that the folds fall into each other thereafter the double sheet is pressured from the longitudinal sides so the folds are pressed to abutment against each other and finally the two longitudinal side edges are welded together if there is need for a through channel in the casing for example to provide space for an inlet or a through rod the middle fold of the sheet is made different from the other ones so that this particular fold does not fail into the meeting fold of the other sheet when expanding the expansion body pressurized fluid led inbetween the two casing portions 16 17 which start to separate from each other the separation starts with one fold and successively continues with the other ones until the casing is totally expanded see fig5 and 6 during the separation the folds in the clamped end parts slightly move about the bolt 21 22 axis the folds 18 19 of the opposed portions being moved in opposed directions the cross section of the expanded casing 11 reaches a diameter which increases the same diameter of the non expanded casing 11 by 3 5 times depending on the thickness of the sheet metal and the number and shape of the folds the expansion body can have a number of different applications for example as a pile a spot footing or an anchor body when used as a pile the non expanded body is inserted in a predrilled hole or is directly hammered into the ground by a driving hammer then the body is expanded by filling it up with pressurized.