2007-01-31_REVISION - M1986015 (2)

Home Browse Search

MINING PLAN DISCUSSION: Unlike the original plan for Phase 1, Phase 2 will be mined after installing a groundwater barrier wall completely around the pit. There are two choices for this wall. The first choice is to use a standard slurry wall about three feet thick and keyed into the Pierre Shale bedrock. The other, and second choice, is to use a driven sheet piling wall. This is made of thick steel, sometimes about 3/8" thick, [ha[ is shaped with large "corrugations." Each sheet is driven into the ground with adjacent sheets overlapping some at their edges. When properly done this produces a tight and very strong barrier wall that does no[ have some of the problems associated with slurry walls. If [he slurry wall is selected, the installation will not be done by the operator, but rather by a contractor experienced in the process of installing such structures. (The original plan for a slurry wall around Phase l no longer applies.) If the driven sheet piling is installed it is likely the operator will do the installation. The installation of these steel walls is much simpler than the process used for constructing a slurry wall. The wall, whether slurry or eel she piling, will be installed a short distance inside the affected land boundary and the groundwater barrier wall installation disturbances will essentially define the affected land boundary. This groundwater barrier wall will extend to the bottom of the deposit and a short distance into the bedrock to form a good seal throughout the wall height. Of course, [he groundwater barrier wall will not be totally impermeable. Some seepage will occur and is expected. However, before it can be considered successful it must meet certain standards for leakage. That degree of success will be sought. Initially, there will be groundwater inside and outside of the wall. Once [he excavation of the gravel begins inside the groundwater barrier wall the groundwater will become exposed. That water will need [o be pumped out. Please refer to the section on dewatering for a description of how each phase is dewatered. One of the large advantages of using a groundwater barrier wall, besides the obvious advantage of being able to mine an almost dry pit, is the elimination of the cone of depression that usually surrounds dewatered gravel pits. Because Phase 2 will be in close proximity to the homes and domestic wells on 25'" Lane, dewatering by conventional means would undoubtedly remove essentially all the groundwater from under 25`" Lane. Wells there would totally dry up. W ith a groundwater barrier wall in place that severe drawdown is eliminated, This is further discussed elsewhere. Requirement: The groundwater barrier wall shall not enclose the northeastern corner of the Phase 2 area [o allow for a smooth outflow of groundwater from 25`" Lane to [be river. DISCUSSION: In [he development of this plan, surrounding the entire pit with a groundwater barrier wall looked very good until i[ was realized there are septic systems along 25ih Lane in addition to the domestic wells. Because one parcel of property on 25'" Lane would be surrounded on three sides by Pueblo East Pit Amendment 3 (2007) Exhibit D Page 25 of 34