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
E21D15/18—Telescopic props with parts held together by positive means, with or without relative sliding movement when the prop is subject to excessive pressure with one part resting on a supporting medium, e.g. rubber, sand, bitumen, lead, located in the other part, with or without expulsion or displacement of the medium upon excessive pressure