Creep deformation of rocks

Strains increase with elapsed time when a constant load, which is smaller than strength, was applied to rocks. The strain increase sometime leads rocks to failure. This phenomenon is so called 'creep phenomenon'. There have been numerous studies paying attentions to the axial strain-time relationship. However, we are paying attentions to the behavior of the circumferential strain. This is because the circumferential strain is sensitive to rock damage but insensitive to environmental conditions, as described in New Criterion for Brittle Rock Based on Principal Tensile Strain.

It was confirmed that the creep strain in circumferential direction is about ten times that in the axial direction. This means that in-situ measurement can be carried out easily and precisely. It was also confirmed that there are the primary, secondary and tertiary creep in the circumferential strain just as in the axial strain.

The circumferential strain rate magnitude decreased first, then increased, leading to the failure. The creep strain in the circumferential direction was so large that the circumferential strain acceleration was able to be evaluated. It began to increase just after the tertiary creep was observed on the strain-time curve, leading to failure.

The circumferential strain values at the points where strain rate magnitude shows the minimum and where acceleration magnitude begins to increase were insensitive to environmental conditions and almost equaled the critical extensile strain in constant strain rate tests. Load increased in the cases where the reloading took place before the distinctive points appeared. The peak load and the circumferential strain value at the peak load point during the reloading equaled those in constant rate tests. On the other hand, load did not increase when reloading took place after the points.

From these points of view, the following three methods were proposed for stability monitoring of rocks.

  • Rock will failure if the absolute value of the circumferential strain rate begins to increase.
  • Rock failure is approaching if the circumferential strain acceleration begins to increase.
  • Rock failures when the circumferential strain reaches the critical extensile strain.