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iii riiiiiiiiii~~~~~ <br />MINED LAND RECLAMATION DIVISION <br />Department of Natural Resources <br />1313 Sherman SI., Room 215 <br />Denver. CO 80203 <br />303 666-3567 <br />FA%.303 832-6106 <br />T _ :_ ._ <br />I <br />~J` 'i.flRADO <br />~~ ~ ~<o <br />~Pr.. k`Si~+~~.•~T <br />rr ~$ <br />.1 ~7 ~ (7 . <br />~ ra76 ~ <br />Foy Romer, <br />Govemar <br />Michael B Lang, <br />Division D~reaor <br />DATE: April 29, 1992 <br />T0: Bruce Humphries L/~ /j / <br />FROM: Allen Sorenson ~~ <br />RE: Hydrostatic Head Applied to Composite Liner Underlying SanjLuis <br />Project Tailing Pond, File No. M-88-112 <br />This analysis is prompted by the memorandum from Harry Posey to Bru e <br />Humphries dated April 24, 1992. The chief factor in limiting seepa a through <br />the composite liner, is the ability of the sub-drainage layer to mi imize the <br />hydrostatic head applied to the liner. The head applied to the tin r is <br />dependent on the permeability (or hydraulic conductivity) of the dr in layer <br />material, the rate at which fluid is entering the drain layer, and he spacing <br />and effectiveness of the drain pipes. <br />Attached is a calculation of the drain pipe spacing necessary to li it the <br />head on the composite layer to an acceptable 1.2 feet, at selected oints in <br />the tail deposition sequence. The table at the bottom the the atta hed page <br />shows that the design spacing of 40 feet does not adequately serve o limit <br />head until the conductivity of the tailings reaches 3.84 x 10-6 cm/ ec. <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 o the total <br />tailings impoundment, so the total amount of seepage due to higher ead at <br />start-up, may have been small enough to be acceptable. <br />The calculations attached to this memo are based on a hydraulic cone <br />within the drainage layer of 5 x 10-4 cm/sec. This value was suppl <br />the operator, and is referenced in the permit documents, however, tl <br />which it was determined are not known. I have contacted SRK to fins <br />sort of analysis was conducted to come up with this value. Sieve a1 <br />conducted on the "type 2" material used in the construction of the ~ <br />layer, and based on this analysis, the material falls within the SC <br />the Unified Soil Classification System. Sherard (1963) gives a prof <br />for the permeability of this soil as 10-8 cm/sec to 5 x 10-5 cm/sec <br />attached table 5). These values obviously conflict with the value <br />by SRK for drainage layer permeability, and if the values for K tak! <br />table 5 are accurate, the ability of the drain layer to limit hydro <br />would be severely curtailed. It should be noted that the "type 2" r <br />uctivity <br />ed by <br />e means by <br />out what <br />alysis was <br />rainage <br />group of <br />able range <br />(see <br />supplied <br />n from <br />tatic head <br />Material is <br />