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• III III III III IIII III • STATE OF COLORADO MINED LAND RECLAMATION DIVISION of ~o Department of Natural Resources Nr`gr$ 1313 Sherman St., Room 215 , 7 ~ Fa Denver, CO 60203 . ~~. 303 866-3567 ' /876' FA x. 303 832-8106 Roy Ramer. Governor Miohael B. Long, Division Drtector 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 continupus 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. Onlly several localized arroyos were filled to establish the final grade within th impoundment. As a result differential settling of the foundation up~n loading will be minimized. If differential ,settling does occur, the 40 mil ~LDPE geomembrane is specified to with stand a 9007. change in elongation. During excavation of the drain blanket channel through the splitter embankment foundation, a perched aquifer was encountered. As a result, the des'gn was modified to raise the channel by 13 feet to be above this saturated one. The operator performed density tests on the saturated materials and stat~s that they are well consolidated, however no numerical values for the material densities are given. A three-foot deep layer of gravel was installep on the invert of the channel excavation to allow ground water to travel uni peded under the foundation. During excavation of the upper impoundment, several localized zones ~f sandy clay were encountered in the impoundment floor. The materials in th se zones were saturated, the zones of saturated materials are described as be ng "relatively small and localized." These saturated materials, when encountered, were over-excavated and backfilled with clean sands and gravels. Also encountered during excavation of the upper impoundment, was a condemnation drilling hole. Once the excavation was at depth, the skeel casing was cut off, and an underdrain trench was constructed from th~ casing to the splitter embankment underdrain mentioned above. During the final grading operations the drill casing was regrouted and sealed.