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Memo to Christine Johnston West Elk Coal Refuse Compaction page 2 Technical Background The regulations require that coal refuse structures be compacted and that the operator provide quarterly testing results to attest to compaction in accordance with the specific design specs for the structure. Compaction control involves two basic steps: (1) Determining the comparative standard for the specific soil being compacted (Proctor test); and, (2) determining the field density achieved at the structure. The compaction of a soil (moisture - density relationship) is quantified in terms of dry density achieved versus the maximum dry density determined by an arbitrary standardized test procedure called the Proctor Test, named for its Corp of Engineer's inventor. There are actually two Proctor Test standards, the "normal" and the "modified". Each of these procedures has at least four sub-methods (A, B, C & D). In greatly simplified version, the "normal" procedure is used for fine to coarse-grained soils or processed aggregates of silt, sand, gravel, or crushed rock which will be exposed to moderate compaction loads. The "modified" is used for coarser granular materials which will be exposed to greater compactive effort to achieve design-required high density. As in the case of most coal refuse structures, MCC's lower refuse pile is required to apply the "normal" proctor standard method for compaction control. The different sub-methods are intended to deal with different size gradations, generally coarser materials as you move from A to D. Experience has shown that the limitations of the testing apparatus effect the test results. As the soil being tested becomes coarser, the large pebbles interfere with compaction of the sample. To compensate for this effect, a larger diameter test mold is used, the larger fraction is removed (greater than 1/4 inch), or the larger fraction is replaced with an equivalent weight of material in the size range from 1/4" to 3/4". This is a gross simplification. For the actual requirements see section 5 of the attached copy of ASTM Standard Method D-698. The West Elk coal refuse, like most Colorado non-wash plant generated coal refuse, is a hybrid mixture of contaminant competent rock, including roof rock, partings, and floor rock, mixed with reject coal. The reject coal tends to be relatively low density, 65 to 80 pounds per cubic foot (pcf), while the contaminant rock tends to be contrastingly higher density, 115 - 130 pcf. As a result, changes in the proportionate mix tends to significantly alter the density of the composite soil material. Therefore it is important that the competent coarse fraction be included when determining the proctor compaction standard for West Elk coal refuse. Candidly, my review of the pre-1995 West Elk compaction reports suggests to me that the coarse fraction was not