Devices with pistons, plungers, or the like, pressed locally against the wall of the borehole

Full Title

Other methods or devices for dislodging with or without loading (breaking-down by means inserted in slits E21C27/14) > by making use of hydraulic or pneumatic pressure in a borehole > Devices with pistons, plungers, or the like, pressed locally against the wall of the borehole