Valley-unloading stress
The stress field formed beneath stream valleys is called “valley-unloading” stress. This type of stress field is common in Appalachian coal fields, where there is high topographic relief and deeply incised valleys. The process of stream erosion to create a valley tends to concentrate horizontal stresses beneath the valley axis. Fractures, slips, and bedding-plane shears (local minor thrust faults) may develop beneath valley bottoms as a result of compressional stresses oriented perpendicular to the valley axis. Hence, weakened roof conditions may occur in underground mines which cross beneath stream valleys, even in mines that otherwise have good roof conditions (Ferguson, 1967; Moebs, 1977; Moebs and Stateham, 1984; Hill, 1986; Sames and Moebs, 1991; Molinda and others, 1992; Hasenfus and Su, 2006).
Molinda and others (1992) found:
- Valleys oriented at large angles to the direction of maximum regional stress are at greatest risk to high shear stresses.
- Valleys that have existing stress-relief failures and are still subjected to horizontal compression will also undergo the most significant compressional roof failure.
- Thickness of cover, shape of the valley, shear strength of rock strata, and the magnitude and orientation of the regional stress all influence the stress field at the mine level.
- Bedding plane faults and low-angle thrust faults beneath valley bottoms can occur to depths of at least 300 feet.
Moebs (1977) and Sames and Moebs (1991) reported roof falls beneath valley bottoms to depths of 600 feet. Also, in many parts of Appalachia, streams valleys themselves are controlled by regional stresses. When streams align with regional stresses, water can percolate down through fractures, causing weathering and shrinking and swelling of clays, which can weaken roof rocks for some distance beneath valley bottoms (see, for example, McCulloch and others, 1975).
