Rock Creep Test: Why Even Strong Rock Can Fail Over Time

Rock looks permanent. A solid core sample can feel almost impossible to break, making it tempting to assume that a rock formation will remain stable indefinitely.

However, strength measured today does not explain everything about tomorrow. Under sustained stress, rock can gradually deform. That movement may be difficult to notice initially, yet it can matter greatly for structures expected to operate for decades.

A Rock Creep Test investigates this behaviour by measuring deformation over time under controlled loading conditions.

In this Avantech guide, we explain what the test measures, how it works, and which questions engineers and laboratory teams should ask before using the results.

What Is a Rock Creep Test?

A Rock Creep Test is a laboratory test that measures how a rock specimen deforms over time under sustained stress. The specimen is monitored while selected conditions, such as temperature and confinement, are controlled.

The key result is a relationship between strain and elapsed time. Strain describes deformation relative to the specimen’s original dimensions.

For example, a specimen may shorten immediately when loaded, then continue shortening gradually while the applied stress remains unchanged. That continuing movement is the behaviour creep testing investigates.

ISRM publishes suggested methods specifically addressing the creep characteristics of rock. These provide a recognised methodological reference for planning appropriate investigations. 

Why Does Rock Creep Matter?

A structure can experience sustained loading throughout much of its working life. Understanding how the surrounding material responds over time helps engineers assess whether deformation could become operationally significant.

Consider a tunnel. Even when excavation is complete, surrounding rock remains stressed. Gradual movement may affect available clearance, support interaction, or maintenance requirements.

Laboratory creep results can contribute quantitative information to long-term stability assessments. However, an intact laboratory specimen does not reproduce an entire rock mass. Natural joints, faults, weak layers, and geological variability require separate consideration. 

The practical question is therefore broader than whether a sample survives loading: how does its deformation develop, and what does that mean for the project?

Rock Creep Test vs Compressive Strength Test

These tests answer different engineering questions.

Knowing compressive strength helps establish a useful material baseline. Creep testing adds information about the role of time.

A testing programme may need both, depending on its objectives.

What Are the Three Stages of Rock Creep?

Creep behaviour is commonly described using three stages. These provide a useful framework, although every experiment will not display three clearly separated regions.

1. Primary Creep

During primary creep, deformation continues but its rate generally decreases.

On a strain–time plot, the curve becomes progressively less steep. The specimen is still changing shape, but more slowly.

2. Secondary Creep

Secondary creep is commonly associated with an approximately steady deformation rate.

The corresponding part of the curve may appear nearly straight. This does not mean deformation has stopped; it means that strain is accumulating at a broadly consistent rate.

3. Tertiary Creep

During tertiary creep, deformation accelerates and may progress towards failure.

The strain–time curve steepens as movement develops more rapidly.

Whether these stages appear depends on the material and test conditions. A specimen tested at sufficiently low stress may show slowing deformation without reaching tertiary creep during the experiment. 

How Is a Rock Creep Test Performed?

The detailed procedure depends on the chosen method and project requirements. A typical laboratory workflow includes the following steps.

Step 1: Define the Testing Objective

Start by identifying the decision the results must support.

Is the investigation comparing different rock types, examining temperature effects, or developing parameters for a numerical model? A clear objective helps determine the specimen requirements, loading conditions, and observation period.

Step 2: Prepare and Document the Specimen

The laboratory prepares suitable specimens and records their dimensions, condition, and identification.

Sample orientation matters when the material contains bedding or other directional features. Recording this information makes comparisons easier to interpret.

Step 3: Install the Measurement System

The specimen is positioned in the loading arrangement, and suitable instruments are installed.

The measurement approach should distinguish specimen deformation from movement elsewhere in the equipment wherever practicable.

Step 4: Apply and Maintain Loading

Loading is applied according to the selected procedure.

During the observation period, the laboratory monitors whether the intended loading conditions remain stable. Unexpected changes must be recorded because they can influence interpretation.

Step 5: Record and Review the Results

Measurements are collected throughout the test.

The laboratory examines the deformation history, checks the data for irregularities, and reports the conditions under which the results were obtained. Agreed stopping criteria should be documented before testing begins.

What Equipment Is Needed?

A useful creep-testing arrangement combines loading control, sensitive measurement, and dependable recording.

Depending on the programme, it may include:

• A suitable loading frame and loading system.
• Load measurement and control equipment.
• Axial displacement or strain instruments.
• Lateral deformation measurement where required.
• A data acquisition system.
• Temperature monitoring or environmental control.
• A pressure cell for tests requiring confinement.

Equipment selection should follow the measurement problem.

For instance, a system with adequate load capacity may still be unsuitable if it cannot resolve the small movements expected during the experiment. Laboratories should also consider instrument drift, calibration, recording reliability, and access for maintenance.

What Factors Affect Rock Creep Behaviour?

Rock type alone does not determine the result. Loading and environmental conditions also influence how deformation develops.

Research identifies stress state, confining pressure, pore pressure, temperature, water content, and the rock’s composition and structure as relevant factors. 

When comparing results, ask:

• Were specimens taken from comparable material?
• Were their orientations consistent?
• Were moisture conditions documented?
• Were loading and temperature histories comparable?
• Were the same measurement methods used?

A difference between two curves may reflect different test conditions rather than an underlying material difference.

For this reason, the supporting test records are as valuable as the final graph.

How Long Does a Rock Creep Test Take?

There is no single duration suitable for every Rock Creep Test.

The observation period should reflect the material, loading conditions, study objective, and selected procedure.

A programme designed to investigate relatively rapid deformation will differ from one intended to resolve slow movement. Laboratory availability also matters, since a specimen may occupy a testing system for an extended period.

Before starting, agree on the intended duration, recording schedule, stopping criteria, and response to interruptions.

Stopping a test before failure provides information about the observed period. It does not establish that failure could never occur later.

How Should Rock Creep Results Be Interpreted?

Start with the strain–time curve, then examine the conditions behind it.

Useful questions include:

• Is deformation slowing, approximately steady, or accelerating?
• Are changes associated with loading or temperature disturbances?
• Is the measured movement larger than the instrument’s uncertainty?
• Do repeat specimens show a comparable pattern?

Researchers also examine strain rate when investigating long-term behaviour. However, natural variability and experimental conditions complicate predictions of long-term strength and time to failure. 

A mathematical model may help describe the observations. Its usefulness depends on whether the model represents the tested behaviour and whether its intended application is justified.

A smooth fitted curve alone is insufficient evidence of a reliable prediction.

Which Standards Apply to Rock Creep Testing?

The specification should identify the agreed method and edition.

ISRM’s suggested methods include guidance on determining rock creep characteristics. ASTM D7070-16 is another reference readers may encounter, but ASTM currently lists it as withdrawn in 2025, with no replacement shown on its product page. It should therefore not be described as a current active standard. 

For a new programme, confirm which procedure the client, laboratory, and responsible engineer accept.

Where a historical method is contractually specified, clarify its status and any agreed departures before testing.

What Should a Useful Test Report Include?

A report should let another qualified reader understand what was tested and how.

Request specimen identification, dimensions, orientation, conditioning, equipment details, calibration information, loading history, environmental records, test duration, and strain–time plots.

The report should also describe interruptions, unusual observations, calculations, and limitations.

For Avantech enquiries, prepare these requirements before discussing equipment or a testing configuration. A clear specification supports a more productive conversation than requesting a machine based on load capacity alone.

Frequently Asked Questions

Can strong rock still experience creep?

Yes. A high short-term strength does not eliminate the possibility of time-dependent deformation. Behaviour depends on the material and conditions.

Is creep the same as stress relaxation?

No. Creep concerns deformation developing under sustained stress. Stress relaxation concerns stress decreasing while deformation is held approximately constant.

Does every creep test end in failure?

No. A test may stop after an agreed observation period or another defined criterion without specimen failure.

Can one specimen represent an entire site?

One specimen provides limited evidence. A representative investigation must consider geological variation and the project’s wider ground model.

What should I discuss with Avantech first?

Share the rock type, specimen dimensions, intended loading, environmental conditions, measurement needs, and required procedure so the enquiry has a clear technical basis.