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6. Slope Stability of Highwall Remnant <br />The highwall remnants shown for all portal areas range from 60 feet <br />to 120 feet in height. These remnants are all located in the <br />sandstones, shales and siltstones of the Bowie Shale of the <br />Mesaverde. <br />Any discussion of the stability of the highwall remnants must be <br />based on what period of time is considered and what degree of <br />sloughage is considered acceptable. <br />Inspections of the highwalls show that the strata is competent and <br />does not tend to fail in massive blocks. The shaley nature of the <br />strata prevents water infiltration, hence, most erosion and <br />sloughage is from the exposed surface, and not deep- seated in the <br />mountainside. Also, the bedding of the strata is near horizontal, <br />which is best for stability. There are no indications of major <br />cracks or other problems that would indicate a potential massive <br />f a i l u r e in the natural ground above the h i g h w a l l s . See the enclosed <br />pictures. With the exception of Mine #5, the thicknesses of <br />colluvium at the top of the highwalls are less than 10 feet and as <br />such, do not contribute significantly to highwall instability. <br />This coincides with the historical record since the portal areas <br />were excavated in the 1950's. No massive failure has occurred at <br />any of the portal areas. Occasionally, large rocks do fall to the <br />bench and surface sloughage occurs on an ongoing basis. The partial <br />highwall elimination (25% to 40%) for reclamation will lessen the <br />rockfalls and surface sloughage. The surface sloughage after <br />reclamation w i l l probably not be significantly greater than that of <br />the surrounding "natural highwalls". <br />Over very long periods of time, the remaining benches will tend to <br />disappear as talus from above settles on the flat areas and <br />gradually fills them to blend with the surrounding terrain. <br />As previously stated, the proposed partial highwall reclamation <br />plan is best for the amounts of reasonably available spoil are <br />present. <br />