Determining coal directional mechanical properties using true triaxial testing facility

Knowledge of coal mechanical properties and strength is critical in modelling and understanding pillar stability, gateroads stability, gas drainage borehole integrity as well as coal responses to hydraulic fracturing stimulation. However, due to the complexity of coal structures and difficulties in obtaining decent coal specimens, measurements of coal mechanical properties have been limited to the application of traditional triaxial and UCS tests, which in turn has shown adverse influence on the design confidence and reliability in practice. In addition, coal is an anisotropic material and such conventional testing techniques are clearly not capable of directly capturing coal anisotropic features. In this work, a true tri-axial testing facility was used to quantify coal strength and its anisotropic characteristics. Eight 50 mm side cube coal blocks were prepared and three types of tests were implemented. The proposed testing procedure measured successfully the mean values of coal young’s moduli in three different x, y and z (vertical) directions as 1,025 MPa, 1,887 MPa, and 2,543 MPa, respectively, which gives the ratio of 1.00: 1.84: 2.48. The mean Poisson’s ratio is also measured as 0.098, 0.038, and 0.091 in x, y and z directions. Coal strength follows the Hoek-Brown criterion reasonably well, and the m value is found to be 23.9. These findings suggest that the implementation of true-triaxial testing techniques for coal mechanical properties can effectively capture its anisotropic characteristics, which could enhance analysis confidence for future designs.