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<br />ATTACH6IENT 6 <br />weathering of the overburden at other mines indicates that some shales <br />crumble and disintegrate during a winter season. Extensive weathering <br />of shales occurs over a five-year period. The sandstones show slight <br />weathering during this interval. <br />4.1 Particle size distribution <br />The results appear in table 4. Type and depth of the layer are shown <br />and the screen sizes. The last column shows the percentage o*" the sample which <br />passed through a -270 mesh screen. This fraction of the sample would contain <br />the silt and clay size particles. Data in this last column indicate [hat the <br />artificial weathering caused a considerable breakdown of larger particles to <br />smaller particles. The combination of sandstone plus siltstone and [he very <br />slightly calcareous sandstone (no. 92) were the most resistant to the weathering <br />process. Some of the shales showed a considerable resistance to the weathering <br />process, but other shales had over two-thirds of the sample in the smallest <br />fraction. <br />Artificial weathering caused 69 percent of the coarse fragments to change <br />to smaller sizes. The smallest fraction (~~~ mPShl chowed an average gain to <br />34 persent_nf *he total weight. A comparison between type of material and West <br />vs. East Panels is shown in table 4a. The shale shows a greater degree of <br />~ disintegration than the sandstone plus siltstone and over twice as much of the <br />"' shale appears in the smallest size fraction compared with the sandstone plus <br />siltstone. Before weathering an average of 51 percent of the total weight <br />appeared in the fraction less than 270 mesh. <br />The distribution of particle sizes has an important effect on the <br />erosional hazards of [he overburden. The unweathered overburden contains a <br />very low percentage (average of 3.8%) of silt and clav size particles which are <br />more subject to erosion than the sand fraction. Three holes, CR 220, 222, <br />and 223, which showed the largest increases in fine particles (-270 mesh) from <br />weathering, contained only 1.7% of this fraction in the unweathered material. <br />Thus, the initial overburden should be quite resistant to erosion, but as <br />weathering proceeds the erosion potential will increase. Therefore, signifi- <br />cant predictions about erosion hazards of [he unweathered overburden depend <br />on the degree of weathering. <br />Artificial weathering was induced by freeze-thaw cycles of water-aatur- <br />ated overburden. Possibly, the 40 cycles simulate weathering that might occur <br />naturally over 2 to 5 years. Therefore, if grading and revegetation follow <br />within a year after mining, erosion of the overburden should be minimal. <br />4.2 Water content, hydraulic conductivity and settling volume <br />These data appear in table 5. Large differences occurred in the water <br />contents at 1/3 bar suction and at 15 bars suction. The difference between <br />these two values represents a measure of [he amount of water available to plants. <br />The water contents in each column reflect differences in the overburden due <br />~~ mainly to texture. From regression curves the clay contents may be estimated <br />'.~ to range from 2.G in a layer (71' - 96') in CR 213 to 64 Y, in a layer (64' - 109') <br />15 <br />