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(1) Highwalls must be eliminated and the reduced highwall slope must be no greater than whatever slope is necessary to achieve a minimum long-term static safety factor of 1.3. The department may specify a lesser slope whenever necessary to achieve postmining slope stability. Highwall reduction must be commenced at or beyond the top of the highwall and sloped to the graded spoil bank.

(2) Highwall reduction alternatives may be permitted only to replace bluff features that existed before mining and where the department determines that:

(a) postmining bluffs are compatible with the proposed postmining land use;

(b) postmining bluffs are stable, achieving a minimum long-term static safety factor of 1.3;

(c) similar geometry and function exists between pre- and postmining bluffs;

(d) the horizontal linear extent of postmining bluffs does not exceed that of the premining condition; and

(e) highwalls will be backfilled to the extent that the uppermost mineable coal seam is buried in accordance with ARM 17.24.505(1) .

History: 82-4-204, 82-4-231, MCA; IMP, 82-4-232, MCA; NEW, 1980 MAR p. 725, Eff. 4/1/80; AMD, 1989 MAR p. 30, Eff. 1/13/89; TRANS, from DSL, 1996 MAR p. 3042; AMD, 2004 MAR p. 2548, Eff. 10/22/04.

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