Except for the approved mine entries, a barrier of solid coal shall be provided around the perimeter of the underground mine area. The barriers shall be: Of sufficient width to withstand the weight of supported overburden without squeezing, crushing, or punching into the floor or roof of the mine; and Designed to withstand anticipated hydrostatic pressure and deterioration over time without failure. Barriers shall be a minimum of 50 feet in width or greater as determined by the formula W = 20 + 4T + 0.1D, when W = width of the barrier, T=thickness of the extraction, and D=depth of overburden. Perimeter barriers shall have a minimum of 50 feet of overburden. Barriers subject to a hydrostatic head shall be at least 1 foot wide for each foot to head. When mining is proposed to be conducted beneath barriers in an existing underground mine, the barriers in the existing upper mine shall be provided subsidence protection sufficient to assure preservation of their design integrity.