Surrounding Rock Failure Characteristics and Water Inrush Mechanism of Roadway above the Aquifer in Nonuniform Stress Field

The expansion of the roadway surrounding rock failure zone may connect the floor aquifer and cause the roadway water inrush. In order to reveal the mechanism of the roadway water inrush above confined water, we established the force model of the roadway surrounding rock above confined water in nonuniform stress field and studied the shape characteristics and expansion law of the roadway surrounding rock plastic zone. The results show that the roadway surrounding rock will form three kinds of plastic zone under different lateral pressure coefficients: circular, elliptical, and butterfly; when the shape of plastic zone is circular or elliptic, the maximum radius increases linearly with the increase of regional stress; when the shape is butterfly, the maximum radius increases exponentially with the increase of stress. Under the condition of a larger bidirectional stress ratio, the surrounding rock of the roadway will show butterfly-shaped failure, and small stress change will cause malignant expansion of the plastic zone; when the plastic zone is connected with the underlying aquifer, confined water of floor will enter the rock mass from the water diversion point and eventually flood into the roadway, causing floor water inrush.

Elliptic Equation of Plastic Area Boundary around the Circular Laneway in Nonuniform Stress Field

In order to obtain the analytical solution of the plastic area boundary of circular laneway surrounding rock in nonuniform stress field, we studied the evolution of the plastic area shapes of the circular laneway surrounding rock from circular to elliptical and derived the analytical solutions of the boundary radii in the elliptical shape. The results show that (1) with the increase of the confining pressure ratio from 1, the major axis radius of the plastic area increases gradually, the minor axis radius decreases gradually, and the shape of the plastic area gradually evolves from circular to elliptical; (2) on the basis of the Mohr–Coulomb strength criterion, the analytical expressions of major axis and minor axis radii of the elliptical plastic area are derived, and the elliptic equation of the plastic area boundary of circular laneway in nonuniform stress field is established; and (3) the confining pressure ratio is the key factor affecting the shape of the plastic area. When the confining pressure ratio is less than 1.6, the plastic area of the circular laneway surrounding rock is elliptical, and the elliptic boundary equation is applicable. When the confining pressure ratio is greater than 1.6, the plastic area is butterfly shaped, and the elliptic boundary equation is no longer applicable.