Investigation on mechanical properties and damage evolution of coal after hydraulic slotting

Hydraulic slotting technique has been widely adopted in China for underground enhanced coal bed methane (ECBM) recovery. However, the related fundamental research of this technique is insufficient enough to get a comprehensive understanding of its action mechanism. In the current work, numerical models of specimens with various flaw inclination angles are built with particle flow code in two dimensions (PFC2D). The relationship between mechanical properties and flaw inclination angle is analyzed, and the results show that (1) with an increase in the flaw inclination angle theta, the peak axial strain appears a decreasing trend after an initial increase and the peak lateral strain decreases gradually. With regard to the peak volumetric strain, when theta < 30 degrees, it is positive, which implies that the specimen is in the state of volume shrinkage. In contrast, when theta > 30 degrees, the peak volumetric strain is negative and the dilatancy occurs. (2) The elastic modulus and the initial damage coefficient show opposite variation trend with theta. Both of the relationships satisfy the Boltzmann function. (3) The peak strength and the initiation stress increase with theta. Furthermore, the damage constitutive equation of specimens with different flaw inclination angles under uniaxial compression is established with the consideration of the initial damage, loading damage, the stress-strain curves and crack number. The theoretical curves match well with the numerical curves and the achievements reported by other authors. The results of this study are expected to provide crucial guidance for the application of hydraulic slotting in ECBM recovery. (C) 2015 Elsevier B.V. All rights reserved.