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Manb 2006 Prt Naekfi/l ,SJabilrty * 11 U%' ~ PuEe 1 • INTRODUCTION Seneca Coal Company is currently backfilling the pits at the IIW Mine with spoils removed from the pits during mining operations. Generally, Seneca plans to place these spoils at slopes up to 2.SH:1 V, in order to match the pre-mining aspect of the terrain. Seneca contracted MWH m catty out field and laboratory testing on the spoils material at the IIW Mine and perform a stability analysis to check the suitability of the 2.SH:1V grading plan for post mining reclamation slopes. FIELD TESTING In-sicu moisture and density tests were performed on the spoils piles at the IIW mine, on February 1, 2005, using a nuclear density gauge. Twenty-two moisture-density tests were performed at the Wolf Creek, Sage Creek, North Wadge, and South Wadge pits. The locations of the tests performed are shown on Figvre 1, Moisture-Density 1~est Locations. The in-situ moisture content of the spoils ranged from 71.7% to 22.G%. The in-situ density of the spoils ranged from 91.5 pcf to 112.0 pcf. The mean values of the in-situ moisture content and dry density were 15.7% and 103.1 pcf respectively. In additioq bulk spoIls material samples were taken From the area directly underneath the nuclear density gauge at eight of the test locations. Each bulk sample consisted of approximately 5 gallons of soil taken from the site of a density test. Table 1, IIW Mine .Spoilr Field Testing Results and Corrected Results shows the moisture-density testing performed and the bulk samples collected. LABORATORY TESTING • Six of the bulk samples from the field program were sent to Northwest Colorado Consultants, Inc. for index testing, that included the following tests: Natural Moisture Content, Grain Size Analysis, and Atterberg Limits. The results of the index testing are summarized in Table 2, IIW Mme Spoils Index Testing Summary. The test results are shown in full in Appendix A, Laboratory Test Results. Based on the index testing results, the natural moisture contents were used to calculate a corrected field moisture content. The corrected moisture contents were then used [o correct the moisture and density readings generated with the nuclear density gauge. The corrected field moisture and density tests are shown in Table 1, IIiY/Mine Spoils Field Tenting Rerultr and Corrected Results. The mean values of the corrected moisture content and dry density were 12.2% and 106.3 pcE respectively. Bulk sample IIW-1 was selected for triaxial testing because it has the highest Plasticity Index (PI) and Liquid Limit (LL), as shown in Figure 2, Plasticity Chart for Samples from IIW Mine, and a high percentage of fines (material < #200 sieve). This sample was sent to Golder Associates, Inc. Eor a three stage Consolidated Undrained Triaxial test. The sample material was remolded to a target dry density and moisture content similar to the corrected average from the field testing. A dry density of 105.5 pcf at a moisture content of 12.8% was achieved, which is representative of the corrected field densities and moistures. The results of this test are shown in Appendix A, Laboratory Tert Results. Two tests performed during the February 1, 2005 field visit showed low dry densities (Test #4 and Test #19). The uncorrected densities of these tests were 91.5 pcf and 95.8 pcf. The Erst test was performed in freshly spread spoils which had not been compacred by placement equipment or had time to consolidate. The second test was performed in the Sage Creek Pit spoils pile, which was very muddy overall and specifically in an area that was Freshly disturbed. These two tests are not representative of the material that will exist in the final backfilled slopes of the reclaimed pits, therefore a density value close to the overall corrected average was targeted for the tdaxial testing. • With a representative density value and high plasticity this material represents an appropriately conservative strength value for the material used to backfill the reclaimed pits. Al1Y/H * iJ75 Prne Grove Rand, .Sane 109 * Steamboat SpnaKr, CO 80J87 u' uvlf..xd 11°`lN~hvkld/d~+bMl~lu',v,..v,.. nmmr •n