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<br /> <br /> <br />1 <br /> <br />1 <br /> <br /> <br />1 <br /> <br /> <br />LJ <br /> <br />1 <br /> <br />1 <br />1 <br />1 <br />Third Party Oversight i Water, Waste & Land, Inc. <br />San Luis Mine January 10, 1993 <br />EXECUTIVE SUMMARY <br />This document contains results of the third-patty review for the design and construction <br />of the Phase II Raise I Expansion at the San Luis Mine in Costilla Ccunty, Colorado. This document <br />has been prepared on behalf of the Colorado Division of Minerals and Geology and Brittle Mountain <br />Resources, Inc., to provide technical review of issues related to the expansion. This document <br />has been organized to address the specific issues which concerned the DMG regarding the <br />construction of the expansion. <br />The third-party review consisted of an initial site visit, review of the original design and <br />geotechnical issues, review of technical submittals, construction oversight, and evaluation of <br />quality assurance testing. Discussions on the design issues, the intent of the design, and the initial <br />and final phases of construction are provided in this document. Daily field logs and weekly <br />construction activity updates are included as appendices to this document. <br />The tailings disposal facility is a managed discharge, thin lift, valley impoundment. The <br />expansion consisted of increasing the height of the tailings embankment by construction of the <br />arse Z <br />Raise I Embankment to a height of 8 05 feet, and increasing the lined impoundment area to the <br />ultimate tailings elevation. Major components of the expansion included the embankment and the <br />liner system. The embankment was constructed in an upstream manner over previowsly deposited <br />tailings, and the liner system consisted of a geosynthetic liner and low permeability compacted soil <br />liner system, a drain pipe system, and a drainage layerlliner cover. <br />Although materials utilized for the construction of the Raise I Embankment were not within <br />the specifications stated in the original design report, it is WWL's opinion that thq embankment <br />will generally function as intended. This conclusion is based on acceptable quality assurance <br />testing during construction, and the assumption of appropriate monitoring after construction. <br />Performance of the embankment and tailings disposal facility should be verified by the installation <br />and monitoring of piezometers. From a review of the settlement data provided for the Raise I <br />Embankment, the embankment appeared to be experiencing insignificant settlement. <br />Approximately 0.75 acres within the expansion area contained drainage layerlliner cover <br />materials which were out of specification for particle size distribution. This area is small in <br />comparison to the overall expansion and would not significantly affect the overall performance of <br />the drainage blanket/liner cover. A final opinion on the actual in-place permeapility, and the <br />resulting performance of the drainage layer/liner cover material, has not been reaghed as not all <br />requested laboratory data have been received. <br />From preliminary field quality assurance test results, it appears that the low permeability <br />liner would perform as intended in the original design. A final opinion on the expected <br /> <br />