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• iii iiiiiiiiiiiu iii • <br />STATE OF COLORADO <br />MINED LAND RECLAMATION DIVISION <br />Department of Natural Resources <br />1313 Sherman St.. Room 215 <br />Denver, CO 80203 <br />303 866-3567 p .+~ <br />Fnx: 303 8328106 ~ r <br />' r -, <br />„` J <br />`~ . r'i <br />DATE: April 29, 1992 <br />T0: Bruce Humphries <br />FROM: Allen Sorenson <br />~ . ,~n <br />~ V J _ <br />.~ -' r' <br />p ii ~ <br />~~ s <br />5 '~" ~ <br />L// /~ / ~ <br />(~/I~~C___/ ~, <br />.~F'OF CO~~9 <br />py~,~} 9~ <br />•~ ~.i/+k <br />`revs <br />Roy Romer, <br />Governor <br />J 1 ~ Michael B. long, <br />a, Drvis~on Dveclor <br />~l 1 ~ <br />;, . <br />1 <br />\\i• <br />V ~ n [~ <br />t ~~t (~ 1' <br />RE: Hydrostatic Head Applied to Composite Liner Underlying San Luis <br />Project Tailing Pond, File No. M-88-112 <br />This analysis is prompted by the memorandum from Harry Posey to Bruce <br />Humphries dated April 24, 1992. The chief factor in limiting seepage through <br />the composite liner, is the ability of the sub-drainage layer to minimize the <br />hydrostatic head applied to the liner. The head applied to the liner is <br />dependent on the permeability (or hydraulic conductivity) of the drain layer <br />material, the rate at which fluid is entering the drain layer, and the spacing <br />and effectiveness of the drain pipes. <br />Attached is a calculation of the drain pipe spacing necessary to limit the <br />head on the composite layer to an acceptable 1.2 feet, at selected points in <br />the tail deposition sequence. The table at the bottom the the attached page <br />shows that the design spacing of 40 feet does not adequately serve to limit <br />head until the conductivity of the tailings reaches 3.84 x 10-6 cm/sec. <br />This conductivity is not attained until some time after year one of mill <br />start-up. However, initial deposition is over a small percentage of the total <br />tailings impoundment, so the total amount of seepage due to higher head at <br />start-up, may have been small enough to be acceptable. <br />The calculations attached to this memo are based on a hydraulic conductivity <br />within the drainage layer of 5 x 10-4 cm/sec. This value was supplied by <br />the operator, and is referenced in the permit documents, however, the means by <br />which it was determined are not known. I have contacted SRK to find out what <br />sort of analysis was conducted to come up with this value. Sieve analysis was <br />conducted on the "type 2" material used in the construction of the drainage <br />layer, and based on this analysis, the material falls within the SC group of <br />the Unified Soil Classification System. Sherard (1963) gives a probable range <br />Ifor the permeability of this soil as 10-8 cm/sec to 5 x 10-5 cm/sec (see <br />attached table 5). These values obviously conflict with the value supplied <br />by SRK for drainage layer permeability, and if the values for K taken from <br />table 5 are accurate, the ability of the drain layer to limit hydrostatic head <br />would be severely curtailed. It should be noted that the "type 2" material is <br />