Experimental study on the confinement-dependent characteristics of a Utah coal considering the anisotropy by cleats

Abstract

Characterizing a coal from an engineering perspective for design of mining excavations is critical in order to prevent fatalities, as underground coal mines are often developed in highly stressed ground conditions. Coal pillar bursts involve the sudden expulsion of coal and rock into the mine opening. These events occur when relatively high stresses in a coal pillar, left for support in underground workings, exceed the pillar’s load capacity causing the pillar to rupture without warning. This process may be influenced by cleating, which is a type of joint system that can be found in coal rock masses. As such, it is important to consider the anisotropy of coal mechanical behavior. Additionally, if coal is expected to fail in a brittle manner, then behavior changes, such as the transition from extensional to shear failure, have to be considered and reflected in the adopted failure criteria. It must be anticipated that a different failure mechanism occurs as the confinement level increases and conditions for tensile failure are prevented or strongly diminished. The anisotropy and confinement dependency of coal behavior previously mentioned merit extensive investigation. In this study, a total of 84 samples obtained from a Utah coal mine were investigated by conducting both unconfined and triaxial compressive tests. The results showed that the confining pressure dictated not only the peak compressive strength but also the brittleness as a function of the major to the minor principal stress ratio. Additionally, an s-shaped brittle failure criterion was fitted to the results, showing the development of confinement-dependent strength. Moreover, these mechanical characteristics were found to be strongly anisotropic, which was associated with the orientation of the cleats relative to the loading direction.

Investigation of the anisotropic confinement-dependent brittleness of a Utah coal

“…From the investigation, the mechanical characteristics were found to be highly anisotropic and controlled by the orientation of the cleats relative to the loading direction. However, the transition from extensional to shear failure at the given confinements was not clearly observed as anticipated based on the s-shaped brittle-failure criterion (Kim et al 2018b). This criterion has clear divisions between extensional failure, transitional failure, and shear failure, and it was our objective to use this criterion to model the coal strength.…”

“…For this study, 14 triaxial tests and 6 UCS tests were performed at Montana Tech in Butte, MT, USA. The purpose of the triaxial tests was to examine the characteristics of brittle/ductile transition in coal, and the UCS tests were used to confirm consistency and comparability with the results from the previous study conducted by Kim et al (2018b). A total of 20 cylindrical specimens were cored from the same Utah coal boulders from which the cores in the previous study were obtained and then tested by Kim et al (2018b).…”

“…Kim et al (2018b) did not clearly observe the transition from extensional to shear failure in specimen failures at the confinements originally considered during testing. The authors presume that this transition was not observed because the original confining stresses as a ratio of the mean UCS (i.e., 1%, 2%, 10% or 50%) used by Kim et al (2018b) might not be suitable to fully characterize the strength characteristics of the coal specimens. Although (Hoek 2007) suggested that the confinement range considered should be at least up to one-half of the mean UCS of a rock, this was based primarily on data from rocks much less brittle than coal, where extensional failures are more easily suppressed by lower levels of confinement.…”

“…Investigation of the anisotropic confinement-dependent brittleness of a Utah coal where r 1 and r 3 are the principal stresses, UCS is the unconfined compressive strength, b' and b are constants, r c is the compressive strength of pillar specimens, UCS is the unconfined compressive strength, and A', B' and a are constants. Using these equations, all the testing data including the results studied by Kim et al 2018b were analyzed to estimate the exponents of confining stress and W/H ratio. The results are shown in Figs.…”

Study on Efficient Utilization Technology of Coal Pillar Based on Gob-Side Entry Driving in a Coal Mine with Great Depth and High Production

“…According to previous studies and theories on the failure of coal pillar structure, there are two modes of failures: tensile failure and compression and shear failure [31]. Quite rare efforts have ever been made on the statistics of dangerous zones caused by the failure of coal pillar structure.…”