Disposal of underground development waste and excess spoil in the head-of-hollow fill shall meet all standards set forth in §12.531 and §12.532 of this title (relating to Disposal of Underground Development Waste and Excess Spoil: General Requirements, and to Disposal of Underground Development Waste and Excess Spoil: Valley Fills) and the additional requirements of this section.
  (1) The fill shall be designed to completely fill the disposal site to the approximate elevation of the ridgeline. A rock-core chimney drain may be utilized instead of the subdrain and surface diversion system required for valley fills. If the crest of the fill is not approximately at the same elevation as the low point of the adjacent ridgeline, the fill must be designed as specified in §12.532 of this title (relating to Disposal of Underground Development Waste and Excess Spoil: Valley Fills), with diversion of runoff around the fill.
  (2) The alternative rock-core chimney drain system shall be designed and incorporated into the construction of head-of-hollow fills as follows:
    (A) the fill shall have, along the vertical projection of the main buried stream channel or rill, a vertical core of durable rock at least 16 feet thick which shall extend from the toe of the fill to the head of the fill, and from the base of the fill to the surface of the fill. A system of lateral rock underdrains shall connect this rock core to each area of potential drainage or seepage in the disposal area. Rocks used in the rock core and underdrains shall meet the requirements of §12.532(2) of this title (relating to Disposal of Underground Development Waste and Excess Spoil: Valley Fills);
    (B) a filter system to ensure the proper functioning of the rock core shall be designed and constructed using standard geotechnical engineering methods; and
    (C) the grading may drain surface water away from the outslope of the fill and toward the rock core. The maximum slope of the top of the fill shall be 33h:1v (3%). Instead of the requirements of §12.531(g) of this title (relating to Disposal of Underground Development Waste and Excess Spoil: General Requirements), a drainage pocket may be maintained at the head of the fill during and after construction, to intercept surface runoff and discharge the runoff through or over the rock drain, if stability of the fill is not impaired. In no case shall this pocket or sump have a potential for impounding more than 10,000 cubic feet of water. Terraces on the fill shall be graded with a 3% to 5% grade toward the fill and a 1% slope toward the rock core.
  (3) The drainage control system shall be capable of safely passing the runoff from a 100-year, 24-hour precipitation event, or larger event specified by the Commission.

Source Note: The provisions of this §12.533 adopted to be effective April 7, 1997, 22 TexReg 3093; amended to be effective November 4, 1997, 22 TexReg 10640.