Underground excavation operations shall not be left in a condition so as to be or become hazardous. Mine shafts, air courses, inclines, or horizontal working temporarily unused or deserted shall be blocked by solid bulkheads constructed of concrete, wood, or steel. A locked manway or door may be installed as part of the bulkhead. Where shafts, air courses, inclines or horizontal working are to be permanently abandoned in accordance with good mining practice, the collar portal to such workings shall be completely blocked by permanent bulkheads constructed of concrete and/or steel or by causing the collapse of solid rock at such collar or portal in such manner as to prohibit the reopening of said workings by natural movement of the collapsed rock by gravity down inclined workings. (Added by Amended Ord. 02-064, Dec. 9, 2002, Eff date Feb. 1, 2003; Amended by Amended Ord. 21-060, Oct. 6, 2021, Eff date Oct. 22, 2021. Formerly 30.31D.150).