Section 045. Mine Tailings Impoundment Structures Design Criteria (Rule 45)

The following minimum design criteria shall be used for all mine tailings impoundment structures designed fo r installation in Idaho. These limitations are intended to serve as guidelines for a broad range of circumstances, and engineers should not consider them as a restriction to the use of other sound design criteria. Deviation from thi s established criteria will be considered by the Director in approving plans and specifications. (3-30-23) 01. Embankment Slopes. (3-30-23) a. For construction of borrowed fill embankments, in the absence of a stability analysis, the slope s shall be: Upstream slope 2:1 or flatter Downstream slope 2:1 or flatter (3-30-23) b. Construction by the upstream method shall not be used in the area of the state east of Range 22 E. , Boise Meridian, unless the engineer can provide evidence that the construction and operation of the tailing s impoundment will achieve a relative density of sixty percent (60%) or greater in the embankment and tailings t o prevent liquefaction during earthquake loading. (3-30-23) c. Safety factors for the embankment shall be at least one and five-tenths (1.5) for static loads and a minimum of one (1) for the static plus the appropriate earthquake load. (3-30-23) d. To insure sufficient permeability and stability of the embankment, designs will require utilizin g materials other than the tailings, when the tailings materials: (3-30-23) i. Contain greater than seventy-five percent (75%) passing the #200 standard U.S. sieve, or fift y percent (50%) passing the #325 standard U.S. sieve; (3-30-23) ii. Contain phosphate clays; (3-30-23) iii. The design calls for the water to be impounded against the embankment; (3-30-23) iv. Have other properties which makes them unsuitable for use as construction materials. (3-30-23) e

ls: (3-30-23) i. Contain greater than seventy-five percent (75%) passing the #200 standard U.S. sieve, or fift y percent (50%) passing the #325 standard U.S. sieve; (3-30-23) ii. Contain phosphate clays; (3-30-23) iii. The design calls for the water to be impounded against the embankment; (3-30-23) iv. Have other properties which makes them unsuitable for use as construction materials. (3-30-23) e. Embankments designed for the storage of hazardous levels of radioactive materials shall, i n addition to any requirements of these regulations, meet the criteria outlined in the Nuclear Regulatory Commissio n Regulatory Guide 3.11 and the Idaho Radiation Control Regulations administered by the Idaho Department o f Environmental Quality. (3-30-23) f. The design shall consider the need for drains and/or operational procedures to promote consolidation and insure that a low phreatic surface is maintained within the embankment. Drainage pipe shall not be used beneath embankments where excessive or differential settlement may cause failure of the pipes and subsequen t piping of the tailings or embankment. When the quality of the mine tailings slurry is such that it will adversely affec t the quality of the existing ground water, the design should be coordinated with the Department and the Department o f Environmental Quality to insure that all applicable permits are obtained. (3-30-23) g. Instrumentation of the embankment and/or foundation will be required to insure that the structure i s functioning satisfactorily. Standpipe piezometers with an inside diameter greater than one-half (1/2) inch will not b e acceptable for use in fine-grained or cohesive soils in order to minimize response time. (3-30-23) h. Tailings impoundment structures which are constructed using the tailings shall not be constructe d or raised during freezing weather to prevent frost lenses in the embankment. Sufficient freeboard must be provided during the summer construction season if the disposal operation is to continue during the winter

or use in fine-grained or cohesive soils in order to minimize response time. (3-30-23) h. Tailings impoundment structures which are constructed using the tailings shall not be constructe d or raised during freezing weather to prevent frost lenses in the embankment. Sufficient freeboard must be provided during the summer construction season if the disposal operation is to continue during the winter. (3-30-23) i. If tailings are to be discharged during times of freezing weather and the embankment is to b e constructed using either the upstream or centerline method, the pond shall be of sufficient size to insure that any ice formed in the tailings pond area melts during the next warm season. (3-30-23) 02. Top Width Embankment. (3-30-23) a. In the absence of a stability analysis, the minimum top width for mine tailings impoundmen t structures shall be: W = 2 (H to 1/2 power) + 4, minimum W = Top width H = Embankment height (3-30-23) b. The minimum top width for any tailings embankment is ten (10) feet. (3-30-23) 03. Cutoff Trenches or Walls. (3-30-23) a. Cutoff trenches, if needed, shall be used to bond the fill through relatively pervious material to a n impervious stratum or zone. The bond area shall extend up the abutments to the maximum high water or tailing s impoundment elevation. Cutoff (keylock) trenches which are to be backfilled with compacted fill shall be wid e enough to allow the free movement of excavation and compaction equipment. Side slopes shall be no steeper than 1:1 for depths up to twelve (12) feet, and no steeper than one and one-half (1 1/2) to one (1) for greater depths to provide for proper compaction. Flatter slopes may be required for safety and stability. (3-30-23) b. Concrete cutoff walls may be used to bond fills to smooth rock surfaces in a similar manner a s cutoff trenches and they shall be entrenched in the rock to a depth approximately one-half (1/2) the thickness of th e cutoff wall

o steeper than one and one-half (1 1/2) to one (1) for greater depths to provide for proper compaction. Flatter slopes may be required for safety and stability. (3-30-23) b. Concrete cutoff walls may be used to bond fills to smooth rock surfaces in a similar manner a s cutoff trenches and they shall be entrenched in the rock to a depth approximately one-half (1/2) the thickness of th e cutoff wall. Concrete cutoff walls shall be doweled into the rock a minimum of twelve (12) inches with a maximu m spacing of eighteen (18) inches for three-quarter (3/4) inch steel dowels. Concrete walls shall have a minimu m projection of three (3) feet perpendicular to the rock surface and shall have a minimum thickness of twelve (12 ) inches. (3-30-23) 04. Borrowed Fill Embankment. (3-30-23) a. The approved earth materials (silt soils are seldom acceptable) shall be zoned as shown in the plans and placed in the embankment in continuous, approximately level layers. Compaction shall be based on ASTM D- 698 for cohesive soils and a minimum compaction of ninety-five percent (95%) of the laboratory Standard Proctor dry density is required. Compaction of cohesionless soils shall insure a relative density of sixty percent (60%) o r greater. (3-30-23) b. An acceptable working range of moisture content for the fill material shall be established an d maintained. (3-30-23) c. The material shall be compacted by means of a loaded sheepsfoot roller, vibratory roller, or othe r acceptable means, to the required density. (3-30-23) d. No rock shall be left in the fill material which has a maximum dimension exceeding the lif t thickness. The fill material shall be free of brush and organic materials. (3-30-23) e. The fill shall be carried up simultaneously the full design width of the structure, and the top of th e fill shall be kept substantially level at all times or slope slightly toward the reservoir. (3-30-23) f. No frozen or cloddy fill material shall be used, and no material shall be place upon frozen, mudd y or unscarified surfaces

material shall be free of brush and organic materials. (3-30-23) e. The fill shall be carried up simultaneously the full design width of the structure, and the top of th e fill shall be kept substantially level at all times or slope slightly toward the reservoir. (3-30-23) f. No frozen or cloddy fill material shall be used, and no material shall be place upon frozen, mudd y or unscarified surfaces. (3-30-23) g. All materials used in the embankment shall meet all the stability and seepage requirements a s shown by a design analysis of the structure and shall be properly installed to meet these requirements. (3-30-23) 05. Riprap. (3-30-23) a. All dams shall be protected from wave action. In cases where water is stored directly against th e mine tailings impoundment structure or where wave action at maximum pool level during design inflow events woul d affect the integrity of the embankment, the Director may require use of riprap or other protective measures.(3-30-23) b. If riprap is used the design shall specify the rock size and extent of blanket required to preven t erosion. (3-30-23) 06. Outlet Systems. (3-30-23) a. Reservoirs must safely handle the design inflow for all areas draining into the reservoir. This ma y be done either by storing the entire design inflow or by having an outlet system or combination of systems adequat e to safely pass the design inflow. If the tailings reservoir is situated on a stream channel, an outlet system or a n approved alternative system capable of meeting downstream flow requirements must be provided. (3-30-23) b. The minimum design inflow for all reservoirs shall be the flood with one percent (1%) probabilit y of occurrence. The Director may require a greater design inflow be used in instances of high hazard, for larger min e tailings impoundment structures, or when the inflow is to be entirely stored in the reservoir during the flood period. (3-30-23) c

eam flow requirements must be provided. (3-30-23) b. The minimum design inflow for all reservoirs shall be the flood with one percent (1%) probabilit y of occurrence. The Director may require a greater design inflow be used in instances of high hazard, for larger min e tailings impoundment structures, or when the inflow is to be entirely stored in the reservoir during the flood period. (3-30-23) c. The outlet system may be composed of one (1) or a combination of the following: decant line , spillway, or stream channel diversion to bypass the reservoir. The system will be determined by individual reservoi r conditions. Unless removal of the mine tailings impoundment structure and reservoir is part of the abandonment plan , the outlet system shall be maintained in perpetuity, unless it is demonstrated that an outlet system is not needed. (3-30-23) d. Outlet systems will not be allowed if their use would release toxic, highly turbid, radioactive o r otherwise hazardous flows from the reservoir. In these cases the design inflow must either be entirely stored o r diverted around the reservoir. (3-30-23) e. All spillways shall be stabilized to discharge flow through the use of concrete, masonry, riprap o r sod, if not constructed in resistant rock. (3-30-23) f. Wherever possible, the spillway shall be constructed independent of the impoundment structure. It shall lead the water far enough away from the mine tailings impoundment structure so as not to endanger th e structure. (3-30-23) g. A diversion system must not subject the mine tailings impoundment structure to erosion during the design inflow event. All stream diversions shall conform to the minimum standards for stream channel alterations a s written by this Department. (3-30-23) h. Decant conduits, if under the embankment, shall be laid on a firm, stable foundation and normall y must not be placed on fill. They shall have a minimum inside diameter of twelve (12) inches and one (1) of th e following provisions included in the design: (3-30-23) i

stream diversions shall conform to the minimum standards for stream channel alterations a s written by this Department. (3-30-23) h. Decant conduits, if under the embankment, shall be laid on a firm, stable foundation and normall y must not be placed on fill. They shall have a minimum inside diameter of twelve (12) inches and one (1) of th e following provisions included in the design: (3-30-23) i. The owner shall have the conduit inspected by photographic or video tape equipment and a copy o f the inspection provided to the Department, if a problem is suspected; or (3-30-23) ii. The conduit shall be completely plugged with concrete and/or suitable material, for that portio n which extends through the embankment, if a nonrepairable problem occurs within the conduit. The conduit shal l consist of material which has been shown to possess the qualities necessary to perform in the environment of th e specific tailings impoundment. The design life of the conduit shall be greater than the life of the mine tailing s impoundment structure. The portion of the conduit through the embankment shall be completely filled with concrete, or other suitable material, and the riser portion of the conduit capped, upon abandonment of the mine tailing s impoundment structure. (3-30-23) i. All decant conduits, if under the embankment, shall have a seepage path through the imperviou s zone at least equivalent in length to the maximum head above the downstream end of the system. Only one third (1/3) the horizontal distance through the impervious zone will be utilized when calculating the length of the seepage path . Collars may be used to satisfy this requirement, but all collars shall extend a minimum of three (3) feet outside th e conduit. Collars shall be spaced at intervals of at least seven (7) times their height and no collar may be closer to th e outer surface of the impervious zone than the distance it extends out from the conduit. (3-30-23) j

utilized when calculating the length of the seepage path . Collars may be used to satisfy this requirement, but all collars shall extend a minimum of three (3) feet outside th e conduit. Collars shall be spaced at intervals of at least seven (7) times their height and no collar may be closer to th e outer surface of the impervious zone than the distance it extends out from the conduit. (3-30-23) j. More than two (2) decant conduits are not to be used, unless special conditions warrant. (3-30-23) 07. Freeboard. A minimum freeboard of two (2) feet plus wave height (H) shall be provided on th e crest of the mine tailings impoundment structure during passage of the design inflow. H = 1.95 (F to 1/2 power) F = Fetch in miles across water surface at a design maximum level. (3-30-23) 08. Records. All instrumentation shall be read and recorded on a regular basis, and all records must b e available for inspection by Department personnel on request. (3-30-23) 09. Inspection and Completion Reports. (3-30-23) a. It is the responsibility of the engineer to submit test reports along with periodic inspection and progress reports to the Director. (3-30-23) b. Upon completion of each approved stage of construction, a letter shall be sent to the Director , giving a short, narrative account covering all items of work. As-built plans shall be submitted to the Director if th e completed project was substantially changed from the plans originally approved. (3-30-23) 10. Abandonment. An abandonment plan which provides a stable, maintenance-free condition when the mine tailings impoundment is no longer being regularly maintained by the owner or the owner has ceased to us e the site for disposal of mine tailings slurry, shall be submitted to the Director by the owner. The plan shall provide a safe condition by providing for removal of the tailings, or construction of a maintenance-free spillway or diversio n works where needed to accommodate runoff

when the mine tailings impoundment is no longer being regularly maintained by the owner or the owner has ceased to us e the site for disposal of mine tailings slurry, shall be submitted to the Director by the owner. The plan shall provide a safe condition by providing for removal of the tailings, or construction of a maintenance-free spillway or diversio n works where needed to accommodate runoff. The plan shall include provisions to prevent water storage behind, an d erosion of, the mine tailings impoundment structure and the impounded tailing. A conceptual plan which includes a n engineering design report, detailed enough to provide the required cost estimate for bonding purposes, will b e required prior to the approval of the proposed project. Detailed construction plans must be approved by the Directo r prior to implementation of any abandonment work. The Director shall notify the owner upon acceptance o f completion of abandonment in accordance with the approved plan. (3-30-23) 046. -- 049. (RESERVED)