Abstract: The water injection into coal seam is an effective means of increasing and discharging pressure during coal mining. The sealing length is a key factor affecting the effect of water injection fracturing. To determine the reasonable sealing length, six different elastic constitutive equations were selected according to the basic theory of Lippmann coal seam instability. The expression of nonlinear dynamic gas pressure distribution was introduced, and the mechanical theory model of gas-containing coal seam was constructed. Accordingly, this study obtained the formulas for the vertical, horizontal stress and plastic zone width of coal seam. Combined with the Gaojiapu coal mine, this study compared and analysed the stress distribution curves and the plastic zone widths of the coal seam calculated by three elastic constitutive equations. Moreover, the field sealing experiment was carried out. The results show that the hole leakage is small and not abnormal when the sealing hole length is 9.5 m, which indicates the sealing effect is good and a reasonable sealing length is 9.5 m. Compared with the other three elastic constitutive equations, the calculated results by i, v and vi elastic constitutive equations well confirm with the actual length of the sealing.

Key words: water injection, sealing length, elastic constitutive equation, plastic zone width