Weak shale mine roofs
Definition and formation: Shale is probably the most common roof rock above coal beds. Many shales offer good, or at least predictable, roof conditions. Weak or soft shales, however, include moisture-sensitive shales, claystones, coaly shales, or rash, and shales immediately above coal beds, termed “draw slates” or “draw rock,” which are prone to small falls. Weak-shale roofs include roof rocks separated into categories by Sames and Moebs (1991) on the basis of low rock strength, moisture sensitivity, and bedding-plane spacing. Most weak shales occur in gray silty shales and shales with higher contents of swelling clays, rather than in black shales, but pyrite in black shales can also be moisture-sensitive and lead to breakdown near mine openings. Black shales, such as the shales above the Springfield (W. Ky. No. 9) and Herrin (W. Ky No. 11) coal beds, may be brittle and break easily under tension.
Discontinuities and obstacles: A weak-shale mine roof is not a discontinuity within the coal, and is not specifically associated with discontinuities, although clay veins and coal riders and claystones commonly occur in association with weak-shale mine roofs.
Potential roof-fall hazards: Roof quality is variable in shales, but many mines have some amount of weak shale. Where the base of shales is slightly coaly or rashy, some amount of the immediate roof must be taken as draw rock. Some shales are also subject to deterioration by moisture. Moisture-sensitive shales and claystones can lose strength through progressive softening and slaking (Haynes, 1975; Cummings and others, 1983; Sames and Moebs, 1991). Humidity in surface air is highest in summer, and when vented through underground mines causes sweating on rock surfaces and swelling of the shales, especially shales that contain appreciable amounts of swelling clays such as montmorillinite, illite, and mixed-layer clays. In winter, the air is dryer and rock strata lose moisture to vented air (Chugh and Missavage, 1981). These seasonal changes lead to deterioration of mine roof strength; roof falls are common in humid late spring and summer months (see, for example, Aughenbaugh and Bruzewski, 1973; Stateham and Radcliffe, 1978). Swelling pressures as high as 14,000 psi have been reported in the Illinois Basin (Chugh and Missavage, 1981).
Trends: Falls in weak shales are generally small, consisting of spalling or sloughing, but can be continuous and grow through time. Shale falls are most common toward openings near the surface, where moisture and humidity changes are most prevalent (Haynes, 1975; Chugh and Missavage, 1981; Sames, 1985; Sames and Moebs, 1991) and where near-surface fracturing can add additional weakness to the roof. Roof cutters from horizontal stresses and other factors are most common in shales and thinly laminated roof rocks (see, for example, Hill, 1986), but fractured shales, including shales with cutters and kink zones are discussed separately under fractures in roof.
Known Kentucky occurrences:Shales are extremely common roof rocks. Not all shales are inherently weak, but weak shales occur above most coal beds. Many mines have to deal with shale spalling and sloughing. One example is gray shales with plant fossils above the Fire Clay coal in eastern Kentucky (Greb, 1991). Gray shale with plant fossils and carbonaceous (coaly) concentrations are common above many coal beds, especially in draw rock, or draw “slate.” Draw rock or miner’s “slate” is the rock immediately above coal beds which must be removed along with the coal bed during underground mining.
In western Kentucky, coalbeds of the Carbondale Formation, commonly have black shale roofs. The black shale above the Springfield (W. Ky. No. 9) coal is called the Turner Mine Shale in Illinois. The black shale above the Herrin (W. Ky. No. 11) coal is called the Anna Shale Member. These shales are extensive across the Illinois Basin and tend to be harder and more brittle than gray shales or gray silty shales. Western Kentucky black shales above coal beds may contain calcareous concretions.
Planning and mitigation: Because this type of roof is so common, most mines plan for shale deterioration in mains and belt lines near mine openings. Testing shales in core (slake durability, clay content) can determine if shales in a planned mine roof are soft or will be susceptible to moisture changes in advance of mining. Roof maps can be made of shale thickness and combined with results from testing or visual inspection of sloughing and slaking. Tempering or cooling ventilation air through old mine works and other methods (where available) can also reduce humidity/temperature fluctuations in shale roofs (Chugh and Missavage, 1981; Cummings and others, 1983; Sames, 1985; Sames and Moebs, 1991). In some cases, where brittle or moisture-sensitive shales in the immediate roof are thin, and more stable roof rocks overly the immediate roof, potentially weak shales may be removed during mining to avoid future spalling, clean-up, and re-bolting.
Roof support: In weak shales, full-column resin bolts are often effective in roof control (see, for example, Chugh and Missavage, 1981), but as moisture progresses, bolt headers, straps, and wire mesh may be needed to support slaking roof, especially in travelways and beltways near the mine opening that must remain open for many years (Ingram and Molinda, 1988; Sames and Moebs, 1991; Molinda and others, 2008). In some problem areas, weak shales at four-way intersections have required supplemental support, including cable bolts, trusses, and timbers (Molinda and others, 2008). Leaving 4 to 6 inches of the top of the coal in the roof and bolting it to the roof has also been suggested in some cases to inhibit moisture penetration into overlying shales and claystones (Cummings and others, 1983; Sames and Moebs, 1991). In the Herrin (W. Ky. No. 11) coal of the Illinois Basin, tension bolts have been anchored into limestone above shale roof to suspend weak shales (Molinda and others, 2008).
Weak shales are also susceptible to compressional horizontal stresses. Shale falls related to horizontal stresses and other fractures are discussed separately under Fractures in mine roof.