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

• Damage evolution of pre-cracked specimen was analyzed.
• The damage coefficient presented in this paper contains initial damage and loading damage.
• Damage constitutive equation of pre-cracked specimen was established.
• The proposed damage constitutive model is in good agreement with physical experimental data and numerical simulation result.

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 θ, 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 θ < 30°, it is positive, which implies that the specimen is in the state of volume shrinkage. In contrast, when θ > 30°, the peak volumetric strain is negative and the dilatancy occurs. (2) The elastic modulus and the initial damage coefficient show opposite variation trend with θ. Both of the relationships satisfy the Boltzmann function. (3) The peak strength and the initiation stress increase with θ. 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.