the mine prop prestressing device has a tube , a cup - shaped member inside the tube which has a base and a cylindrical side wall fixed internally to the wall of the tube . a counter - member is also fixed to the tube in opposition to the cup - shaped member , and a valved inlet leads through the wall of the tube for the purpose of introducing a pressurised fluid into the tube between the base of the cup - shaped member and the counter - member . if the fluid is under sufficiently high pressure , the base of the cup - shaped member and the counter member are deformed apart from one another . the counter - member is typically in the form of an identical or similar cup - shaped member . the device is a component part of a mine prop which includes a plunger inserted slidably into the tube so as to locate in a cup - shaped member . when pressurised fluid is introduced into the tube , the plunger is driven telescopically out of the tube .