U S Patent 4,394,051, 1983 Jul; :1-5
A method of breaking rock from a free surface which uses hydrofracturing to induce rock failure. Initially, a hole is cut in the rock face to a depth suitable for spalling by a high pressure water jet drill. Next, at the bottom of this hole a thin circular slot is hydraulically cut into the rock. The slot's circular axis is cut parallel to the transverse axis of the hole and the slot is made larger than the hole diameter. Following this step, a high pressure packer, with a high pressure tube passing through its center, is inserted into the drill hole. This packer is placed near the bottom of the hole above the slot and inflated. A fluid, like water, under high pressure is pumped down the hole past the packer into the slotted area. This high pressure fluid initiates a tensile fracture in the rock at the circular periphery of the slot. Tension is induced in the rock at this peripheral location due to the small radius of curvature existing there. This circular tensile fracture propagates outward away from the drill hole and upward to the free rock surface. After the rock fragment is broken free, pressure is released from the packer and it is withdrawn from the hole letting the fragment drop. To advance through the rock, the process is continuously repeated with the high pressure fluid being applied to the slotted area over a very short time period.