Uniaxial and triaxial single and multistage creep tests on coal-measure shale rocks

• Investigate time-dependent progression of cutter-failure in roof of coal mine
• Uniaxial creep tests with single and multistage tests show time-dependent behavior.
• Triaxial multistage creep tests show the effect of confinement on time-dependent strain.
• Results show that the roof rock with shale formation will deform and fail with time.

We investigated the time-dependent behavior of laminated shale that is prone to massive roof falls. One particular compressional failure known as “Cutter” roof failure often transforms into massive roof failure within a few days or months. High horizontal stresses also increase the propensity of these failures. In the past few decades, the effects of high horizontal stresses have been studied using numerical models, and these studies have been successful in providing significant insight into their role. However, the models did not include any time-dependency analysis of the roof rocks. The main explanation for this has been the absence of data on the time-dependent properties of roof rocks. Therefore, a series of uniaxial and triaxial creep tests were performed on shale specimens to investigate whether roof rock deforms with time and how it fails under complex stress conditions. In uniaxial tests, the load applied to specimens varied from 70 to 85% of their uniaxial compressive strength. When the load was 75 and 85% of the strength, the specimens failed instantaneously. The strain rate changed throughout the test period and did not attain a steady state. Results showed that rocks will deform with time and increasing strain will result in potential failure. In triaxial creep tests, both axial and confining stresses were incremented in stages. Finally, results from these tests showed that the rocks under complex stress conditions will also strain with time and will transform into complete failure of the specimen.