Stress Distribution and Propagation Mechanism of Crack Tip in Directional Fracturing Blasting under the Influence of Free Boundary

Natural rock masses often contain free boundaries, which can interfere with directional fracturing blasting. To investigate effects of free boundary on directional fracturing blasting, the caustics method and high-speed photography were used to study the crack-tip stress distribution and propagation of directional blast-induced cracks. The reflected P/S waves from the free boundary act on a directional blast-induced crack, and change the crack-tip stress distribution and generate an “arc shaped” crack path. Directional blast-induced crack propagation can be divided into three stages. Stage one: before the action of reflected waves, the crack tip is subjected to the action of a blast-induced gas wedge, resulting in a mode Ⅰ crack that propagates along a straight line. Stage two: under the action of reflected waves, both reflected P and S waves cause the crack tip to be subjected to tension and shear action, resulting in a mixed mode Ⅰ-Ⅱ crack which deflects towards the free boundary. Under reflected P waves, the crack tip produces distorted caustics, and crack-tip stress changes from K-dominated field to non-K-dominated field, while under reflected S waves, crack-tip stress returns to K-dominated field. Stage three: after the action of reflected waves, the crack tip is subjected to inertial action and then returns to a mode Ⅰ crack which propagates along a straight line. On the basis of clarifying effects of reflected P/S waves on the tip of directional blast-induced cracks, a calculation formula for the distance between two directional fracturing blasting holes under the influence of free boundary is derived, providing a theoretical basis for refined directional fracturing blasting.