PURPOSE: To make an impermeably layer easily inspectable and repairable by boring many parallel tunnels in an adjoining state by means of a tunnel boring shield machine and forming the external walls of the tunnels of inner pipes and external layers packed with a mineral sealant.

CONSTITUTION: Many parallel tunnels are bored in an adjoining state below a dumping area, etc., by using a tunnel boring shield machine. At the time of boring the tunnels, inner pipes 2 composed of steel pipes, polyethylene pipes, etc., are arranged behind the shield machine. Then the annular spaces between the inner pipes 2 and the external walls behind the shield machine are filled up with a mineral sealant 29 composed of viscous earth, bentonite-containing sandy earth, etc. In addition, an impermeable layer is formed composed of a lining element 1 by compacting the sealant with the advancing reaction of the shield machine. Moreover, the groundwater level is lowered by providing perforated rances 4 and, when required, the insides of the pipes 2 are alternately filled up with a sealant 3. Therefore, the sealing effects of the impermeably layer can be inspected and repaired any time. Since the rances 3 are provided, in addition, the cleaning water of the dumping area can be collected and the sealing effect of the impermeably layer can be improved by lowering the groundwater level.