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- 3- is higher. The slake durability test is a method which allows judgment to be made on the weatherability of the intermediate quality rocks. The type of rocks that occur at the Kerr Coal Mine tend to deteriorate from atmospheric exposure. The purpose of this test is to try to identify materials which will deteriorate very rapidly. Slake durabilities of the range of 90 to 96 percent, as indicated by samples No. 2, 3, and 4 tend to indicate a material with a moderate to high resistance to erosion. These tests were conducted in clean water. A more acidic water may result in a higher degree of degrad- ation of the samples. Based on the results of these tests and observations of the existing cuts at the Kerr Coal Mine as well as road cuts which have been exposed fora number of years in the North Park area, we do not believe that erosion will be a large problem. If these rocks were exposed to the weathering process for an extended period of time it is likely that the ma- terials would break down to a soil-like material on the surface. The likeli- hood of this occurring is relatively remote. The mining sequence at this particular mine generally involves hauling material from the area of active mining directly to an area that has been previously mined. The only materials which would be subject to erosion problems would be areas where the overburden is used to construct extensive hail roads, or the materials stockpiled from the initial box cut. To date, erosion of materials stockpiled from the initial cut has been minimal and is controlled with sedimentation ponds located • downstream of the stockpile. In 1978 a number of deep borings were made which were, in some locations, continuously cored and in other locations cored for partial depths within the overburden and the coal. These borings were drilled to evaluate conditions which would be encountered in the existing mining operations. Highly de- tailed logs of the materials encountered in each boring were prepared as a part of our report No. 3725, dated March 21, 1978. As an appendix to this _-- ---1 we are Laching_d_copy of Fi~(~from this previous report which shows a general subsurface pro~i`le of the Kerr Coal Mine including borings 77-11C, 77-12C, 77-15C, and 78-37. We have also included copies of the de- s c~ tailed boring logs for 77-15C. This detailed boring log is a good description b~k ~ 3 of the typical types of material encountered as excavation progresses from ,~~ ~ the ground surface to the coal. In-place densities were estimated by per- ~~~ forming bulk specific gravity tests on relatively intact pieces of core ob- tained from these borings. These test results indicate an in-place moist density ranging generally from 136 to 145 pounds. Natural moisture contents range typically from 9 to 12 percent. Dry densities range from 119 to 129 pcf with most of the samples around 122 to 124 pcf, dry. The observed in-place densities after the material has been excavated and replaced using the current mining procedures indicate a swell factor in the range of 24 per- ,a....,_ cent. We believe the data cited above will generally satisfy your request for information regarding the physical properties of the overburden materials at •