100. Reinforced concrete slabs used to seal off the access to a mine shall have the following characteristics:
(1) where they are cast in place, be equipped with an opening of 100 mm in diameter topped with a metal tube 1 m high curved downward allowing ventilation;
(2) comply with or be equivalent to the standard specified in the plan appearing in Schedule III and have a minimum thickness of 150 mm where the concrete used in their fabrication has a strength of at least 30 MPa and neither the opening nor the slab have a width exceeding 1.50 m;
(3) be equipped with an engraved inscription indicating the year of their fabrication and the name of the mine where they are installed.
Reinforced concrete slabs must be proportionately thicker than what is prescribed under subparagraph 2 of the first paragraph in order to give them strength equivalent to that prescribed in that subparagraph where the concrete used in their fabrication has a strength of less than 30 MPa or when the width of the opening or slab used is greater than 1.50 m.