Cutters
"Roof cutters" are fractures that form after mining takes place along the rib; they propagate upward into the roof, usually at angles greater than 60 degrees from the horizontal (see, for example, Hill, 1986). These can intersect other weaknesses in roofs above the level of roof-bolt anchors and cause sections of the roof to cantilever and fall. Cutters occur in stress fields where horizontal stress is greater than vertical stress along the rib or at an entry corner (Aggson, 1979b; Kripakov, 1982; Hill, 1986). When cutter roof fractures occur in consistently similar orientations (such as along main headings, or crosscuts, but not both) in deep mines, but are not concentrated beneath valleys, or along the outer margins of the hillslope (in drift mines), the cause of the cutters is generally horizontal stress (Hill, 1986; Sames and Moebs, 1991). Cutters can also be concentrated and oriented along trends parallel to sandstone rolls and cutouts (Hylbert, 1974, 1984; Kertis, 1985; Hill, 1986).
