Current through Register Vol. XLI, No. 50, December 13, 2024
Section 38-4-8 - Subsidence Evaluation8.1. A subsidence evaluation of the site and the dam and its storage area will be required considering past and proposed mining. No dams shall be constructed over underground workings or other voids unless those underground workings or other voids have been stabilized or it has been demonstrated that the coal pillars, roofs and floor are strong enough to withstand the superincumbent weight of the strata above the workings or are otherwise capable of preventing significant subsidence impacts in accordance with 8.2 and 8.3 of this rule.8.2. No plan shall be approved unless there is a minimum safe cover to be determined by the Secretary, based on test holes drilled by the applicant in a manner to be prescribed by the Secretary. Such permits shall require in accordance with a plan to be approved by the Secretary, a safety zone be established beneath and adjacent to the impoundment storage area. 8.2.a. Basin. There shall be no underground mining in a safety zone that extends horizontally 200 feet from the high water mark of an impoundment and vertically to a depth that provides for a minimum thickness of 100 feet of solid strata between the bottom of the pool and any mining. The presence of any mine workings within this safety zone is prohibited unless the potential subsidence effects are mitigated by injection grouting or otherwise filling the mine related voids completely. Alternately, such risk can be mitigated by providing a combination of constructed barriers and grouting to establish equivalent distances that will comply with the safety zone dimensions. Coal extraction of 80 percent or more is prohibited unless at a depth greater than 60 times the coal extraction thickness or at a depth where the maximum tensile strain at original ground is less than 5.0 mm/m (0.5%), whichever is greater. The Secretary may impose other limitations as specified by BM IC 8741, barrier analysis, other pertinent analysis or due to conditions such as fracturing, which may require a larger safety zone or further limitations in coal extraction.8.2.b. Embankment. There shall be no mining in a safety zone under the structural embankment measured outward 200 feet in all directions, downward 350 feet and then outward at a dip of 650 from the horizontal, unless acceptable pillar stability and/or strain effects are confirmed by a design evaluation to be certified by an RPE. Also, the related AHCF must clearly demonstrate that the facility will have a low risk of impact to the public and the environment. Existing mine workings within this safety zone having the potential to cause significant subsidence impacts are prohibited unless those effects are mitigated by grouting, filling the mine related voids or providing comparable protection. Additional underground mining may be subsequently approved in the embankment safety zone only if a design evaluation, certified by an RPE, demonstrates that no significant impacts from subsidence can result.8.2.c. Existing Impoundments. Existing impoundments that currently have mining within the safety zones must be evaluated in accordance with this section and 3.4.c. of this rule. Remedial measures shall be implemented as necessary to eliminate or reduce the potentialimpact on the public and/or the environment. Remedial measures may include, but are not limited to, constructed barriers, grouting of underground works and back stowing of mines.8.3. Safety Factor Applicable to new, revised, and existing impoundment facilities a detailed engineering design evaluation of the embankment and impoundment basin areas shall be conducted to assure protection of the environment and public. The engineering design analysis shall demonstrate that appropriate safety factors exist. Major design considerations of this engineering analysis are embankment stability, pillar design, outcrop barrier design, and any other design aspects as necessary to manage risk. The adequacy of calculated safety factors should be determined by applying appropriate regulatory standards. For design applications where regulatory standards do not exist, the AHCF should be the basis used to derive acceptable safety factors.