Date of Graduation
Statler College of Engineering and Mineral Resources
Keith A. Heasley
Edward M. Sabolsky
Gabriel S. Esterhuizen
In underground coal mines, mining activity disturbs the natural equilibrium state of in-situ stresses. The induced and in-situ stresses deform the rockmass surrounding the mine openings. Primary roof supports impede the deformation of the rockmass overlying the entries. However, failure can occur in the bolted rockmass, causing fatalities and injuries. The rockmass failure is erratic, and its occurrence often varies from a few days to months and years after the opening of the entry. One of the often-neglected factors in this process is the effect of time on the stability of mine openings, which can often be observed in the propagation of failure in the rockmass. Coal-measure rocks have exhibited time-dependent strength reduction in laboratory creep tests. However, the exact mechanism responsible for this time-dependent reduction in strength is not clear. Therefore, this thesis investigated the fundamental reason for macroscopic creep deformation in shale through microscopic visualization using an X-ray computed tomography (CT) technique. An approach was formulated based on the following experiments.
· Conduct compressive strength tests and X-ray powdered diffraction (XRD) analysis on core specimens
· Perform triaxial creep tests at different triaxial stress states
· Determine the geometry of the microcracks using X-ray CT image processing
· Perform triaxial creep and recovery test with X-ray CT scan of shale specimens
The mineralogical analysis, using XRD, showed that the shale contained calcite, as well as a minor percentage of quartz, feldspar, and illite. The uniaxial and triaxial compressive strength tests showed brittle failure, with a high modulus of stiffness and a low Poisson’s ratio in the elastic region. The creep behavior of shale was analyzed under constant triaxial stress. As shale is an anisotropic rock, specimens were tested with both parallel and perpendicular bedding orientations to the major principal stress. The triaxial experiments showed that both types of specimens experienced creep failure; however, the creep deformation was unpredictable. The time to reach creep failure did not show any correlation with the level of applied stress. The results of secondary creep strain rate did not follow Norton’s creep law; the degree of determination (R2) between the logarithm of secondary creep strain rate and logarithm of applied differential stress was significantly small. In addition, the nature of creep deformation in brittle shale was different from other brittle rocks, such as quartz and granite. In both parallel and perpendicular-bedded shale specimens, the increase in confining pressure changed the inelastic volumetric strain, at the onset of the tertiary creep stage, from dilation to the compression. The results of creep experiments proved that shale exhibited creep deformation; however, its nature did not follow the established empirical creep law.
The X-ray CT technique enabled the microscopic visualization of shale to a pixel resolution of 29.9 microns. However, limited research is available on the procedure for X-ray CT image processing. Therefore, a “standard” procedure was developed to analyze the geometry of microcracks in X-ray CT images using an open-source image analysis software, called FIJI. The user-defined macros in FIJI efficiently processed the images and determined the geometry of microcracks, such as volume, aperture, and area of the plane of the microcracks. Further, triaxial creep and recovery tests were conducted on shale specimens and changes in the geometry of microcracks was visualized between the pre- to post-test states using X-ray CT equipment. The laboratory creep and recovery tests showed that the time-dependent strain in shale was a combination of visco-elastic and visco-plastic strain. The X-ray CT scan in the pre-test state showed that shale specimens contained a significant volume of pre-existing microcracks, which varied among specimens. The X-ray CT scan in the post-test state showed that the volume of microcracks after creep and recovery test depends on the time under constant stress, as well as the volume of pre-existing microcracks. The growth of both plane and aperture of microcracks caused the inelastic strain in the axial and radial directions, respectively. The orientation of bedding planes influenced the nature of the change in the geometry of microcracks. Therefore, it was concluded that the microcracking was the fundamental reason for creep deformation in shale.
Gupta, Neel, "Fundamental Mechanism of Time Dependent Failure in Shale" (2019). Graduate Theses, Dissertations, and Problem Reports. 7445.