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2.7.2.1 Physical Characteristics <br />In an effort to determine how exposed overburden would react to weathering, <br />selected samples were artificially weathered by Stearns -Roger, Inc. (1974). <br />Artificial weathering was accomplished by exposing 21 selected samples to forty <br />freeze -thaw cycles in the presence of water. Changes induced by this kind of <br />weathering should indicate some of the expected changes likely to occur naturally <br />and assist in diagnosing the properties of the overburden. Changes in particle <br />size distribution (texture) and chemical properties were measured in addition to <br />the water content at one third bar and 15 bars suction. The changes in chemical <br />properties after weathering is presented by parameter in Section 2.7.2.2. <br />Although it is impossible to be certain, it is believed that the 40 cycles simu- <br />lated weathering that might occur naturally over 2 to 5 years for materials <br />exposed at the surface. <br />Texture <br />The results of analyses for particle size distribution appear in Table 2.7-3. <br />Type and depth of the layers are shown as well as screen sizes. The last column <br />shows the percentage of the sample that passed through a -0.053 mm screen. This <br />fraction of the sample would contain the silt and clay size particles. Data in <br />this last column indicate that artificial weathering caused a considerable break- <br />down of larger particles to smaller particles. The combination of sandstone plus <br />siltstone (samples W-11, W-15 and W-22) and the very slightly calcareous sand- <br />stone (W-92), were most resistant to the weathering process. Some of the shales <br />(W-13 and W-75) also showed a considerable resistance to the weathering process, <br />but other shales had over two-thirds of the sample in the smallest fraction <br />(W-30, W-49 and W-84). <br />The distribution of particle sizes has an important effect on the erosion poten- <br />tial of the overburden. The unweathered overburden contains a very low percent- <br />age (average of 3.8%) of silt and clay size particles which are more subject to <br />erosion than the sand fraction. The three holes, CR220, 222 and 223, which <br />showed the largest increases in fine particles (-0.053 mm) from weathering, con - <br />2 -362 <br />