E21D—SHAFTS; TUNNELS; GALLERIES; LARGE UNDERGROUND CHAMBERS
E21D15/00—Props; Chocks, e.g. made of flexible containers filled with backfilling material
E21D15/14—Telescopic props
E21D15/16—Telescopic props with parts held together by positive means, with or without relative sliding movement when the prop is subject to excessive pressure
E21D15/24—Telescopic props with parts held together by positive means, with or without relative sliding movement when the prop is subject to excessive pressure with axial screw-and-nut, rack-and-worm, or like mechanism
E21D15/26—Telescopic props with parts held together by positive means, with or without relative sliding movement when the prop is subject to excessive pressure with axial screw-and-nut, rack-and-worm, or like mechanism with screw, worm, or the like, not self-locking but normally prevented from rotation by friction members which slip upon excessive pressure