Abstract: To investigate the structure transformation and permeability evolution law of coal seam under the effect of solid-liquid coupling and seepage water pressure (hydraulic gradient), the permeability test of coal during the overall stress-strain process has been carried out on MTS 815.02 electro-hydraulic servo rock mechanics test system with the method of transient pressure pulse. The test results show that the coal permeability is closely related to its internal structure, and the permeability evolution is consistent with its variation tendency of internal fractures in the overall stress-strain process. When the strain softening is 24.98 percent of the peak strength, the coal permeability reaches to 15.69×10-13cm/s, which is 79.30 and 16.34 times of that in elastic stage and peak value, respectively. When the solid-liquid coupling doesn’t produce damage, the transient permeability coefficient is proportional to hydraulic gradient on the whole, and the seepage water pressure can cause dislocation and closure to structural planes or make rupture debris gather and block up seepage channel. After the deformation of coal sample enters into elastic-plastic stage, the transient permeability coefficient first decreases with time on the whole and increases rapidly to a certain peak value after it decreases to a certain value, then, it gradually decreases again and tends to be stable. In addition, when the coal sample goes from elas-tic-plastic stage, plastic stage to failure (residual strength) stage, the difference between the mutations peak and initial transient permeability coefficient becomes smaller and smaller, and the emergence of middle mutations peak needs longer and longer time. Moreover, by using transient pressure pulse method to measure and evaluate the rock permeability, the data should be adopted when the upper and lower water pressure difference becomes stable at the first time, to calculate the permeability.

Key words: coal mass, solid-liquid coupling, structure, hydraulic gradient, permeability coefficient