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ACZ • hfs. Cathy Bcgcj January 14, 1992 Page 2 The proportional mix of coarse and fine refuse varies dependent oo changes in geologic and mining conditions. Given torten[ layered BII cons[ruuioa methods, the potential for arcular failure is signitican[ly reduced and a sliding block failure through the fine refuse material may be the more critical failure mechanism. Actual strength testing of fine «fuse materials indicates a higher ctfcctive Crictioo angle, at the densities documented by field sampling, than assumed far previous stability analyses. 2.0 Existing Conditions and Material Properties The current refuse placement practice at the mine consists of a layered placement with the coarse and Cme components being placed is individual layers. The fine refuse is considered to be a lower strength material than the coarse refuse. The stability evaluation will, therefore, focus oa the fine refuse layers az the primary structural units influencing overall stability. The presence of the coarse material in individual layers may be an advantage, is that [hey will tend to alt as drainage layers for the fine refuse, intercepting and conducting infiltration Bows to points oa the pile surface where these layers daylight. • Figure 1 indicates the general geometry and layout of the refuse pile az of the end of 19g9 and it's planed final coafiguratioo. Review of this figure indicates that the pile haz beta coastruaed with approximately 3H:1 V side slopes to a maximum height of 80 feet Figure 2 indicates two uoss-seaioas through the pile [hat were taken at critiral locations (steepest side slopes and highest overall slope). These sections indicate the extent of refuse materials, the loratioa of underdrains, and the assumed subsurface stratigraphy. These two cross-sections were used to conduct liability analyses of the refuse pile is it's current and planned final configuration. The material properties used for the refuse and the foundation soils are presented is Table 1. Additional testing (GeoWest, 19g8) focused oa measurement of in-place densities and included strength testing of fine refuse materials. A series of direct shear tests were conducted oa the fine refuse materials recompacted to varying densities. The results of the d'vcu shear tests indicated that [he friction angle varied from 33 degrees to 38 degrees for recompaded densities of 45 to 63 pcf, respectively. While the triaion angles determined by direct testing appear relatively high (or fine refuse, it should be noted [hat EFCI designates fine refuse az any material smaller than ?$ mesh which is a medium sand. Shear test results also appear to be consistent with Gthologic and tamtral charaaerizatioa of roof and Boor materials az summarized by Table 4, Textural Analysis of Roof/Floor Material, Table 7, Jack O'I.antern Roof/Floor Analyses (Parts 1 and 4), and Table g, Red Arrow Roof/Floor Analyses (Parts 1 and 4) of Southfield permit document. These tables indicate that coal refuse materials are primarily saadstoau and shales with a « latively high percentage of sand-sized particles and low clay content. Given the low rlay content, plastiaty is not a significant coasideratioa and the friction angles indicated by testing are within the normal range for 28 mesh granular fines. The lower bound of the test values was chorea for the stability analyses. Material properties for the sandy clay • material encountered at the original ground surface (foundation material) are based oa values reported by Roclty Mountain Geotechnit:al, Inc 1982.