Abstract: A form front positioned between the shield of a tunnel excavator and a tunnel-lining form movable relative to both the shield and the tunnel-lining form. The form front comprises an annular support positioned with clearance space from the shield and the tunnel-lining form, wherein the annular support has a lipped inner seal pressing on the tunnel-lining form and is held thereto by pressure adjacent the inner covering surface of the annular support, and also has an outer seal held under pressure against the shield adjacent the outer covering surface of the annular support.