30 CFR § 816.104 - Backfilling and grading: Thin overburden.
(a) Definition. Thin overburden means insufficient spoil and other waste materials available from the entire permit area to restore the disturbed area to its approximate original contour. Insufficient spoil and other waste materials occur where the overburden thickness times the swell factor, plus the thickness of other available waste materials, is less than the combined thickness of the overburden and coal bed prior to removing the coal, so that after backfilling and grading the surface configuration of the reclaimed area would not:
(1) Closely resemble the surface configuration of the land prior to mining; or
(2) Blend into and complement the drainage pattern of the surrounding terrain.
(2) Meet the requirements of §§ 816.102(a)(2) through (j) of this part.