(a) Valley fills shall meet all of the requirements of §12.531 of this title (relating to Disposal of Underground Development Waste and Excess Spoil: General Requirements) and the additional requirements of this section.
(b) The fill shall be designed to attain a long-term static safety factor of 1.5, based upon data obtained from subsurface exploration, geotechnical testing, foundation design, and accepted engineering analyses.
(c) A sub-drainage system for the fill shall be constructed in accordance with the following:
  (1) a system of underdrains constructed of durable rock shall meet the requirements of paragraph (4) of this subsection, and:
    (A) be installed along the natural drainage system;
    (B) extend from the toe to the head of the fill; and
    (C) contain lateral drains to each area of potential drainage or seepage;
  (2) a filter system to ensure the proper functioning of the rock underdrain system shall be designed and constructed using standard geotechnical engineering methods;
  (3) in constructing the underdrains, no more than 10% of the rock may be less than 12 inches in size and no single rock may be larger than 25% of the width of the drain. Rock used in underdrains shall meet the requirements paragraph (4) of this subsection. The minimum size of the main underdrain shall be:
Attached Graphic   (4) underdrains shall consist of nondegradable, non-acid forming and non-toxic forming rock such as natural sand and gravel, sandstone, limestone, or other durable rock that will not slake in water and will be free of coal, clay, or shale.
(d) Underground development waste and excess spoil shall be hauled or conveyed and placed in a controlled manner and concurrently compacted as specified by the Commission, in lifts no greater than four feet, or less if required by the Commission, to:
  (1) achieve the densities designed to ensure mass stability;
  (2) prevent mass movement;
  (3) avoid contamination of the rock underdrain or rock core; and
  (4) prevent formation of voids.
(e) Surface-water runoff from the area above the fill shall be diverted away from the fill and into stabilized diversion channels designed to pass safely the runoff from the 100-year, 24-hour precipitation event, or larger event specified by the Commission. Surface runoff from the fill surface shall be diverted to stabilized channels off the fill which will safely pass runoff from a 100-year, 24-hour precipitation event. Diversion design shall comply with the requirements of §12.511(6) of this title (relating to Hydrologic Balance: Diversions).
(f) The tops of the fill and any terrace constructed to stabilize the face shall be graded no steeper than 20h:1v (5%). The vertical distance between terraces shall not exceed 50 feet.
(g) Drainage shall not be directed over the outslope of the fill.
(h) The outslope of the fill shall not exceed 2h:1v (50%). The Commission may require a flatter slope.

Source Note: The provisions of this §12.532 adopted to be effective April 7, 1997, 22 TexReg 3093; amended to be effective November 4, 1997, 22 TexReg 10640.