1994-01-01_PERMIT FILE - C1981017 (5)

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5. Nature of the Material Available for Road Backfill During the Fall of 1993, samples of fill material were gathered from the roads and the mine portal areas. At each sample area, approximately 100 to 200 l bs . of material was gathered over roughly 1/2 acre, gathering some material from the surface and some from digging with a shovel. In all cases, the material gathered was representative of the material which would be used in backfilling. All sample locations are shown on Exhibit 1. All samples were sent to CTL Thompson in Denver for sieve analysis, plasticity indexes, liquid limits, optimum moisture content, maximum dry density and direct shear tests to determine the cohesion and internal angle of friction of the material. All lab test data is included in Appendix A. Only the samples from Mine Roads #3, #4, #5 and #2 Mine Bin Pad are considered for this report. The sample from the #2 Mine Bin Pad is actually on the road to Mines #1 and #2 in the cut through the Rollins Sandstone. As can be seen from the analyses, the percent silt and clay of the materials (passing No. 200 sieve) range from 7% to 17 %. This demonstrates the lack of fines in much of the material. Also, the low plasticity indexes, ranging from 6 to 11, confirm the lack of appreciable clay in the material. For this reason, it is not likely that the material will expand or contract to a large extent after backfilling; therefore, consolidated swell tests have not been performed. Slake d u r a b i l i t y tests were performed on the samples and the results show that the material is not readily weathered and does not exhibit much shrink swell potential. However, water infiltration is very likely due to the coarse nature of the material and this is accounted for in the slope stability analyses through use of a pore pressure ratio. The hard, durable and angular nature of the material with little clay indicates that it should possess little cohesion but should have relatively high internal angles of friction. The results of the direct shear tests show this to be the case. These tests were performed under the consolidated - undrained condition and the samples were compacted to 90% of their maximum dry density prior to testing. Only that material that passed the #4 sieve was tested in the direct shear apparatus. Larger particles cannot be used. Therefore, it must be considered that, for each test, a large portion of the material is not being tested. This larger size material will have large friction angles and very low cohesion. 9