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5. Long -Term Stability of Backfilled Slope <br />During the Fall of 1993, samples of fill material were gathered <br />from the roads and the mine portal areas. At each sample area, <br />approximately 100 to 200 l bs . of material was gathered over roughly <br />1/2 acre, gathering some material from the surface and some from <br />digging with a shovel. in all cases, the material gathered was <br />representative of the material which would be used in backfilling. <br />A l l samples were sent to CTL Thompson in Denver for sieve a n a l y s i s , <br />plasticity indexes, liquid limits, optimum moisture content, <br />maximum dry density and direct shear tests to determine the <br />cohesion and internal angle of friction of the material. All lab <br />test data is included in Appendix A. <br />As can be seen from the analyses, the percent silt and clay of the <br />materials (passing No. 200 sieve) range from 11% to 32%. This <br />demonstrates the lack of fines in much of the material. Also, the <br />low plasticity indexes, ranging from 9 to 11, confirm the lack of <br />appreciable clay in the material. For this reason, it is not likely <br />that the material will expand or contract to a large extent after <br />backfilling; therefore, consolidated swell tests have not been <br />performed. Slake durability tests were performed on the samples and <br />the results show that the material is not readily weathered and <br />does not exhibit much shrink swell potential. However, water <br />infiltration is very likely due to the coarse nature of the <br />material and this is accounted for in the slope stability analyses <br />through use of a pore pressure ratio. Overall, the test results <br />show that the materials from portal excavation have slightly better <br />strength properties than that of the roads. The tests show a <br />surprisingly high cohesion for the material from Mine #4. <br />The hard, durable and angular nature of the material with little <br />clay indicates that it should possess little cohesion but should <br />have relatively high internal angles of friction. The results of <br />the direct shear tests show this to be the case. These tests were <br />performed under the consolidated - undrained condition and the <br />samples were compacted to 90% of their maximum dry density prior to <br />testing. Only that material that passed the #4 sieve was tested in <br />the direct shear apparatus. Larger particles cannot be used. <br />Therefore, it must be considered that, for each test, a large <br />portion of the material is not being tested. This larger size <br />material will have large friction angles and very low cohesion. <br />Since some moisture is present in the samples during shear testing, <br />the resulting failure envelope, when plotted, can show an "apparent <br />