Fractures in mine roofs
Definition and formation: Fractures and joints are natural cracks or planes of weakness in mine roofs. Fracture is a general term for any crack or break in a rock caused by mechanical stress. Joints are types of fractures without displacement (Bates and Jackson, 1980). Fractures and joints can occur in any rock type, but are more of a roof-support concern in shale roofs roofs (Horne and others, 1978; Nelson, 1983; Hylbert, 1984; Millici and Gathright, 1985). Fractures can occur naturally in roof rocks or can form from roof sag and related mining pressures. Fractures and joints commonly occur as straight, vertical to near-vertical breaks in the rock, but also can be curved. Some special types of fractures found in coal mines include:
Natural fractures and joints form from:
Discontinuities and obstacles: Fractures do not cause discontinuities. Fractures are natural in coal and are called cleats. If the coal is offset along a linear crack or break, the feature is a fault.
Potential roof-fall hazards: Fractures and joints are vertical weaknesses in mine roofs. As a vertical disruption in bedding, they can interfere with horizontal “beam” strength accomplished by conventional bolting. Fractures can act as pathways for moisture, especially where they occur near the topographic surface of the mine. This can weaken moisture-sensitive rocks such as weak shales and claystones beneath coal riders. Rock type, fracture spacing, stress fields, and other factors influence the potential weaknesses fractures impose on roof rock.
Trends: Fracture occurrence and density vary considerably in different mines in different areas. Likewise, some mine roofs have abundant fractures and joints, which are oriented in the same direction, and others have fractures with considerable variability.
Fractures occurring under deeper cover tend to orient along stress fields formed by:
- Regional (horizontal, in situ) stress fields
- Valley-unloading stress (under valley fractures)
In areas with major structural overprints such as southeastern Virginia, fracture/joint density and orientation are commonly associated with larger tectonic folds and faults (Millici and others, 1982; Moebs and Ellenberger, 1982). In other areas, where major folds and faults are less common, regional stresses and valley-unloading stresses may control fracture patterns. In eastern Kentucky, the horizontal stress field strikes northeast-southwest. In western Kentucky the stress field strikes essentially east-west. Local discontinuities, such as paleochannels, can also impose local fracture orientations on more regional trends.
Known Kentucky occurrences: Regional, horizontal-stress-induced fracturing can occur in any mine, but may be more common in deeper mines. Valley-unloading fractures and hill seams are common in eastern Kentucky and occur in most drift mines. Valley unloading is most common in mines developed below drainage, or in drift mines that cross under stream drainages or are developed in narrow ridges. Greb and Cobb (1989) and Greb (1991) provided examples from mines in the Hazard No. 8 coal. Sames and Moebs (1989, 1992) provided detailed examples of hill seams from several eastern Kentucky mines, but the mine names and seams were withheld.
Planning and mitigation: Planning for regional stresses can be done by examining known horizontal-stress indicators in a mining area. In eastern Kentucky, most areas have northeast–southwest-striking horizontal-stress directions (World Stress Map, 2017), with some variability, whereas in western Kentucky, east-west horizontal stresses are common (; Dart, 1985; Nelson and Bauer, 1991; World Stress Map, 2017). Many reports recommend reorienting entries obliquely (to 45 degrees) from theprincipal regional stress (see, for example, Hill, 1986), so entries are not subparallel to the stress field and pillars can take up more support of the roof. Staggering pillars to isolate roof failure in crosscuts, and reducing the width of crosscut entries can also reduce roof failures related to regional stresses (Aggson, 1979a; Hill, 1986; Sames and Moebs, 1991). Turning entries obliquely to oriented fractures has also been used for fractures related to known tectonic trends such as faults and folds (see, for example, Millici and others, 1982) or more local discontinuities such as paleochannels (see, for example, Ingram and Chase, 1987).
Valley-unloading fractures can be anticipated and planned for by making topographic overlays of mine plans to determine areas of mining that might be beneath valley axes, especially valleys with steep valley walls. Trends of steep valley walls that are oriented along preferred regional trends are common in Appalachia. Recording these trends with topographic overlays and air photo overlays is the basis of lineament studies (Overbey and others, 1973; Jansky and Valane, 1983; Hylbert, 1984).
Roof support: Different types and spacings of fractures require different support methods, depending on site conditions. Once cutters form, cribbing, additional bolting, angle bolting, truss bolting, and other supplemental support are often required (Thomas, 1950; Hill, 1986; Liu and others, 2005). Roof support of fractures related to valley-unloading stress often requires angled bolts and tensioned angle bolts near the roof-rib intersection (Hill, 1986; Sames and Moebs, 1992). Post bars and cribs may also be needed.