and the point 22 where they meet the surface 18 helps resist net forces in the direction of arrow 28 and the surface 20 resists forces in the direction of arrow 30 the plug 16 can be a shape memory polymer that tends to get softer when swelling to a larger dimension and needs fixation assistance that the projection 14 can provide other materials that swell can also be used other alternatives are swelling rubber or foams or retained mineral clays such as bentonite while two surfaces 18 and 20 that intersect have been illustrated either surface can be omitted so that fixation boost occurs in a single preferred direction rather than opposed directions while surfaces 18 and 20 are shown flat they can be rounded or irregular and can also feature a surface roughness to aid retention of the plug 16 instead of the projection 14 the inside wall 12 can be surface roughened to enhance the grip of the plug 16 on the inside diameter or wall 12 alternatively the projection 14 can be flipped so it is a recess while still being subject to the alternatives described above when it is in the form of a recess depending on the length of the plug 16 and the surfaces 18.