Method of joining tubes having a corrugated wall of plastic material

Method of joining first and second double-walled tubes, each having a mating end and a corrugated outer wall forming corrugation ribs and an inner wall joined to said outer wall between said ribs, both walls being made of a fusible plastic material, the outer wall of each tube having a diameter larger than the inner wall of the other tube, comprising the steps of:

• a) cutting each tube through a corrugation rib at the tube mating end to open an annular groove formed between the walls at said corrugation rib;
• b) positioning an annular body expandable when energy is supplied to it, in said annular groove of one of said tubes;
• c) putting the tube ends together to form a closed annular space with said body enclosed therein, such that one portion of the annular body is proximate said outer wall of said first tube, and another portion of the annular body is proximate said inner wall of said second tube;
• and
• d) heating said body to expand the body and to fuse the body to the first and second tubes.