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The New Horizon North Mine will have 3 sedimentation ponds: NHN -001, NHN -002 and NHN -003. All 3 ponds <br />have the following design and construction parameters: <br />1. No higher than 10 feet measured from pond bottom and the top of embankment. <br />2. Top width of the embankment no less than 10 feet. <br />3. Side slopes no steeper than 2.5H:1 V on both outer and inner slopes. <br />4. High compaction due to the material and equipment available on site. <br />5. A principal spillway pipe and an emergency spillway cutout in the dam. <br />The construction material for the embankments of all 3 ponds will use the subsoils at the pond locations. The <br />USDA -NRCS soil map units have their corresponding Unified Classification soil type shown in Table 1. By <br />examining the NHN pond locations and the soil map, the material for the embankment will be SM -SC soil type for <br />the slope stability analysis and GM soil type for the foundation material. <br />A slope stability analysis was completed as part of the New Horizon North Mine permit and is included in <br />Appendix 2.05.3(4) -2A. All three ponds have the same design parameters and similar soil types. The analysis <br />covers a full range of cases to account for a wide range of possible factors of safety. The minimum factor of safety <br />for the worst case is 1.52 which is higher than the required 1.3. <br />In summary, the construction specifications for 3 sedimentation ponds to meet the safety factor are described as <br />follows: <br />1) The gated sedimentation ponds are designed to provide adequate runoff and sediment storage volume <br />for 10 -year 24 -hour precipitation event in compliance with the DRMS regulations. <br />2) All 3 ponds are designed with a combination of principal and emergency spillways. The principal spillway <br />(riser and barrel) design is an 18" diameter CMP pipe which has a valve on the entrance. This pipe will be <br />stopped down in the inlet side to a 6" valve which will be controlled by a wheel on the pond embankment, <br />as shown in Figure 5. The valves remain shut normally and will be opened when the water meets the <br />effluent standards to discharge. The operator can decide to open or shut the valve depending on the <br />water quality conditions. The elevation of the principal spillway is slightly higher than the 10 -year 24 -hour <br />peak water surface elevation in order to have sufficient capacity of required storage. <br />3) The combination of principal and emergency spillways are designed to safely contain or discharge the <br />runoff from 25 -year, 24 -hour precipitation event. Pond freeboard will be a minimum of 1.0 feet from the <br />maximum water surface elevation of 25 -year, 24 -hour precipitation event to the top of embankment. <br />4) All 3 ponds are designed to have a height no more than 10 feet from the pond bottom to the top of the <br />embankment. Top of embankment widths are 10 feet. <br />5) All principal spillways will have at least 2 seep collars installed within the dam which will be at least 2x the <br />pipe diameter. All dam fill will be compacted around the seep collars and principal spillways pipe to 90% <br />of the maximum dry density of the fill material. <br />6) The emergency spillway will be a rip -rap lined trapezoidal channel of 10 feet bottom width and 2:1 side <br />slopes. <br />