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• <br />© MONTGOMERY WATSON <br />Mining Group <br />I <br />December 29, 2000 <br />Seneca Mining, Inc. <br />P.O. Drawer D <br />Hayden, Colorado 81639 <br />RE: Landslide at Pond 12 -Stability Analysis <br />Dear Michael Altavilla, <br />JAN 12 RECD <br />Seneca Mining requested that Montgomery Watson (MW) evaluate the stability of the slide <br />area adjacent to Pond 12 (Pond ]2 Slide) in its current configuration (re-graded post slide) and <br />make recommendation regarding the need for and/or type of further remedial action to stabilize <br />the slope. This letter report details the results of the slope stability analysis performed on the <br />Pond 12 Slide, and the subsequent conclusions and recommendations. Included in this report is a <br />summary of the properties of the soil found in the slide area, and the stability analysis performed <br />on the pre slide and re-graded post slide topography (Drawings l-4). <br />1. Slope and Slide Characterization <br />• <br />The Pond 12 Slide occurred in February 2000 after a period of significant precipitation and <br />after the excavation of Pond 12. Prior to the slide and after the excavation of Pond 12 a survey of <br />the azea had been performed. The area was again surveyed right after the slide. After this survey <br />Seneca re-graded the area to stabilize the slope and realign the haul road which had been <br />impacted by the slope failure. After Seneca re-graded the slide area was once again surveyed to <br />record the current topography. <br />From an analysis of the various surveys it is evident that the Pond 12 Slide impacted an area <br />of approximately 2.5 acres. The resulting head scarp was approximately 596 feet long and was <br />approximately 25 feet tall at its tallest point. Reviewing the locations of the head scarp and the toe <br />of the slide generated an approximation of the failure surface. These locations were evident in <br />both the field observations and the survey data. This approximate failure surface was utilized in <br />further analysis <br />2. Soil Characterization <br />u <br />In August 2000 Montgomery Watson performed a geotechnical investigation of the azea. The <br />geotechnical investigation included nuclear moisture and density measurements, and disturbed <br />sampling in two test pits. The test pits were located at the bottom of the slide and in the upper <br />portion of the body of the slide, as shown on Drawings No. L-3. In addition to the samples of <br />material taken from the test pits, a disturbed sample was retrieved from the face of the head scarp. <br />Cohesive brown clay with some sand was encountered throughout test pit located in the body of <br />the slide and in the head-scarp. The material encountered in the test pit at the tce of the slide <br />consisted of a black weathered shale clay, as the test pit progressed deeper into the material it <br />uansitioned from a saturated black clay to black hardpan or shale. Sample material was subjected <br />to laboratory testing. A summary of the results is located at the beginning of Attachment 2 (Soil <br />1655outh Union Blvd. Te1:3037635140 <br />Suite 410 Far' 3037fi38003 Serving the World's Fnnronm ental Needs <br />Lakewootl. Colorado <br />90229 <br />