(1) Disposal of underground development waste in valley fills must meet all the requirements of ARM 17.24.924, and the following additional requirements of this rule.

(2) The fill must be designed to attain a long-term static safety factor of 1.5 and the design must be based upon data obtained from subsurface prospecting, geotechnical testing, foundation design, and accepted engineering analyses.

(3) Underground development waste must be hauled or conveyed and placed in a controlled manner and concurrently compacted as specified by the department, in lifts no greater than 4 feet, or less if required by the department, to:

(a) achieve densities designed to ensure mass stability;

(b) prevent mass movement;

(c) avoid contamination of the rock underdrain; and

(d) prevent formation of voids.

(4) The top of the fill must be graded no steeper than 1v:5h, unless otherwise approved in writing by the department.

(5) To control surface runoff, each terrace bench must be graded to a slope of 1v:20h toward the fill.   A ditch must be constructed on the inside of each terrace to intercept runoff and divert it toward the channels specified in ARM 17.24.924(16).