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ACZ <br />Ms. Cathy Bcgcj <br />January 14, 1992 <br />Page 2 <br />The proportional mix of coarse and fine refuse varies dependent on changes in geologic and <br />mining conditions. <br />Given current layered fill construction methods, [he potential for circular failure is significantly <br />reduced and a sliding block failure through [he fine refuse material may be the more critical <br />failure mechanism. <br />Actual strength testing of fine refuse materials indicates a higher effective friction angle, a[ the <br />densities documented by field sampling, than assumed for previous stability analyses. <br />2.0 Existing Conditions and Material Properties <br />The current refuse placement practice at the mine consists of a layered placement with [he coarse and fine <br />components being placed in individual layers. The fine refuse is considered [o be a lower strength material than <br />the coarse refuse. The stability evaluation will, therefore, focus on [he fine refuse layers as the primary <br />structural units influencing overall stability. The presence of the coarse material in individual layers may be <br />an advantage, in that they will tend to act as drainage layers for the fine refuse, intercepting and conducting <br />infiltration flows to points on the pile surface where these layers daylight. <br />Figure 1 indicates the general geometry and layout of [he refuse pile as of [he end of 1989 and it's planned final <br />configuration. Review of [his figure indicates [hat the pile has been constructed with approximately 3H:1V side <br />slopes to a maximum height of 80 feet. Figure 2 indicates two cross-sections through the pile that were taken <br />at critical locations (steepest side slopes and highest overall slope). These sections indicate [he extent of refuse <br />materials, the location of underdrains, and [he assumed subsurface s[ratigraphy. These two cross-sections were <br />used to conduct stability analyses of [he refuse pile in iPs current and planned final configuration. <br />The material properties used for the refuse and [he foundation soils are presented in Table 1. Additional <br />testing (GeoWest, 1988) focused on measurement of in-place densities and included strength testing of tine <br />refuse materials. A series of direct shear tests were conducted on the fine refuse materials recompacted to <br />varying densities. The results of [he direct shear tests indicated [ha[ [he friction angle varied From 33 degrees <br />[0 38 degrees for recompacted densities of 45 to 64 pcf, respectively. <br />While the friction angles determined by direct testing appear relatively high for fine refuse, it should be noted <br />[ha[ EFCI designates fine refuse as any material smaller than 28 mesh which is a medium sand. Shear test <br />results also appear to be consistent with lithologic and textural characterization of roof and Moor materials as <br />summarized by Table 4, Textural Analysis of Roof/Floor Material, Table 7, lack O'Lan[ern Roof/Floor <br />Analyses (Parts 1 and 4), and Table 8, Red Arrow Roof/Floor Analyses (Parts 1 and 4) of Southfield permit <br />document. These tables indicate that coal refuse materials are primarily sandstones and shales with a relatively <br />high percentage of sand-sized particles and low clay content. Given the low clay content, plasticity is not a <br />significant consideration and the friction angles indicated by testing are within the normal range for 28 mesh <br />granular fines. <br />The lower bound of the test values was chosen for the stability analyses. Material properties for the sandy clay <br />material encountered a[ [he original ground surface (foundation material) are based on values reported by <br />Rocky Mountain Geotechnical, Inc. 1982. <br />