Block off

1. To fill and seal undesirable openings, fissures, or caving zones in a borehole by cementation or by lining the borehole with pipe or casing. Also called blank off; case off; seal off. Long
2. To secure a mine opening against the flow or escape of gas, air, or liquid by erecting rock, concrete, steel, wood, or cloth barriers. Long
3. To erect barriers to prevent workers from entering unsafe areas in underground workings. Long

Source: Dictionary of Mining, Mineral, and Related Terms