2007-01-31_REVISION - M1986015 (2)

Home Browse Search

altered. Approval of whateverchanges are necessary will be sought if this dredge approach or another significant change in method is implemented. SPECIFIC METHODS: Requirement: Aggregates will be removed from the deposit by front-end loader and will be transported directly to the processing plant via conveyor. No specific conveyor routes are shown on the mining plan map because these will frequently change as the deposit is mined. DISCUSSION: Extraction of the aggregate, of course, will only occur where the topsoil and overburden has been previously removed. Mining aggregate prior to such removal would not only waste the soil and overburden resource, but would necessarily create a severe contamination of the final product and render washing far more difficult. Requirement: Pit slopes in Phase 1 will be left in a near vertical condition because Phase I will be backfilled. DISCUSSION: In Phase 1, because the area will be backfilled, pi[ edge slopes are no[ critical. In general, these are mined to be vertical or nearly so. There is always some sloughing of the wall which produces a small "berm" at the base of the wall. These walls are usually amazingly secure. Large scale collapse of the wall is rare. Requirement: Pit slopes inside [he groundwater barrier wall in Phase 2 will be mined to a grade not steeper than 3:1. If a steel wall rather than a slurry wall is installed and fill is available that will not adversely affect the reclamation of Phase 1, then constructing part or all of the slopes with backfill is acceptable. However, under no circumstances may [he steel wall itself be exposed directly by mining or through sloughing of the mined face. DISCUSS N: Phas~equires a completely different approach to the edges of the pit adjacent to the oundwater barner wall. Because of the groundwater barrier wall the edge of the pit must ( be carefully med [o maintain maximum stability of [he slope. Therefore, the pi[ edge will be mined to a slope of 3:1 as a minimum. Thus, native material will form the pit slope rather than creating the final slope using backfllling. Backfilled slopes, in these situations, are not as stable as native material, especially before water is placed in [he closed basin. Severe thunderstorms as well as possible flood overflows can quickly erode backfilled slopes unless they are constructed with considerable compaction. Overflow can cause headward erosion that can quickly cut back into a slurry wall, if that choice is Pueblo East Pit Amendment 3 (2007) Exhibit D Page 31 of 34