Definition and formation: Sandstone is a common roof rock above many coal beds. It is especially common beneath paleochannel deposits, and transitional with some stackrock deposits. Thick-sandstone mine roofs commonly offer good roof conditions. Sandstone tops may have local weaknesses along bedding, however, which can lead to roof falls. Sandstone roofs can also be intersected by fractures or underlain by local pockets of slickensided shales, which are discussed separately.

Small roof fall in sandstone roof. Fall height is controlled by thin, discontinuous, undulating shale laminae between sandstone beds.

Discontinuities and obstacles: A weak-sandstone roof is not a discontinuity within the coal, unless it rolls or truncates the coal, which is discussed separately in cutouts and rolls.

Weak sandstone can cause roof falls in underground coal mines (from Greb and Cobb, 1987).

Potential roof-fall hazards: Roof quality is usually good in sandstone, but local weaknesses can occur along bedding. The most commonly reported falls in sandstone, which are not caused by combinations of fractures or other features, are bedding-plane weaknesses. Concentrations of mica, or coaly plant fossils between sandstone beds, can be a natural weakness in sandstone. In some sandstones, thin shale or mudstone laminae are common between beds, and can also cause a bedding-plane weakness. These shale-draped beds may be transitional with stackrock roofs.

Example roof fall in crossbedded sandstones of the Pond Creek coal bed in eastern Kentucky. The roof breaks along dipping weaknesses in the sandstone caused by thin, discontinuous shale laminae.

Some sandstone roof falls occur along inclined bedding, such as these examples from the Jellico and Harlan coal beds in eastern Kentucky.

Trends: Local falls in sandstone generally occur along bedding weaknesses, which may follow overall bed dip or may be caused by internal scours. Some sandstones are actually composites of different types of beds, with many internal scour surfaces. Thick sandstones may have formed from multiple channels within a larger valley. Weaknesses along these scours may follow linear to slightly sinuous trends.

Known Kentucky occurrences: Sandstones are extremely common roof rocks. Local weaknesses can occur in any sandstone roof.

Planning and mitigation: Local sandstone weaknesses are difficult to plan for. If falls are encountered, care should be taken to look for possible weaknesses in the roof that might be correlated in advance of mining, or recognized in a bore scope. Care should also be taken to determine if the reason for a fall is inherent in the sandstone, or caused by superimposed fractures or other features.

Roof support: Roof support for weak sandstones depends on the weakness, but in general would be similar to support methods for sandy stackrock conditions.

Sandstone fall with sharp breaks along and across bedding. This example is from a mine in the Hazard No. 8 coal bed in eastern Kentucky. Straps were used to support the roof (from Greb and Cobb, 1987).

Sandstone fall that required wood cribs for support. This example is from a mine in the Pond Creek coal bed in eastern Kentucky. Note the small roll (deformation) at the base of the low-angle dipping crossbeds, and numerous breaks along shaly laminae in the crossbeds.

 

 

Last Modified on 2017-11-01
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