Abstract: A method and apparatus for closing or sealing the gap space defined between a tunnel support frame and the ground during the tunneling formation of adits in connection with underground mining operations and the construction of underground tunnels, wherein backfill concrete is introduced into the gap space, comprises the disposition of inflatable bags within the gap space so as to be in abutment with the backfill concrete. In this manner, proper and complete backfilling is able to be achieved whereby the cured backfill concrete will exhibit proper structural integrity. The bags may be fabricated from a suitable plastic foil and are provided with an inflating-deflating valve. The bags can be reused upon subsequent support frame sections as the tunneling operation progresses.