PERMFILE46723 - Laserfiche WebLink

Home Browse Search

1 ' COMMENTS AND CONCERNS BUTTRESS AND SLURRY WALL DESIGNS MARCH 10, 1989 1. It is estimated the pit inflows from the alluvial mar.erials would be abut 125 gallons per minute, given alluvia) permeabilities of 4x10 cm/sec. Battle Mountain is proposing to contra; this inflow by use of a horizontal drain system. The inflow and proposed drainage t system discussion indicates that the potential for alluvial inflows may be more prevalent than previously anticipated. Therefore, we would suggest that the slurry wal) design be incorporated into the permit at this time. The design cou)d be deleted from tAe permit via a technical revision if it becomes apparent that the alluvial inflows are less significant than 125 gallons per minute. ' Response: The 125 gallons per minute figure was not developed as along-term water seepage calculation. This figure was developed as a short- term, worst case scenario for initial water inflow to the pit based on available hydraulic conductivity values and the following assumptions: 1. A short-term, relatively high, hydraulic gradient near ' the pit face of 0.63 ft/ft. This gradienl. represents probable conditions as the pit wall is initially exposed, however, with time the hydraulic gradient will decrease as the phreatic surface assumes its long-term steady state position; 2. A pit wall length of 1500 ft (actual alluvial exposure is estimated at 850 ft based on drilling data); and 3. A saturated alluvium thickness of 25 ft. 1 The assumed values were used to place an upper bound on augmentation requirements and were not intended for use as long-germ seepage calculations. Therefore, the 125 gallons per minute figure is not meaningful for use in evaluating seepage cutoff or stability requirements. Based on more realistic long-term conditions, significantly lower seepage quantities are anticipated than those quantities calculated for initial augmentation purposes. The slurry wall designs submitted by BMR in a March 2, 1989 letter from SRK to your office ,were preliminary and conceptual in nature. BMR believes that the development of additional site specific information during the first two years of mining will be necessary to determine the actual rate and extent of seepage encountered, if any. This information is essential to the final determination regarding the actual necessity of a slurry wall and the final site specific design of such a structure. BMR's development and submittal of the conceptual design reflects its formal commitment to minimize and control seepage if necessary through construction of a slurry wall. During the first two years of mining activity BMR will submit an evaluation report as a technical revision to the permit. This