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• <br />© MONTGOMERY WATSON <br />Mining Group <br />May 16, 2000 <br />Mr. Mike Altavilla <br />Engineer Manager <br />Seneca Coal Company <br />P.O. Drawer D <br />Hayden, CO 81639 <br />Re.: Stabilization of Pond 12 Slide <br />Dear Mike, <br />MAY 18 RECD <br />The following letter presents our plan and methodology to analyze and design stabilization measures for the <br />(ailed slopes on the North side of Pond 12 a[ the Yoast Mine. This plan has been prepazed according [o our <br />site visit performed on May 5, site visit performed by John Pellicer immediately after the occurrence of the <br />slide, information on the design of Pond 12, observations provided by Seneca Coal Company, and <br />Montgomery Watson's experience. <br />The Collowing sections present some background information on the slide under analysis, analyses <br />proposed to determine [he stability of the existing slope, and the plan and methods proposed [o select and <br />implement the preferted slope stabilization procedure. <br />1.0 BACKGROUND <br />• <br />Sediment control Pond 12 is located at the Yoast mine. This pond was built by partial excavation of the <br />basin and placement and compaction of the excavated material in the retention dam. In addition, an access <br />road was placed along the top of the Nonh slope of the pond. <br />The North slope of Pond 12 failed March 6th. The area that failed was approximately 540 h long with a <br />width vazying between 110 to 220 h. Preliminary site investigations suggest that the slope failure occurred <br />primarily due to the following: <br />• excavations performed at the base of the slope during the construction of the pond; <br />• increase in weight of the soil mass due to saturation caused by uncontrolled tun-on Flowing <br />on to the slide area; and, <br />• decrease in shear strength of the soils due to saturation and increase in pore water pressures. <br />Temporary stabilization works have been or are curtently being performed based on the observations <br />completed during the site visit and discussions with Seneca Coal Company. These works are detailed as <br />follows. <br />• Completion of topographic profiles of the slide area. <br />• improve run-on control along the top of the slope. <br />• Regrading of the failed slope to minimize accumulation of water and infiltration. These <br />regrading works should be performed carefully to avoid leaving voids are large pockets within <br />the soil mass. <br />2.0 ADDITIONAL SITE INVESTIGATIONS <br />• <br />Additional site investigations are recommended to complete the stability analyses of the slide. These <br />investigations should consist of test pits or trenches along the slide area. The purpose of these test pits will <br />be to determine depths of failure surface, soil, groundwater and moisture conditions of the soil mass within <br />the failed slope mass and along the failure surface. Detailed logs of [he test pits should be completed by an <br />165 South Union Blvd. Te I. 303 763 5140 <br />Suite 416 FaK~ 303 763 9003 <br />Lakewood, Colorado <br />86228 <br />Serving fhe World's Environmental Neetls <br />