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• -III IIIIIIIIIIIIIIII ,~ _,~ _ _ S 1_A_ 1__~ O_F COLORADO _ _ MINED LAND RECLAMATION DIVISION Department of Natural Resources 1313 Sherman 51., Room 215 Denver, CO 60203 303 866-3567 RA x: 303 832-8106 of coy '~°~ ~4-a ~~~~ ~ c .{'~. Roy Romer, Goverrpr Michael B. Long, Division Director DATE: April 14, 1992 T0: Larry Oehler FROM: Allen Sorenson RE: San Luis Project Tailings Impoundment and Collection Pond The main features of the tailings disposal area consist of a continuous composite liner system consisting of compacted, low permeability soil and a geosynthetic membrane overlaid with an under-drain system, two major earthern embankments, a seepage collection pond, a tailings delivery pipeline, a runoff diversion channel, and a water return pump back system. IMPOUNDMENT EXCAVATIONS The majority of the tailings impoundment was established in cut. Qnly several localized arroyos were filled to establish the final grade within the impoundment. As a result differential settling of the foundation upon loading will be minimized. If differential settling does occur, the 40 mil VLDPE geomembrane is specified to with stand a 900X change in elongation. During excavation of the drain blanket channel through the splatter embankment foundation, a perched aquifer was encountered. As a result, the design was modified to raise the channel by 13 feet to be above this saturated zone. The operator performed density tests on the saturated materials and states that they are well consolidated, however no numerical values for the material densities are given. A three-foot deep layer of gravel was installed on the invert of the channel excavation to allow ground water to travel unimpeded under the foundation. During excavation of the upper impoundment, several localized zones of sandy clay were encountered in the impoundment floor. The materials in these zones were saturated, the zenes of saturated materials are described as being "relatively small and localized." These saturated materials, when encountered, were over-excavated and backfiiled with clean sands and gravels. Also encountered during excavation of the upper impoundment, was a condemnation drilling hose. Once the excavation was at depth, the steel casing was cut off, and an underdrain trench was constructed from t;he casing to the sDlitter embankment underdrain mentioned above. During the final grad~~7 oeer=.tiens the ;rill casing was re grouted and sealed.