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<br />-z- <br />is a zone of brown, weathered claystone and sandstone which generally <br />occurs in the upper 40 to 50 feet of the excavation. We do not have any <br />recent testing on this material, but would estimate the strength to range <br />from 50 to 200 psi. Atterberg limits tests on the shales and claystones <br />have not indicated high plasticity. Many of the samples tested have exhi- <br />bited liquid limits in the range of 30 to 40 with a plasticity index in the <br />range from 8 to 20. These test results tend to indicate a silty clay. <br />Some recent standard Proctor compaction tests indicate the upper brown sandy <br />claystone to have a maximum dry density of 114 pcf with an optimum moisture <br />content of 15.2 percent when compacted using ASTM D 698-70, Method "D". <br />The gray shale with occasional pieces of coal has a maximum dry density of <br />109 pcf with an optimum moisture content of 14.2 using the same compaction <br />procedures. The gray sandstone has a maximum density of 120 pcf and an op- <br />timum moisture content of 11 percent using ASTM D 698-70, Method "D". A <br />number of tests have been performed at various locations within the existing <br />mine to evaluate the effect of placement conditions on the densities ob- <br />tained from the procedures presently used in the mine. For the present pro- <br />cedures in-place densities of backfilled areas are on the order of 10~1~cf <br />with moisture content on the order of 16.3 percent in the backfille3 areas <br />where brown claystone was tested. The gray shale tends to have a slightly <br />higher density, probably because it does not break down as much as the <br />more weathered materials near the surface. The measured densities which <br />are influenced by the large particles within the backfill indicate an in-place <br />density of 104 pcf at an average moisture content of 13.2 percent. In-place <br />densities of the gray sandstone generally are on the order of 108 pcf at <br />moisture contents of 10 percent. These densities were determin~n areas <br />that have been backfilled with material excavated during the mining process. <br />The densities are, in our opinion, somewhat higher than might be normally <br />expected. We attribute the higher measured densities to the scraper placement <br />operations at the Kerr Coal Mine. The method of placement is such that heavy <br />lifts are not possible. The scraper operation as is presently used to back- <br />fill the pit areas generally results in lifts approximately 2 feet, or so, <br />in thickness which have to be regularly levelled to allow high production <br />from the scrapers. This process results in some compaction of the material <br />by the traffic. The net result of the placement procedures from a mining <br />viewpoint is to reduce the swell factor. <br />Four random samples of the gray shale and sandstone were recently tested to <br />evaluate the slake durability of these overburden materials. The test results <br />were as follows: <br />•~Sample <br />Y No. 1 - 84.1 percent <br />C 1~ <br />- Sample No. 2 - 93.3 percent <br />Sample No. 3 - 90.1 percent ... ,,, ,,...~rfo~~ <br />Sample No. 4 - 95.7 percent f' <br />The lower indicated durabilities are of the more clayey materials. Samples <br />2 and 4 had more sandstone particles, therefore, the indicated slake durability <br /> <br />