane-polarized. The universal stage allows the rock-forming minerals in the thin sections to be studied at different inclinations from vertical and horizontal axes.

Applications for Structural and Engineering Geology

Understanding creep, or the effect of time on rock properties, is important to the structural geologist studying rock mechanics as well as to the engineering geologist concerned with landslide prediction and control or with the stability of earth-retaining structures. Deformation of earth materials may occur as brittle failures or after considerable ductile or plastic strain has resulted. Intuitively, it is easy to understand rock failure through fracturing or cracking, given that one tends to think of rocks as brittle substances. However, under the influence of high confining pressures, elevated temperatures, and stresses acting over long periods of time, rocks can and do undergo considerable ductile or plastic deformation. Entire mountain chains of visibly folded rock are common throughout the planet.

Studying the process of folding, creep, and rock flowage has a number of practical applications. For example, it is common to find commercial quantities of oil and gas in folded structures known as anticlines. Therefore, understanding how and where rocks fold and which rock types are likely to develop the best porosity and permeability during the process is of critical significance in the search for new petroleum reserves. Similarly, quantifying creep strain and rates is very important in predicting, preventing, and correcting earth hazards such as landslides and in the proper design of foundations and retaining walls. On the subject of slope instability, creep can play a key role. Earth creep, or the slow, imperceptible downslope movement of soil and argillaceous rocks, is the main cause of a specific type of landslide recognized worldwide. In this form of creep, considerable volumes of earth move as the sum of a very large number of minute displacements of individual particles and grains that do not necessarily strain at the same rate. This motion may be caused by the expansion and contraction of clay-rich rocks in response to fluctuations in moisture content, which is especially critical in earth materials containing minerals from the smectite or montmorillonite family that expand considerably when wet and contract when dry. The end result of this creep strain is mass flow or landsliding. Similarly, soil creep can exert enormous stresses on retaining walls and foundations. Pressures exceeding 207,000 kilopascals or 2.1 kilobars (where one bar is basically equivalent to one atmosphere of pressure) have been recorded in north-central Texas.

Finally, creep is important in understanding earthquake mechanisms. Earthquakes are classified as shallow, intermediate, and deep based on their focal depth. Shallow earthquakes have focal depths not exceeding 70 kilometers. Intermediate earthquakes occur within a range of 70-300 kilometers. Deep earthquakes occur between 300 and 700 kilometers. The elastic rebound theory and the brittle failure of rock are accepted as the main mechanisms giving rise to earthquakes—but only of the shallower types, because at depths where intermediate and deep earthquakes occur, the environmental conditions are conducive to ductile behavior. Convection currents in the earth's mantle and the thermal instability of creep have been proposed as the major mechanisms responsible for the deeper earthquakes.

Principal Terms

creep tests: experiments that are conducted to assess the effects of time on rock properties, in which environmental conditions (surrounding pressure, temperature) and the deforming stress are held constant

dislocation: a linear defect or imperfection in the atomic structure (arrangement) of rock-forming minerals; virtually all minerals and crystals contain dislocations

ductility: the rock property that expresses total percent deformation prior to rupture; the maximum strain a rock can endure before it finally fails by fracturing or faulting

elastic deformation: a nonpermanent deformation that disappears when the deforming stress is removed

plastic deformation: a nonrecoverable deformation that does not disappear when the deforming stress is removed

strain: the deformation resulting from the stress, calculated from displacements; it may involve change in volume, shape, or both

strain rate: the rate at which deformation occurs, expressed as percent strain per unit time

stress: the force per unit area acting at any point within a solid body such as rock, calculated from a knowledge of force and area

stress-strain test: a common laboratory test utilized in the study of rock and soil deformation; stress is plotted versus strain throughout the test along the vertical and horizontal axes

ultimate strength: the peak or maximum stress recorded in a stress-strain test

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