§ 33.127. Permanent air stoppages.

 When it becomes necessary to permanently close a section underground, all crosscuts connecting the main inlet and outlet air passages shall be substantially closed with brick, hollow tile, concrete or stone set in mortar. If brick or tile is used, the same shall be laid in cement mortar wherever practicable. Under damp conditions, if ordinary mortar would not harden, special types of under water hardening cement shall be used. In no case shall the above air stops be constructed of wood or brattice cloth. Wood or brattice cloth shall only be used for temporary stoppings.