(1) Secondary containment walls constructed of earth shall be allowed at permanent storage facilities when a liner is used: Provided, That only permanent storage facilities having storage containers of one hundred thousand gallons or greater capacity can use clay liners.

(2) Earthen walls shall have a horizontal to vertical slope of at least three to one, unless a steeper slope is consistent with good engineering practice, and shall be packed and protected from erosion.

(3) The top of earthen walls shall be no less than two feet six inches wide.