1983-06-03_REVISION - M1977285 (2)

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43.90(d)(3) (3) the embankment and cover slopes shall be relatively flat after • final stabilization to minimize erosion potential and to provide conservative factors of safety assuring long term stability. The objective should be to contour final slopes to grades which are as close as possible to those which would be provided if tailings were disposed of below grade. Slopes shall not be steeper than 5 horizontal to 1 vertical (Sh:lv), except as specifically autho- rized by the Agency. Where steeper slopes are proposed, reasons why a slope steeper than Sh:ly would be as equally resistant to erosion shall be provided, and compensating factors and condi- tions which make such slopes acceptable shall be identified; (4) a full self-sustaining vegetative cover shall be established or rock cover employed to reduce wind and water erosion to negli- gible levels. Where a full vegetative cover is not likely to be self-sustaining due to climatic conditions, such as in semi-arid and arid regions, rock cover shall be employed on slopes of the impound- ment system. The Agency will consider relaxing this requirement foz extremely gentle slopes, such as those which may exist on the top of the pile. The following factors shall be considered in establishing the final rock cover design to avoid displacement of rock particles by human and animal traffic or by natural processes, and to preclude undercutting and piping: • (i) shape, size, composition, gradation of rock particles (excepting bedding material, average particle size shall be at least cobble size or greater); (ii) rock cover thickness and zoning of particles by size; and (iii) steepness of underlying slopes. Individual rock fragments shall be resistant to erosion and weathering action. Local rock materials are permissible provided the characteristics under local climatic conditions indicate similar long-term performance as a protective layer. Rock covering of slopes may not be required where top covers are very thick (on the order of 10 meters or greater); impoundment slopes are very gentle (on the order of lOh:ly or less); bulk cover materials have inherently favorable erosion resistance characteristics; and there is negligible drainage catchment area upstream of the pile, and there is good wind protection. Furthermore, all impoundment surfaces shall be contoured to avoid areas of concentrated surface runoff or abrupt or sharp changes in elope gradient. In addition to rock cover on slopes, areas toward which surface runoff might be directed shall be well protected with substantial rock cover (rip rap). In addition to • 43 - 16 (April 1990)