PURPOSE: This rule provides guidelines regarding the integrity, design, construction and maintenance of metallic mineral waste management structures including, but not limited to, tailings dams and slag waste piles. This rule includes dams less than thirty-five feet in height not regulated under the Missouri Dam Safety Act, sections 236.400— 236.500, RSMo (1986).

1. (1) For dams located in metallic minerals 10 CSR 45-8

waste management areas that are constructed after the effective date of this regulation and are less than thirty-five feet (35') in height, the operator shall certify using standards that are currently acceptable and available to the engineering profession that the structures meet the requirements for spillway capacity, slope stability, correction of observable defects, maintenance, and inspection in order to ensure the continued integrity of the structure.

1. (2) For dams located in metallic minerals waste management areas that were constructed before the effective date of this regulation and are less than thirty-five feet (35') in height, the operator shall certify using standards that are currently acceptable and available to the engineering profession that the structures meet the requirements for spillway capacity, correction of observable defects, and maintenance and inspection.
2. (3) Sloped faces of slag waste piles or other waste management control structures shall be maintained according to the lines and gradients shown on the approved permit application. Any slope failures, as evidenced by scarp formation, sloughing, bulging, or other indications, shall be reported to the director in writing within ten (10) days of the time when the failure is first noticed. Upon review of the failure, the director will determine what corrective action is to be taken. Corrective action may include repair and stabilization of the failed area.
3. (4) Sloped faces that experience erosion shall be repaired by the operator on an on-going basis. The operator shall keep a record of all these repairs and make these records available to the director upon request. No repairs shall be made that would result in significant deviances from the lines and grades shown on the approved permit application without written approval of these repairs by the director. Areas that experience recurring erosion may require special erosion control measures, such as application of revetment materials, regrading, and so forth. The director and operator will determine the need for these measures during the review of the closure plan. The operator shall prepare plans and specifications for measures in accordance with practices reputable and appropriate in the engineering, geologic, and construction professions. A copy of these plans and specifications will be provided to the director for review. The operator shall not begin construction of erosion control measures without written approval of that work from the director.
4. (5) The operator shall provide a judgment of the effect of subsidence and earthquake loads on the long-term stability and integrity of all tailings dams, slag piles, and other waste management control structures located within the boundary of the waste management area. The judgment shall be based upon engineering analysis and experience in accordance with practices reputable and in current use in the engineering and geologic professions. The operator and director shall determine the need for remedial measures to counteract the effects of potential subsidence.

AUTHORITY: sections 444.362 and 444.380, RSMo 2016.* Original rule filed Oct. 2, 1990, effective April 29, 1991. Amended: Filed March 26, 2018, effective Nov. 30, 2018.

*Original authority: 444.362, RSMo 1989 and 444.380, RSMo 1989, amended 1993, 1995.